MICROSCOPIC ILLUSTRATIONS 
 
 JVING OBJECTS, 
 
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 -[0>T Kr. 
 
 CONSTRUCTING MICROSCOPES, 
 
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THE LIBRARY 
 
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NEW WORKS 
 
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 Mic roSrojw &t (nice & Natural ftfttorg 
 
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 Just published, in 8vo. price 8s. 6d. boards, the 
 
 NATURAL HISTORY 
 
 OF 
 
 ANIMALCULES; 
 
 CONTAINING 
 
 DESCRIPTIONS OF ALL THE KNOWN SPECIES OF INFUSORIA 
 
 WITH 
 
 INSTRUCTIONS FOR PROCURING AND VIEWING THEM. 
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 The whole illustrated by coloured Plates and Woodcuts, price 18s. 
 
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 Contents : History and Description of Reflecting Microscopes On Micro- 
 meters, and their Use Monochromatic Illumination Solar Engiscopes, 
 and the Exhibition of Tests by them On trying Microscopes and 
 Engiscopes against each other, with Rules for ascertaining their compa- 
 rative merits On Eye-Pieces Illustrations of Angular Aperture On 
 the most improved Construction and Management of Solar and Gas 
 Microscopes Cuvier's Method of dissecting Objects in Fluids On a new 
 and simple fine Adjustment for Microscopes On making Drawings of 
 Inanimate Microscopic Objects, by F. Bauer, Esq. F.R.S. &c. On a new 
 Method of Illuminating Objects, by the Rev. J. B. Reade, M.A. 
 
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 Also, price One Shilling, 
 
 A LIST OF 2000 MICROSCOPIC OBJECTS: 
 
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LONDON: 
 
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 57, Skinner Street. 
 
MICROSCOPIC ILLUSTRATIONS 
 
 OF 
 
 LIVING OBJECTS, 
 
 THEIR NATURAL HISTORY, &c. &c. 
 
 WITH 
 
 RESEARCHES CONCERNING THE MOST ELIGIBLE METHODS OF 
 CONSTRUCTING MICROSCOPES, 
 
 AND INSTRUCTIONS FOR USING THEM, 
 BY C. R. GORING, M.D. 
 
 (JJUttum <menlftlr antr 
 
 BY ANDREW PRITCHARD, M.R.I. 
 
 )V. -MKM. SOt. ARTS, EUIV. ; AUTHOR OF "NATURAL HISTORY OF ANIMALCULKS, 
 ETC., ETC. 
 
 LONDON : 
 
 WHITTAKER AND CO. AVE-MARIA LANE. 
 
 1840. 
 
t/L 
 
 OP [N IONS 
 
 OF 
 
 SCIENTIFIC JOURNALS 
 
 ON THK FIRST EDITION OF THE 
 
 MICROSCOPIC ILLUSTRATIONS. 
 
 " Impressed as we are with the high importance of this branch of science, 
 and with the great value of the improvements which these gentlemen have 
 
 introduced, we looked forward with the most sanguine expectations and 
 
 have now no hesitation in stating it as our opinion, that Dr. G. and 
 Mr. P. have both accomplished their difficult tasks with the greatest success. 
 The coloured engravings are executed in such a masterly manner that they 
 will themselves bear to be seen by the microscope." 
 
 Brewster's Edinburgh Journal of Science. 
 
 " The present publication cannot fail to recommend itself generally, for it 
 is associated with elaborate descriptions and beautiful coloured engravings of 
 a variety of diverting and popular objects."- Journal of the Royal Institution. 
 
 " The notices of natural history in the volume before us are well arranged ; 
 the coloured engravings are executed with the most perfect fidelity ; and the 
 entire work will afford a high treat to the curious admirer of nature." Atlas. 
 
 M369318 
 
OPINIONS OF THE PUBLIC PRESS 
 
 ON THE 
 
 MICROSCOPIC CABINET. 
 
 " This is an exceedingly curious and interesting work ; the plates are marvellous.*' 
 
 Spectator. 
 
 " We earnestly recommend the Microscopic Cabinet both to the general and the 
 scientific reader, as an original, a valuable, and an ingenious work." 
 
 London and Edinburgh Philosophical Magazine* 
 
 " We are presented," in the Microscopic Cabinet, " with engravings of the most 
 beautiful or remarkable water-insects, and are moreover informed of their history, habits, 
 and anatomical conformation. We have also directions as to the use of the instru- 
 ments employed in these researches, and a full description of Mr. P.'s new microscope. 
 * * * We cannot speak ton highly of the finished coloured engravings ; they are 
 equally remarkable for the fidelity and the beauty of their execution. A mere inspec- 
 tion of these would be sufficient to create a taste for the study of a department of 
 philosophy which certainly is inferior to none in the interest it is calculated to create, 
 or the elegance of form and the harmony of adaptation which it presents to the view." 
 
 New Monthly Magazine. 
 
 " This work may be termed a practical treatise on the present state of Microscopes. 
 In the descriptive portion, valuable information is imparted on the structure and habits 
 of those most interesting creatures," Animalcules, Crustacea, and Insects ; " and for 
 procuring 1 , feeding, preserving, and the best mode of examining them, useful directions 
 are given ." Magazine of Natural History. 
 
 " This work selects a variety of living objects that have never been before described, 
 and depicts faithfully many others that have hitherto been most incorrectly represented. 
 The descriptions are most elaborately written in a plain and familiar style." 
 
 Monthly Magazine. 
 
 " A vast variety of practical information, popularly detailed, is diffused throughout 
 the work ; and the whole is illustrated by a number of very beautiful plates. * * * 
 It is decidedly the most practical and satisfactory treatise on the microscope that it has 
 hitherto been our good fortune to have met with." London Medical Gazette. 
 
 "This is a very complete and interesting work." Athenceum. 
 
 " Many of the descriptions of the structure and habits of animals, as observed by our 
 Author, will be found curiously interesting. The style in which the work is got up is 
 very creditable, and the illustrations extremely good and copious." 
 
 London Medical and Physical Journal. 
 
 " The lovers of natural history will derive both gratification and instruction by the 
 perusal of this work." Register of Arts. 
 
 " This is a beautiful volume, which must be deeply interesting to those engaged in 
 the study of natural history. The plates are well delineated, beautifully coloured, and 
 afforded at a reasonable rate. It is impossible to speak in too high terms of its execu- 
 tion." . London Medical and Surgical Journal. 
 
 " Some of the best executed plates and cuts that we have seen on the subject. We 
 kave no doubt that the present volume will go far to supply a desideratum ; and not 
 least as to the due appreciation of microscopes, perfection in the qualities of which is 
 obviously essential to the just characterization of the insect tribe." 
 
 Gentleman's Magazine. 
 
 *' The greater part of this volume is taken up with descriptions of the economy and 
 nature of aquatic insects, Crustacea, and animalcules, written in ^.popular style, without 
 any scientific display of terms to perplex the general reader, and is illustrated by some 
 admirably coloured engravings, of whose beauty and fidelity we can bear witness." 
 
 National Omnibus. 
 
 " The Microscopic Cabinet will essentially aid the cause of science." Atlas. 
 
 " This is a valuable work for scientific men, and of great .assistance to the student of 
 botany or natural history." Bell's Old Weekly Messenger. 
 
 " The Microscopic Cabinet is a work which does almost infinite credit to its authors, 
 and is certainly by far the most valuable contribution which has been made to micro- 
 scopic science in modern times." Mechanic.? Magazine. 
 
PREFACE. 
 
 AT the period of the publication of the first edition 
 of this work in 1829, microscopes on the new prin- 
 ciple had only just sprung into being, and some 
 apology for a treatise upon an instrument which 
 then apparently had little but novelty to recommend 
 it, might have been thought requisite; since that 
 time, however, its application to the sciences, and 
 the pleasure derivable from it as a source of amuse- 
 ment, have rendered this unnecessary. 
 
 It will be found that the present arrangement 
 of the papers is different to what it was in the first 
 edition, that having been suited to their being pub- 
 lished in numbers, which occasion being done away 
 with, Dr. Goring' s are now placed together, and 
 with them the description of his instrument. 
 
 In the first chapter of this volume the reader is 
 
10 PREFACE. 
 
 furnished with a brief sketch of the uses of the 
 microscope, and its various improvements. This 
 appeared to be preferable to commencing 1 with the 
 original Exordium, on account of the progress 
 microscopic science has made in public opinion, and 
 the enlarged view which is now taken with respect 
 to the microscope. As, however, there are many 
 persons who desire and expect to meet with the 
 Exordium in the present edition of this work, it 
 has been deemed advisable to give it entire in the 
 Appendix, with Dr. Goring's corrections. 
 
 The three succeeding chapters are appropriated 
 to a description of certain living microscopic objects; 
 and containing, as they do,, some remarkable parti- 
 culars relating to the wonderful metamorphoses 
 which these creatures are ordained to undergo, it 
 is confidently hoped they will not be deficient in 
 interest or instruction. 
 
 The remaining portion of the volume contains an 
 account of the mechanical construction of the micro- 
 scope and its apparatus; and since we have no 
 other work devoted to this branch of the subject, 
 affording to all persons possessing this instrument 
 such practical instructions as are absolutely neces- 
 
PREFACE. 11 
 
 sary for the right using- and managing of it, it is 
 fairly presumed that this will be serviceable. These 
 instructions, it may be as well to mention, are not 
 confined to any particular sort of microscope, but 
 are purposely drawn up in general terms^ in order 
 that some profitable information may be gleaned 
 from them, in the management of any microscope 
 whatever, no matter what may be its optical con- 
 struction. 
 
 From the rapidity with which one improvement 
 has succeeded another, it is necessary, in order to 
 come at a right conclusion as to the relative merits 
 of the productions of different artists, to take the 
 time at which their instruments were fabricated 
 into consideration; for, should some of them have 
 been made three or four years before the others, 
 the latter will in all probability excel the former; 
 whereas, had a comparison been instituted between 
 the simultaneous productions of the artists, the result 
 might have been quite the reverse. 
 
 In the explanation I have given of that remarkable 
 instrument, the Polarizing Compound Microscope, it 
 will be found that I have gone somewhat minutely 
 
12 PREFACE. 
 
 into detail respecting it ; and, with a view of render- 
 ing 1 it as available as possible, have printed in the 
 Appendix Mr. Talbot's highly interesting papers 
 upon the polarizing properties of crystals. Although 
 this is in itself almost a new instrument, a great 
 deal has already been accomplished by its means, 
 and a great variety of the hidden phenomena of 
 nature most beautifully displayed. 
 
 The kind consideration bestowed upon the first 
 edition by men the most distinguished in science, 
 who had turned their attention to this subject, and 
 the interest now so generally felt in microscopic 
 researches, induce me to hope that the present 
 volume a large portion of which has been re- 
 written, expressly that it should contain the latest 
 improvements will be favourably received. 
 
 ANDREW PR1TCHARD. 
 
 162, Fleet Street, London. 
 
CONTENTS. 
 
 Page 
 Preface . 9 
 
 CHAPTER I. 
 
 Introductory Remarks on the Application of the Micro- 
 scope to the Sciences, with an Account of its recent 
 Improvements 17 
 
 CHAPTER II. 
 
 On the Larva and Pupa of a Straw-coloured Plumed Culex 
 or Gnat 5O 
 
 CHAPTER III. 
 
 On the Larva and Crysalis of a Day-Fly 61 
 
 CHAPTER IV. 
 On the Larva of a Species of British Hydrophilus 70 
 
 CHAPTER V. 
 
 On the Terms employed in Microscopic Science 
 
 With a Description of the Vertical Microscope 79 
 
14 CONTENTS, 
 
 CHAPTER VI. 
 
 Page 
 
 A Description of the Achromatic Microscope, together with 
 its Apparatus, and the mode of using it : 
 
 Round Stand Microscope Solid Tripod-stand Microscope 
 On selecting Magnifying Powers Candle- holder, Shade, and 
 Condenser Spring Double-Stage Moveable Stages Spring 
 Phial-Holder Polarizing Microscope Apparatus for Dis- 
 secting Simple Lenses mounted in imitation of Achromatics 
 Garden Clamp for the Microscope Steadying Rods Camera 
 Lucida, for Drawing Micrometer Eye-pieces Method of 
 Viewing Moveable Fluids in the Cavities of Crystals Concave 
 Specula, Silver Cups, &c. Stops, or Diaphragms On View- 
 ing Transparent Objects by Daylight On Viewing Bodies by 
 Intermitting Light Black Ground Illumination for Transpa- 
 rent Objects On Viewing Opaque Bodies by Oblique Reflected 
 Light Method of determining the Forms of Minute Bodies 
 On Viewing the Currents in Fluids during Evaporation 85 
 
 CHAPTER VII. 
 
 Practical Remarks on Microscopes for viewing and drawing 
 Aquatic Larvae, &c. By C. R. Goring, M.D 146 
 
 CHAPTER VIII. 
 
 Whether there is a best possible way of constructing the 
 Stand, or Mounting, &c. of Microscopes (the specific 
 purpose or purposes to which they are to be applied 
 being first determined) By C. R. Goring, M.D 150 
 
 CHAPTER IX. 
 
 Description of Dr. Goring' s Operative Aplanatic Engiscope 168 
 
CONTENTS. 
 
 15 
 
 CHAPTER X. 
 
 Page 
 
 Manner of Observing with and Managing the Operative 
 Aplanatic Engiscope : 
 
 General Observations Method of Mounting for viewing Inani- 
 mate Transparent Objects by pure intercepted Daylight 
 Method of Mounting for viewing Transparent Objects by 
 Artificial Light Mode of Mounting for viewing Diaphonous 
 Bodies by reflected Daylight, either in a horizontal or vertical 
 position Way of Viewing Crystallizations of Salts and various 
 Chemical Actions Way of Mounting for Transparent Living 
 Objects Method of Mounting the Diamond and Sapphire 
 Microscopes for Transparent Objects Opaque Objects 
 Method of observing Opaque Objects by Daylight, plain or 
 condensed Silver Cups or Specula Mounting for Dissections, 
 &c. Mounting the Diamond and Sapphire Microscopes for 
 Opaque Objects The Amician Catadioptric Engiscope. By 
 C. R Goring, M.D 179 
 
 APPENDIX. 
 
 No. 1. 
 
 On the Optical Phenomena of certain Crystals. By H. F. 
 Talbot, Esq. F.R.S 213 
 
 No. 2. 
 Further Observations on the Optical Phenomena of Crystals. 
 
 By H. F. Talbot, Esq. F.R.S. 219 
 
 No, 3. 
 Exordium. By C. R. Goring, M.D 232 
 
 No. 4. 
 
 Swammerdam's Method of Dissecting and Preparing Objects 
 
 for the Microscope 244 
 
ERRATA. 
 
 Page 87, lines 1 and 14, for Chapter VIII. read Chapter IX. 
 93, line 10,/or Chapter VII. read Chapter VIII. 
 
NEW PHILOSOPHICAL INSTRUMENTS, 
 
 MADE BY 
 
 ANDREW PRITCHARD, 
 
 162, FLEET STREET, LONDON. 
 
 PRITCHARD'S POCKET MICROSCOPE. 
 
 THIS Instrument is very steady, its construction simple, and not liable to derange- 
 ment. It is furnished with rack-adjustment, concave mirror, and four magnifiers. 
 Price, in case, 38*. 
 
 PRITCHARD'S GARDEN-FRAME THERMOMETER 
 
 Is inclosed in a strong glass case, mounted with brass, so that it can be inserted into 
 the earth without danger. It will be found of great value for Mushroom beds, and 
 strikine: of delicate FjQger_Plants in frames. It is so graduated, that any person 
 
 NOTICE. 
 
 Mr. PRITCHARD having been for several years engaged in prepar- 
 ing Drawings and Specifier: . :. :onts for new Inventions, 
 offers his assistance to PA ^rs, and 
 others in the various branches connected with Patent b . 
 especially in ascertaining the novelty of Inventions id 
 the Titles for Patents in soliciting Patents making T 
 and Specifications for Enrolment, and enter: i 
 vent others from obtaining Patents wr.". v, 
 
 LONDON: 162, FLEET STREET. 
 
 En tije 
 
 (Price to Subscribers, 24s. ; to Non-Subscribers, 30s.) 
 
 A HISTORY OF INFUSORIA; 
 
 LIVING A X D FOSSIL; 
 
 ARRANGED ACCORDING TO THE 
 
 DIE INFUSIONSTHIERCHEN OF C. G. EHREXBERG. 
 
 ILLUSTRATED BY 
 
 SEVERAL HUNDRED ACCURATE COLOURED ENGRAVINGS OF THESE CURIOUS 
 CREATURES, HIGHLY MAGNIFIED. 
 
 BY ANDREW PRITCHARD, 
 
 Author of the " Microscopic Illustrations," " Natural History of Animalcules," $c. 
 
 This Work will contain descriptions of the external characters, internal structure, 
 instincts, habits, and locomotion of these interesting living atoms, forming a complete 
 Manual on this subject ; and it is believed will become of deep interest, not only to the 
 physiologist, microscopic and general observer, but also to the Botanist, as contain- 
 ing accurate figures of many creatures hitherto considered as belonging to the vege- 
 table kingdom. 
 
 Subscribers' names received at 162, Fleet-street, London. 
 
NEW PHILOSOPHICAL INSTRUMENTS, 
 
 MADE BY 
 
 ANDREW PRITCHARD, 
 
 162, FLJEET STREET, LONDON. 
 
 PRITCHARD'S POCKET MICROSCOPE. 
 
 THIS Instrument is very steady, its construction simple, and not liable to derange- 
 ment. It is furnished with rack-adjustment, concave mirror, and four magnifiers. 
 Price, in case, 38*. 
 
 PRITCHARD'S GARDEN-FRAME THERMOMETER 
 
 Is inclosed in a strong glass case, mounted with brass, so that it can be inserted into 
 the earth without danger. It will be found of great value for Mushroom beds, and 
 the striking of delicate Flower Plants in frames. It is so graduated, that any person 
 may, by mere inspection, know whether the heat is too great or too little, and thus 
 prevent injury to the plants. Price 18*. to 28*. 
 
 PRITCHARD'S THERMO-HYGROMETER. 
 
 This elegant little instrument indicates temperature, dryness, and humidity. It 
 will be found peculiarly useful in winter, especially in the apartments and chamber of 
 the Invalid, as it enables him to regulate with facility the proper state of dryness. In 
 rooms heated by closed stoves, air, or water, it should always be present. Price, on 
 ebony pedestal, 21*. 
 
 PRITCHARD'S GEOLOGICAL SLIDES. 
 
 This set of Slides illustrate, with the Phantasmagoria, or Magic Lanthorn, the 
 principles of Geology, exhibiting the various strata of the earth and their relative 
 positions. Also, the different Vegetables and Animals living and extant, peculiar to 
 each deposit. They will be found peculiarly useful for public and private instruction. 
 Price 3 15*. per set. 
 
 In tfjc 
 
 (Price to Subscribers, 24s. ; to Non-Subscribers, 30s.) 
 
 A HISTORY OF INFUSORIA; 
 
 LIVING AND FOSSIL; 
 
 ARRANGED ACCORDING TO THE 
 
 DIE INFUSIONSTHIERCHEN OF C. G. EHRENBERG. 
 
 ILLUSTRATED BY 
 
 SEVERAL HUNDRED ACCURATE COLOURED ENGRAVINGS OF THESE CURIOUS 
 CREATURES, HIGHLY MAGNIFIED. 
 
 BY ANDREW PRITCHARD, 
 
 Author of the "Microscopic Illustrations," "Natural History of Animalcules," &c. 
 
 This Work will contain descriptions of the external characters, internal structure, 
 instincts, habits, and locomotion of these interesting living atoms, forming a complete 
 Manual on this subject ; and it is believed will become of deep interest, not only to the 
 physiologist, microscopic and general observer, but also to the Botanist, as contain- 
 ing accurate figures of many creatures hitherto considered as belonging to the vege- 
 table kingdom. 
 
 Subscribers' names received at 162, Fleet-street, London. 
 
 J. Uarkio, Printer, 3, Cloude^ley -street, Islington. 
 
CHAPTER I. 
 
 INTRODUCTORY REMARKS 
 
 APPLICATION OF THE MICROSCOPE TO THE SCIENCES, WITH 
 AN ACCOUNT OF ITS RECENT IMPROVEMENTS. 
 
 " THERE IS PROBABLY NO BRANCH OF PRACTICAL SCIENCE WHICH HAS 
 UNDERGONE SUCH ESSENTIAL AND RAPID IMPROVEMENTS AS THAT WHICH 
 RELATES TO THE MICROSCOPE. IT HAS BECOME Q.UITE A NEW INSTRUMENT 
 IN MODERN TIMES, AND IT PROMISES TO BE THE MEANS OF DISCLOSING 
 THE STRUCTURE AND LAWS OF MATTER, AND OF MAKING AS IMPORTANT 
 DISCOVERIES IN THE INFINITELY MINUTE WORLD, AS THE TELESCOPE HAS 
 
 DONE IN THAT WHICH is INFINITELY DISTANT." Sir David Brewster*. 
 
 WHTLST the mind dwells with the highest admiration on 
 the rapid advances which are daily and hourly being 
 made in the pursuits of science, we are recalled almost 
 naturally to what is perhaps one of the most important 
 considerations connected with this deeply interesting 
 subject, namely, how it is that we are thus enabled to 
 make discovery after discovery into the inexhaustible 
 treasures of Nature, and by the help of what machinery 
 it is that we are making this astonishing progress. To 
 investigate the genius and faculties of the human mind, 
 the primum mobile in devising all that is great and all 
 
 * Encyclopaedia Britannica, 7th edit. art. Microscope. 
 C 
 
18 MICROSCOPIC ILLUSTRATIONS. 
 
 that is valuable, would fall within the province of the 
 profound metaphysician, rather than tljat of the practical 
 man of science ; but to elucidate the methods by which 
 genius is aided in its high researches after truth, and in 
 conducting those researches to their desired proficiency, 
 although an occupation of a much humbler grade than 
 the other, is one, nevertheless, that is by no means insig- 
 nificant, or devoid of interest. 
 
 Thus, for example, an illustration of the improvements 
 effected in the optical construction of the telescope, by 
 which the astronomer may attain to a more perfect know- 
 ledge of the celestial bodies by which he may discover 
 worlds beyond worlds innumerable, and read the laws by 
 which all those worlds are governed cannot be deemed 
 either unimportant or uninteresting. The invention of 
 this beautiful instrument gave a powerful impulse to the 
 human mind towards the study of astronomy, and every 
 improvement since made upon it has laid open new 
 sources of knowledge in that science, and created a 
 greater zest for its acquisition. Even the splendid dis- 
 coveries of the immortal Newton on the theory of the 
 Moon, until verified by the observations of a Flamstead, 
 by means of astronomical instruments, could not be 
 received as the actual laws which govern that satellite. 
 Even since these great discoveries, which carried the 
 mind of man into the immensity of the heavens, and 
 gave him a far more extended idea of the creative power 
 than any thing could possibly have given him before, 
 what has not the astronomer been able to effect through 
 
INTRODUCTORY REMARKS. 19 
 
 the vast perfection to which the telescope in more recent 
 times has been brought ? I need only mention the dis- 
 covery of the aberration of light; the resolution of 
 nebulas and of the double stars ; the motions of these 
 binary systems, and their subservience to the same law 
 of gravitation as governs the golar j the position and 
 direction of the latter in space: I might mention many 
 others, all equally important all marvellously grand ! 
 
 And so with respect to the Microscope in aiding to ex- 
 tend our knowledge of the economy of this planet. Every 
 improvement in this instrument has developed some im- 
 portant feature in the structure and design of the almost 
 infinitely varied organized beings of our earth. The 
 achromatic principle, which was at once adapted to the 
 telescope with the most astonishing success, did not 
 produce the same happy result on its first application to 
 the microscope. The recent discovery of Dr. Goring, 
 that the penetration of this instrument was dependent 
 upon the angle of aperture, or the number of rays of 
 light collected by the object-lens from every point on the 
 surface of the object, was requisite for the accomplish- 
 ment of this desirable purpose. By thus applying the 
 achromatic corrections and those for sphericity to this in- 
 strument, it has now been perfected, and elevated to a 
 grade nowise inferior to the telescope. The fruits of 
 this discovery are fast pouring in upon us, and promise 
 to yield a rich and most abundant harvest. We will al- 
 lude cursorily to some of those which have been reaped 
 already ; a few only will suffice to shew their importance, 
 
20 MICROSCOPIC ILLUSTRATIONS. 
 
 and to claim for the microscope a high rank among 
 optical instruments. 
 
 The fact is, that since the modern improvements the 
 microscope has undergone, it is being brought to the 
 assistance, and is at the present time furthering the pro- 
 gress of almost every branch of natural science. To the 
 Geologist it may be said to be a new instrument. But what 
 has it not even now effected for him ? In his study of or- 
 ganic life and structure, it has unfolded to him the pre- 
 cise characters of divers animals and plants which inha- 
 bited and clothed our earth in ages which have long 
 passed away. Look at the discoveries of Agassiz on the 
 fossil creatures of the deep ! By a microscopic investi- 
 gation of such portions of them as have withstood the 
 destructive power of time, namely, their scaly covering, 
 he has been able so to group and class them, that the 
 characters and habits of the genera belonging to each dis- 
 tinct era are clearly demonstrated. A microscopic exa- 
 mination also of the testaceous remains of sundry Ento- 
 mostraceans found in slate-clay formations, now ele- 
 vated much above the level of the sea, prove them to 
 have been at some time or other imbedded in the waters. 
 And the Naturalist may even determine by his inspection 
 of the shell, whether the species were the inhabitants of 
 fresh or salt water, and consequently whether the strata 
 themselves were the indurated beds of the sea, or of some 
 river or lake. -.3* 
 
 The most perfect animal remains which the microscope 
 has disclosed to us, are the various loricated Infusoria 
 
INTRODUCTORY REMARKS. 21 
 
 of the division Bacillaria*. These minute creatures are 
 so inconceivably numerous that they cover many miles 
 of surface with several feet of thickness ; as instanced 
 in the polishing-slate and rotten-stone of Bohemia. In 
 Tuscany whole mountains consist almost entirely of the 
 silicified shells of these creatures ; thus combining with 
 each other in infinite numbers, to counterbalance, as it 
 were, their individual minuteness, and to teach the un- 
 thinking this useful lesson, that Nature, in all her ope- 
 rations, is never employed in vain, and that, what are ap- 
 parently her most insignificant productions, fall not be- 
 neath the notice of the profoundest inquirer after truth. 
 To the Botanist the aid of the microscpoe is indispen- 
 sable. In the investigation of our fossil-flora, what does 
 it not exhibit to us ! How beautiful and delicate is the 
 structure of the envelope of some of the fossil-fruits; those, 
 for instance, of our London clay, when viewed under this 
 instrument! And how important is it, that, by its assis- 
 tance, we can determine with accuracy the natural orders, 
 genera, and sometimes the very species of the trees and 
 plants of former epochs ! How, beyond all question, is 
 now demonstrated the vegetable origin of our coal ! 
 Preserved within a bituminous lump of coal, which has 
 been deposited for thousands of years deep in the bowels 
 of the earth, you may discern not only the woody fibre, 
 its arrangement, and the disposal and form of the 
 medullary rays, but even the most delicate of the vege- 
 table organs, such as the spiral vessels and the beautiful 
 
 * See Natural History of Animalcules, page 59. 
 
22 MICROSCOPIC ILLUSTRATIONS. 
 
 terminations of those vessels ! These are as distinctly 
 discoverable as in the finest preparations of a recent plant. 
 And what can be more amusing and instructive than the 
 examination of the silicified woods, when formed into 
 sections no thicker than the paper of a Bank-note ? 
 Thus rendered pervious to light, the organic structure of 
 the wood becomes plainly distinguishable. And ema- 
 nating from thisj what can be a more interesting subject 
 than the inquiry into the mode in which the siiicifying 
 process has been carried on by which the constituent 
 elements of the inmost and minutest portions are changed 
 whilst their form and situation and colour remain the 
 same ? In investigating also that extinct genus of plants, 
 the Lepidodendra, a similar idea is raised in the mind, as 
 to what must have been the particular state of the earth 
 with respect to atmosphere and temperature at the pe- 
 riod of their growth, and what the changes which have 
 since taken place, in order to bring it to its present 
 condition. 
 
 In our physiological inquiries into the animal and 
 vegetable productions of the present time, the assistance 
 of the microscope is essentially requisite. When Dr. 
 Harvey made his grand discovery of the circulation of 
 the blood, and first lectured upon it, in St. Bartholo- 
 mew's Hospital, in 1619, he was ridiculed, and lost his 
 practice, through maintaining what was then supposed 
 to be so absurd and wild a theory. The idea was 
 suggested to his mind by reflecting on the valves of the 
 heart and veins, which were evidently so planned as to 
 
INTRODUCTORY REMARKS. 23 
 
 allow a fluid to pass but one way. All the philosophical 
 reasoning, however, of this celebrated man could not 
 establish, what appears to us so plain a truth, until it was 
 evidenced in the circulation of cold-blooded animals by 
 means of the microscope, and thus placed beyond a 
 doubt. Discerning, as we can do, the very forms of the 
 globules of that fluid, as they flow through the capillaries 
 from the arteries to the veins, in obedience to the laws 
 impressed upon them by the Almighty Creator viewing 
 this most sublime phenomenon, by which life itself is 
 diffused throughout, and sustained in every part of the 
 system who can resist conviction of the great trutli ? 
 
 Nor is it a matter of less importance in a scientific 
 point of view, or less interesting, that by the same 
 means we can perceive the fibrous structure of the 
 muscles and nerves, the form and arrangement of the 
 canals by which the internal cavities of the bones are 
 lubricated and nourished, the glandular structure of that 
 beautiful and complex apparatus by which the secretions* 
 are carried on all, and each of these, requiring but the 
 aid of one of our improved microscopes to render them 
 distinctly visible. Again : how admirably developed by 
 means of the microscope are the curious and complex 
 structures of the eyes of insects, the crystalline lenses of 
 those of fish, birds, &c.f and many of the other parts of 
 the visual organsj. The eye that useful and delightful 
 
 * Nouvelles Recherches sur la Structure de la Peau, par M. Breschet. 
 f Philosophical Transactions, 1833. 
 J See Langenbeck on the Eye. 
 
24 MICROSCOPIC ILLUSTRATIONS. 
 
 portion of us which furnishes all the endless variety of 
 objects from which we derive so great enjoyment 
 resembles, in its peculiar formation and arrangement, an 
 achromatic optical instrument. And if we descend to 
 the lower classes of animals nay, I would hardly say 
 lower, lest some perhaps might imagine that in their 
 small forms they do not evince as much perfection as is 
 discoverable in beings of a higher scale, and have not 
 all the functions which are necessary to life as full in 
 operation as even man himself if we enter upon an 
 investigation of their minute structures, we can deter- 
 mine absolutely nothing without the microscope ; and 
 our knowledge of the very existence of many highly- 
 organized and active creatures is wholly dependent 
 upon it. 
 
 Vegetable organography, upon which the modern 
 botanist depends so much for his systematic arrangement, 
 and with which the student is so greatly interested and 
 amused, owes almost its very existence to the microscope. 
 This observation will be found to apply in an especial 
 manner both to the cellular and vascular tissues of plants. 
 The membranous cellules of cellular tissue are some- 
 times not more than l-1000th of an inch in diameter; 
 and those of the ordinary size are about 1 -200th or 
 1 -300th. How, then, is it possible that we could become 
 acquainted with their forms and arrangement but by the 
 aid of the microscope? And so with respect to vascular 
 tissue : it is absolutely indispensable toward acquiring 
 an accurate knowledge of the structure and forms of 
 
INTRODUCTORY REMARKS. 25 
 
 these membranous tubes, arid of the spiral or annular 
 fibre which surrounds them. 
 
 A knowledge of the fructification, if I may so express 
 myself, of that numerous and curious class of plants, the 
 Acrogens, could not be obtained without it; nor could 
 the existence of many of them, such as the Fungi, 
 Lichens, Algae, and some of the Musci, be proved. By 
 its powers even the ashes of vegetables may be seen to 
 contain the decisive characteristics of organic structure 5 
 and the long-debated question of the antiquarian, as to 
 whether the " fine linen of Egypt," in the times of the 
 Pharaohs, were of linen or cotton fibre, seeing the 
 latter is now indigenous to that country, is for ever set 
 at rest. 
 
 In many of the larger portions of plants, such as the 
 cuticle of their leaves, the stomata, &c. which require 
 but a shallow magnifying power to display them, there 
 is as great a difference manifested when these are viewed 
 under an achromatic microscope, or under the old com- 
 pound, as is perceptible between the most highly-finished 
 miniature, where the most delicate features and even 
 the down on the skin are correctly depicted, and the 
 mere black and white profile, where we see but the rude 
 contour of the face. Surely, then, as works of art 
 merely, instruments which can effect so much as this 
 are justly entitled to a due share of consideration even 
 from the most refined and polished minds. 
 
 In the foregoing remarks on the application of the 
 microscope to botanical purposes, I have given the 
 
26 MICROSCOPIC ILLUSTRATIONS. 
 
 reader only a general outline ; to enter upon any thing 
 like details would of necessity extend an introductory 
 chapter of this description far beyond its proper bounds. 
 I may, nevertheless, be permitted to make use of one 
 example by way of illustration. But how shall I select 
 one where the materials suited to construct it are so 
 superabundant, and where they all present so many 
 points of interest ? It will be readily admitted that this 
 is by no means an easy choice. I will limit my obser- 
 vations, however, to a single microscopic slider, and see 
 what instruction is derivable from it. Suppose this 
 slider to contain some sections of a recent or fossil wood ; 
 for instance, three specimens or shavings of such ex- 
 treme tenuity, that if they were exposed, they would be 
 wafted away on the slightest breeze. Let them be 
 weighed, and they will not exceed a grain. Hand them 
 to the chemist, and he can only prove to you that their 
 primary constituents are oxygen, hydrogen, carbon, ike. 
 and in so doing he will destroy them. What, then, does 
 an inspection of them under the microscope reveal ? It 
 will tell you, in the first place, whether they grew up, 
 like our forest trees, by yearly additions to the outside of 
 their woody centres, or by internal accessions, like most 
 of the. productions of the tropics. It will tell you 
 whether their leaves were veined or not whether their 
 embryos were dicotyledons or monocotyledons whether 
 the trees from which they were cut had branches or not 
 and, if they had, whether these were thick and sturdy, 
 like the boughs of the oak ; or thin and flexible, like the 
 
. INTRODUCTORY REMARKS. 2? 
 
 branches of the fir tribe. It will tell you whether the 
 wood might be easily cleft asunder, like deal; or would 
 sooner break, like beech ; whether it was elastic, like the 
 pine so admirably suited for the masts of ships; or, like 
 the stubborn oak, would rather snap than yield to the 
 wind. These are some of the ordinary properties deve- 
 loped by the microscope. 
 
 Again, in taking a more minute physiological survey, 
 our information will be by no means less complete. For 
 these same specimens will disclose to us, under the 
 microscope, the form and arrangement of their woody 
 fibres, the disposition of their barks, the beautiful struc- 
 ture of the tubular receptacles, by which their secretions 
 have been carried on and their growth promoted. In 
 short, so much will be revealed by them in this manner, 
 that the actual distinction between a wild and a culti- 
 vated tree may be, in some cases, clearly traced. " Thus, 
 in the cultivated cherry, the plates of the medullary rays 
 are very thin, the adhesions of them to the bark are very 
 slight, and hence a section of the wood of that plant ex- 
 hibits a pale, smooth, homogeneous appearance; but, 
 in the wild cherry, the medullary plates are much thicker, 
 they adhere to the bark by deep broad spaces, and are 
 arranged with great irregularity, so that a section of the 
 wood of that variety has a deeper colour, and a twisted, 
 knotty, very uneven appearance*." 
 
 Should they be specimens of endogenous woods (ver- 
 * Lipdley's Botany, page 241. 
 
28 MICROSCOPIC ILLUSTRATIONS. 
 
 tical sections of which I have at some expense and 
 trouble lately introduced,) what a volume of instruction 
 may be derived from them ! whilst the exquisite beauty 
 arid lace-like form of the transverse cuttings will bid de- 
 fiance to every attempt at describing them ! 
 
 In order that this class of microscopic objects may be 
 rendered as useful as possible, it may not be unimportant 
 to suggest that collections of thin sections of woods should 
 be selected according to the natural arrangement of 
 plants. In the cases I have lately mounted, an example 
 is given from all the principal groups ; and where the 
 genera, in any natural Order, are very dissimilar, two 
 specimens are taken, for the purpose of instituting a 
 comparison between them. 
 
 I need hardly mention how essential is the aid of the 
 microscope to the Mineralogist in determining the 
 crystalline structure of a body. In the study of crys- 
 tallography, which science may be said to have been for 
 a long time almost at a stand-still, a very extensive field 
 of research appears to be now opening by the adaptation 
 of polarized light to a microscopic examination of minute 
 crystals, thus eliciting a great variety of curious and 
 beautiful properties, entirely unknown to the world be- 
 fore. This subject being as yet completely in its infancy, it 
 would be unfair to expect an elucidation of it at the present 
 time. To convey, however, some general idea to the reader 
 of the additional degree of interest which attaches to the 
 phenomena of crystallization by this happy contrivance, it 
 
INTRODUCTORY REMARKS. 29 
 
 is only necessary to state, that we have now displayed 
 to us minute crystals, with a brillancy and richness of 
 colouring that is quite inconceivable. We see the smallest 
 difference in their thickness marked by some exquisite 
 change of colour ; whilst the beautiful black cross in the 
 circularly arranged crystals of zanthate of ammonia, and 
 the cross with the coloured rings in the compound of phos- 
 phoric acid and borate of soda, &c. &c. excite our admi- 
 ration beyond all bounds. These newly-discovered phe- 
 nomena, after a patient investigation, may lead to results 
 highly interesting, and of great importance to science. 
 
 By a simple modification of its polarizing apparatus, 
 the microscope becomes of great value in investigating 
 some of the physical properties of matter : it is capable 
 of being constituted a complete polariscope, and made to 
 exhibit the phenomena of polarization in plates of crystals 
 cut perpendicular to their axes ; for instance, the coloured 
 rings, the black cross, the tints of unannealed glass, of 
 amethyst, &c. 
 
 When relaxing from the stern pursuit after the exi- 
 gencies of life, where, perhaps, we may have been in- 
 dustriously labouring to benefit the condition of our- 
 selves or families, what a delightful and inexhaustible 
 source of amusement is ever open to us where a good 
 microscope is at hand ! It is not always an easy task to 
 find innocent amusement : to find it blended with im- 
 provement is to find it indeed. We may, it is true, be 
 entertained by the veriest trifles in life, but we shall soon 
 grow weary of them ; and even the converse of intimate 
 
30 MICROSCOPIC ILLUSTRATIONS. 
 
 acquaintances at times becomes vapid and dull. A re- 
 creation, to last beyond an hour, must be made to instruct 
 as well as to amuse j it must, to a certain extent, be 
 commensurate with what a rational and intelligent mind 
 is in quest after, or we shall speedily be thrown upon 
 our resources to find out a new one. A good micro- 
 scope, as I have said, is a never- failing source of amuse- 
 ment. Were we permitted to draw a comparison be- 
 tween it and that grand and noble instrument, the tele- 
 scope, we should be compelled to admit that it possesses 
 very many advantages even over that instrument in this 
 respect. We can use it at all times and seasons, and that 
 with the greatest facility. We can invite our friends to 
 pass an evening with us in being gratified with its per- 
 formance, without any fear of encountering a disappoint- 
 ment. Not so, however, with the telescope. A few nights 
 only throughout the year are what we may term good for 
 astronomic purposes, and over these few we can exercise 
 no control. Our friends may be invited they may assem- 
 ble and the weather may prove so inauspicious as to 
 render the finest instrument in the world of no avail. And 
 even on the clearest night, when all our expectations are 
 raised to the highest pitch, the wind may suddenly shift, 
 so as to occasion the greatest interruption $ so that, per- 
 haps, the very means we have taken to expand and ele- 
 vate our notions of the infinite wisdom and power of the 
 Divine Being, may terminate, through some mishap, in 
 ruffling our minds, and causing an effect the very 
 opposite to that we had designed to produce. 
 
INTRODUCTORY REMARKS. 31 
 
 The valetudinarian dare not seek his pastime from the 
 employment of the telescope ; were he to do so, his 
 career would soon be stopped, inasmuch as his remain- 
 ing out exposed to a few cold, damp nights, would at 
 once prove fatal to him. And even those who are in the 
 full enjoyment of health may suffer very materially by 
 prolonging their observations with a view of attaining to 
 some proficiency in astronomical pursuits ; to say 
 nothing of the inconvenience it occasions by breaking in 
 upon that regularity which is so essential to the comfort 
 and happiness of life. To the professed astronomer these 
 remarks are not of course addressed ; with him such dif- 
 ficulties as these do not, and ought not, to have any 
 weight. My object was to institute something like a 
 comparison between the telescope and the microscope 
 as to their capability of affording rational amusement to 
 the generality of mankind, when it is desirable to be- 
 guile a vacant hour, and where our whole time is not at 
 our own disposal. In making out my case in favour of 
 the microscope in this respect, I may be allowed to refer 
 the reader to such proof as is afforded by the subjects 
 treated of in the first three chapters of this work. 
 
 " These," says Dr. Goring, when speaking of livingobjects 
 viewed under the microscope, " experience has shewn to 
 excite the strongest emotions of pleasure and satisfaction 
 in the great mass of observers of all ranks, whom I 
 have always found to be most delighted by comparatively 
 large living objects, seen with medium powers. In fact, 
 they seem to afford the same sort of gratification with a 
 
32 MICROSCOPIC ILLUSTRATIONS. 
 
 menagerie of living wild beasts on the large scale ; and 
 most certainly many of them wonderfully emulate the 
 ferocity, voracity, cunning, and cruelty of the mammalia. 
 They prey on each other, and fight with a degree of 
 determined obstinacy, not inferior to that of any beings 
 whatever. They have likewise a thousand diverting 
 pranks and humours, quite peculiar to themselves. In 
 addition to these amiable and amusing qualities, they 
 possess such a high degree of transparency, that their 
 unique and beautiful internal machinery is as clearly 
 perceptible as if they were made of glass ; so that, with- 
 out any dissection, we can unravel all the mysterious 
 workings of their nature ; such as the circulation of the 
 blood, the pulsations of the heart, the peristaltic motion 
 of the intestines, and the play of every muscular fibre. 
 This property of transparency is not possessed by any 
 other living beings with which I am acquainted, except 
 the Animalcula infusoria. I may observe, that no perfect 
 insects present so many facilities for adaptation to the 
 microscope, or can be so easily preserved and managed; 
 so that the larvae, &c. appear to me peculiarly and ex- 
 clusively devoted to the consideration of microscopists*." 
 Having now given a brief outline of the uses to which 
 the microscope may be applied, we will proceed to con- 
 sider those practical improvements which, in modern 
 times, the instrument itself has undergone, and which, 
 step by step, have at length brought it to its present 
 high state of perfection. 
 
 * Exordium, first edition, p. 6. 
 
INTRODUCTORY REMARKS. 33 
 
 Microscopes may be classed under two denomina- 
 tions, simple and compound. 
 
 The simple microscope presents an object to our view 
 under a greater angle than that which it is seen under 
 when viewed by the naked eye ; whence it is that the 
 impression is derived of it being amplified or magnified. 
 The amount of this amplification will be in precisely the 
 same ratio as that when a person is seen at different 
 distances from us : the nearer he is, the greater is the 
 angle he will be viewed under 5 and so vice versa. 
 Hence it is, that a lens which will admit of an object 
 being placed within one inch of it, will amplify twice* 
 as much as another the focal length of which is two 
 inches. This quality, viz. the focal length of a lens, or 
 its power of admitting an object within a definite distance, 
 so that it may be distinctly visible, is obtained by giving 
 to the surface of any transparent medium (say glass) a 
 spherical or other curved figure. Now it will be evident 
 that, on account of the curvature which is thus given, 
 the rays of light which proceed from any given point in 
 an object to the middle of the lens will pass in a more 
 direct course through it (assuming the lens to be convex) 
 than those which proceed from the same point to the 
 outside or edges ; and consequently the former will be 
 less refracted or bent in their passage than the latter. 
 All the rays, therefore, after permeation, will not proceed 
 towards the same point in the axis of the lens, but 
 crossing it at different distances they will enter the eye 
 
 * See Mierographia, page 63. 
 D 
 
34 MICROSCOPIC ILLUSTRATIONS. 
 
 confusedly, and produce a nebulous or indistinct impres- 
 sion of the magnified object. This defect is commonly 
 designated spherical aberration. Another obstruction to 
 perfect vision is occasioned by the effect produced upon 
 the light itself, when passing from one medium into 
 another of different density. Light (say common white 
 light) is a compound of coloured lights, which are not 
 equally refrangible; that is, some of the colours are 
 more powerfully acted upon by a refracting body than 
 others ; so that a white ray, when it enters a lens, is more 
 or less broken up and separated into its coloured con- 
 stituents, which gives a semblance of colouring to the 
 object we are viewing. This defect is termed chromatic 
 aberration, or dispersion. 
 
 These two great defects are to be obviated in con- 
 structing a microscope. Until within a few years, the 
 lens of a simple microscope consisted of a single piece of 
 glass made into a curved form, or a spherule. In order 
 to increase its magnifying power, i. e. in order to admit 
 an object being placed very near the lens, so that it 
 might be viewed under a greater angle, Di Torre, 
 Leeuwenhoek, and others, constructed lenses of spherules 
 of exceeding minuteness some, it is recorded, with a 
 focal length of not more than 1 -700th or 1- 800th part of 
 an inch. This method of obtaining high magnifying 
 powers was attended with so much inconvenience in 
 practice that it counterbalanced all the advantages which 
 would otherwise have resulted from it. Whilst the 
 obstructions to vision just mentioned, viz. spherical and 
 
INTRODUCTORY REMARKS. 35 
 
 chromatic aberration, were obliged to be remedied in 
 a low power by placing a diaphragm or stop behind the 
 lens, so as to reduce the aperture ; by which means so 
 much of the light transmitted by the lens was cut off, that 
 the delicate structure of an object was hardly visible for 
 want of light. 
 
 The calculations of Euler and other mathematicians 
 were soon directed towards finding out such forms for 
 single lenses as would occasion the minimum quantity 
 of aberration. Still, the microscope derived but little 
 practical advantage from their investigations ; and the 
 reason of it I apprehend was this : their formulae were 
 all calculated for lenses of very small angular aperture, 
 whilst the goodness of a lens for the microscope depends 
 upon the opposite extreme. 
 
 In 1824, in consequence of Sir D. Brewster's remarks 
 on the diamond, I commenced making lenses of precious 
 stones*. These, when the stones are clear, free from 
 flaws, and from double refraction, are the most perfect 
 single magnifiers we at present possess. 
 
 From the period just named, a new era sprang up in 
 the history of the microscope. In 1829f 5 Dr. Wollaston 
 sent forth to the world his doublet lens, which consisted 
 of two plano-convex glasses. This invention, the result 
 of experiment, caused a most astonishing alteration for 
 the better in the constructing of microscopes. It sug- 
 gested the idea of employing more than one glass in 
 
 * See Microscopic Cabinet, chap. 14. 
 t Philosophical Transactions, 1830. 
 
36 MICROSCOPIC ILLUSTRATIONS. 
 
 constituting a deep magnifier ; and the consequence was, 
 that we had soon devised for us a triplet, composed of 
 three plano-convex lenses, which is by far the most 
 perfect construction employed in simple microscopes. 
 I may remark, however, that the use of these compound 
 magnifiers is limited to the examining of delicate trans- 
 parent objects which admit of being brought almost in 
 contact with the lens. I may mention another improve- 
 ment, though perhaps rather prematurely, viz. that the 
 achromatic object-glass may be used as a simple micro- 
 scope, and with very great advantage when the magni- 
 fying power is moderate *. 
 
 We now proceed to consider the improvements which 
 have been made in compound microscopes. The com- 
 pound, unlike the simple microscope, in which we look 
 through the magnifier at the object itself, is furnished 
 with a second magnifier, designated the eye-piece, through 
 which the sight is directed, not to the object, but to an 
 image of it, which has been magnified by the first lens, 
 or, as it is called, the object-glass. Now, we have noticed 
 that the best magnifiers of simple microscopes are in fact 
 compounds, being composed of two or three lenses ; but 
 yet they have not the double action of a compound 
 microscope, even should it be composed of only two 
 lenses. Dr. Goring, however, to prevent any confusion 
 arising between the one and the other, proposed the 
 
 * See Chapter X. 
 
INTRODUCTORY REMARKS. 37 
 
 name of Engiscope* for the instrument we are now 
 considering. 
 
 In this instrument, it is of the utmost importance that 
 the object-glass forming the image, which is to undergo 
 a second amplification by the eye-glass before it meets 
 the eye, should be as perfect as it can possibly be made ; 
 because, every error occasioned by it will be magnified 
 by the eye-glass, and thus cause very great confusion. 
 Dr. Goring having this in view, directed the late Mr. 
 William Tulley, in 1824, to make him a triple achromatic 
 object-glass, for a compound microscope. With this, 
 after many trials, during which the Doctor had contrived 
 to enlarge the aperture of his lens, he found that he 
 could distinguish some delicate markings on certain 
 animal tissues, which, with equal magnifying powers 
 having less aperture, he had not been able to discern. 
 
 Hence originated the discovery, that the penetration 
 of a microscope is dependent upon the angle of aperture 
 of its object-glass. 
 
 As this discovery has tended, in a great measure, to 
 bring the microscope to a high state of perfection, I may 
 be permitted to give a familiar explanation of the reason 
 why an increase of aperture is so efficacious. For the 
 details of the subject, the reader is referred to Dr. 
 Goring's own account, in the Micrographia, Chap. VII. 
 I am the more anxious to give this explanation, because 
 many persons are at a loss to conceive how it is that in 
 a compound achromatic microscope, where the light has 
 
 * See Chapter VII. 
 
38 MICROSCOPIC ILLUSTRATIONS. 
 
 to pass through several glasses, each of which impedes a 
 portion of the rays, whilst others are lost by reflection at 
 the surfaces, that more should remain for the purpose 
 required than when a single glass is employed. 
 
 Every one who has considered the subject at all, will 
 understand that by a series of glasses of different media, 
 aberrations, &c. may be corrected and almost entirely 
 dismissed ; but it is not so readily perceived how a 
 compound microscope, having a series of glasses, can, 
 with less illumination, give a brighter and more vivid 
 picture, with more of the detail or minute structure of 
 an object, than can be obtained with greater illumination, 
 by a single lens. In this particular, however, consists 
 the main advantage of an achromatic. 
 
 Let me premise that, in order to render any object 
 visible, it is necessary that rays of light should proceed 
 from it, either by reflection from its surface, or by trans- 
 mission through it, to the eye. Again, if the number of 
 rays be insufficient, the object cannot be seen, notwith- 
 standing we employ a microscope for the purpose. 
 Bearing this in mind, I will endeavour to explain how 
 an increase of angular aperture in an object-glass, 
 independent of any increase of its magnifying power, 
 will admit a greater quantity of light from any given 
 point on the surface of an object to pass through the lens, 
 so as to render the structure of the object visible. 
 
 Let A, and , represent two objects, in all respects 
 alike, and let us employ two microscopes, of equal 
 magnifying powers, for the purpose of viewing them. 
 
INTRODUCTORY REMARKS. 
 
 39 
 
 Suppose that we are going to look at some spot on the 
 surface of A, or a, which we will imagine to be a delicate 
 
 Fig. i. 
 
 Fig. 2. 
 
 tissue. By a well-known law of light, the rays proceed 
 in right lines, in all directions, from this spot, in the 
 manner shewn by the dotted lines in both figures. 
 Suppose B, B, and b y b, to be two object-glasses, bf equal 
 focal lengths ; the former a single lens, of the best 
 construction, such as was used in the old compound 
 microscope, and the latter a lens of the newest form, 
 termed an achromatic. Now, these object-glasses will 
 form their respective images at i, and i, and they will be 
 of equal dimensions. But if the number of rays pro- 
 ceeding from A, and falling upon the single lens B, B, is 
 
40 MICROSCOPIC ILLUSTRATIONS. 
 
 not enough, when collected at i, sufficiently to stimulate 
 the eye, any minute pore, striae, or other marking at A, 
 will not be rendered visible ; whilst, from the increase 
 of aperture in the achromatic lens, b, b, allowing much 
 more light from a, to fall upon it, and to be transmitted 
 through it and collected at i, every marking, &c. at , 
 will be clearly represented at i ; and the eye, being 
 powerfully acted upon by this increase of ligbt, will 
 become highly sensible of it. 
 
 The angles B, A, B, and b y a, b, are the angles of aperture 
 of the respective object-glasses ; and the quantity of 
 light collected and transmitted by each will be as the 
 squares of B, B, and b, b, the focal lengths being equal. 
 Hence it is that the power of a microscope, or that 
 faculty it possesses to render the structure of an object 
 visible, depends upon the angle of aperture of its object- 
 glass, and not upon its magnifying power alone. 
 
 But it may be supposed, perhaps, from this reasoning, 
 that if we throw a greater quantity of light upon an 
 object, so that more may be collected by the object-glass, 
 we shall be the better able to define its structure ; which 
 would probably be the case if the additional light could 
 be thrown only upon those minute parts which we wish 
 to examine, and not upon the whole object. But as we 
 cannot do this as the increase of illumination cannot be 
 made to increase the relative proportions of light which 
 proceed from these minute parts, the intended advantage 
 will not be derived. 
 
 shewn, however, that the goodness of a 
 
INTRODUCTORY REMARKS. 41 
 
 microscope is chiefly dependant upon the angle of aper- 
 ture of its object-glass, (freed from the aberrations just 
 mentioned), it is of some importance to be able to mea- 
 sure this angle with accuracy. When the object-glass 
 consists of a single lens, or of two cemented together, 
 its angle of aperture may be easily determined by the 
 actual measurement of its acting focal length and 
 diameter. But if it consist of two or three pairs of lenses, 
 not in contact with each other, this mode of measure- 
 ment will not give a correct result. The simplest and 
 best method in such cases, and indeed I may say in all 
 cases, is by means of the Microscope-gnomeometer, de- 
 scribed at page 218 of the Microscopic Cabinet. 
 
 Considering what has been just stated, I proposed, in 
 1835, that achromatic object-glasses should be applied to 
 the micrometer microscopes which were made use of for 
 reading off the divisions whilst making the trigonome- 
 trical survey of Ireland. They have since been used by 
 astronomers for similar purposes in the microscopes 
 attached to transit circles ; the advantage of which is, that 
 observations can be read off much longer in the twilight 
 of an evening without artificial illumination ; thus caus- 
 ing less fatigue to the eye than is experienced by looking 
 alternately at objects illuminated by different coloured 
 lights. 
 
 Lest it should be imagined that I have dwelt somewhat 
 too much upon the subject of angular aperture, I will ad- 
 duce a case in point to shew the necessity of its being 
 better understood than it is, I have in my possession at 
 
42 MICROSCOPIC ILLUSTRATIONS. 
 
 this moment a triple object-glass, evidently made in 
 imitation of one of Mr. Tulley's achromatics. In con- 
 structing it, however, the optician, a person of great 
 respectability, was so unacquainted with what an achro- 
 matic is designed to effect, that he actually placed a stop 
 behind the lenses, so that, notwithstanding the focal 
 length does not exceed half an inch, an angle of aperture 
 of only 7 degrees is obtained. This object-glass of course 
 is inferior to a common lens : and hence has originated 
 the erroneous notion that the introduction of achromatics 
 has been no improvement to microscopes. 
 
 Should the question arise, why a single lens, or a com- 
 bination of convex ones, cannot be mounted so as to have 
 an angle of aperture equal to that in achromatics, the 
 answer is, that it can be done : I have often done it 5 and 
 it affords an excellent method of shewing by comparison 
 the vast superiority of the latter over the former. Long 
 experience has taught the optician that common lenses, 
 when mounted in this manner, occasion such great aber- 
 rations that there is no possibility of obtaining any thing 
 approaching to a distinctness of vision. Hence has 
 arisen the necessity of placing a small stop behind them ; 
 the effect of which being to transmit only a small pencil 
 of the light collected by the glass, the chromatism is re- 
 duced and rendered imperceptible. So effectually, indeed, 
 is this accomplished, that when one of our ablest philoso- 
 phers was told by Dr. Goring that he proposed to make the 
 object-glasses of microscopes achromatic, he exclaimed, 
 " I thought they had always been so constructed/' 
 
INTRODUCTORY R KM ARKS. 43 
 
 The first effective achromatic object-glass for a micros- 
 cope was made, under the direction of Dr. Goring, by the 
 late Mr. W. Tulley, in 1824. It consisted of three lenses, a 
 concave inclosed within two convex ones. From its 
 great thickness, when compared with the ordinary object- 
 glass, it received the appellation of the thick* aplanatic 
 object-glass for diverging rays. 
 
 It appears that the celebrated Professor Amici, of 
 Florence, constructed some achromatics about the year 
 1815, but the happy discovery of test objects not being 
 then made, nor the value of angular aperture understood, 
 they were abandoned as useless at that period. In this 
 country Mr. Tulley went on improving the triple form he 
 had adopted ; and notwithstanding he found it extremely 
 difficult to work it on a small scale, and was thus obliged, 
 in order to get magnifying power, to have recourse to a 
 long body and a deep eye-piece, still he made the angle 
 of aperture as large as he well could. The foci of his 
 object-glasses were about 9-10ths of an inch, and 
 the angular aperture 18 degrees. Subsequently he in- 
 creased the angle to 38 degrees, by placing another 
 triple achromatic before it of 6-10ths, so that the acting 
 focus of the combination was only 3-8ths of an inch. 
 
 The objections to this arrangement are, that it is liable 
 to get out of adjustment that it has twelve reflecting 
 surfaces, the lenses not being cemented together and 
 that it cannot be worked of very short focus. It is con- 
 siderably easier to obtain any given angle of aperture 
 
 * See a section of it, drawn in plate 2 of the Micrographia. 
 
44 MICROSCOPIC ILLUSTRATIONS. 
 
 with combinations of short foci than with shallow lenses ; 
 so much so, indeed, that 60 or 70 degrees with a focal 
 length of l-10th of an inch is gained with as great facility 
 as 20 degrees with an inch focus. Thus, in proportion 
 to the minuteness and delicacy of an object, the focus 
 must be shortened, if we would have a suitable penetra- 
 tion ; otherwise, the labour and time expended upon cor-, 
 reeling the pencils of light, more especially the oblique 
 ones, will be immense. The best proportions of angular 
 aperture to focal length are treated of in another part of 
 this work. 
 
 About the time of Dr. Goring's applying achromatics 
 to the microscope, M. Sellique, in France, invented and 
 executed a new description of object-glass, part of the 
 arrangement of which, on account of the many advantages 
 it possesses, is, and will probably continue to be, adhered 
 to by opticians. Its novelty and chief merit were, that it 
 consisted of several pairs of corrected lenses ; each pail- 
 having a plano-concave of flint glass, and a double convex 
 of plate glass, cemented with mastic varnish. The latter 
 of these lenses was turned towards the object ; by reason 
 of which, as well as from the want of angular aperture, 
 the combination was not so effective as it was afterwards 
 rendered. The genius of M. Chevalier, of Paris, soon 
 led him to perceive some of these defects. He saw at 
 once that the plan of combining thin pairs of lenses 
 greatly diminished the chromatic dispersion, and he saw 
 also that the placing the convex lens next the object must 
 necessarily increase the aberration from sphericity. He 
 
INTRODUCTORY REMARKS. 45 
 
 therefore mounted his glasses with the flat side of the 
 flint lens foremost, and he made the whole still more 
 valuable by the superiority of his workmanship. It was 
 not, however, until these object-glasses arrived in this 
 country, and their aperture was increased by Dr. Goring, 
 that their full effect was brought out. 
 
 In the early productions of M. Chevalier, the foci of 
 the different pairs of lenses composing an object-glass were 
 of unequal lengths ; the anterior pair having the shortest, 
 and the posterior the longest focus. Some of these sets 
 were admirably corrected ; and it is much to be regretted 
 that on account of the comparative difficulty there is in 
 this plan of constructing them, he should have been in- 
 duced to make the pairs of equal foci, by doing which the 
 maximum of angular aperture cannot be obtained together 
 with a fine definition. When we consider the errors which 
 are to be corrected in an object-glass, in order that it may 
 produce a magnified representation of an object suffi- 
 ciently perfect for it to undergo a second amplification by 
 the eye-piece ; and when we consider further that many 
 errors, which under ordinary circumstances are barely 
 appreciable, become very evident as soon as some prin- 
 cipal error is subdued, it is not surprising that the achro- 
 matic construction has been so long progressing towards 
 its present state of perfection. To say nothing of sphe- 
 rical and chromatic aberration, before mentioned, it is 
 no easy task to correct for the central and oblique pencils, 
 so as to obtain a moderately flat field ; the more especially 
 when we consider how restricted we are to certain forms 
 
46 MICROSCOPIC ILLUSTRATIONS. 
 
 in our lenses, through the necessity of their being 
 cemented together to prevent loss of light. To say 
 nothing of the errors arising from the irrationality of 
 the spectrum of the difficulty there is in centering and 
 adjusting the lenses of the minuteness of the scale upon 
 which the whole is constructed, insomuch that the 
 slightest variation in the thickness of a lens (imperceptible 
 to the eye of a workman) is quite enough to alter the 
 state of the aberrations in finely corrected object-glasses 
 of great angular aperture ; nay, that even the thickness 
 of the film of glass, or mica, used for the purpose of pre- 
 serving an object, may do this it is not difficult to under- 
 stand that there have been great obstacles to surmount 
 in constructing an achromatic. 
 
 The talented Professor Amici, about the period of his 
 first attempt at achromatics, invented a reflecting micros- 
 cope. This instrument not having any chromatic disper- 
 sion to contend with, and only one surface to be figured, 
 was soon constructed with a considerable angle of aper- 
 ture. Hence its performance so far excelled that of any 
 other, that he was induced to lay aside his refracting 
 microscope at that time. 
 
 When an account of this microscope reached this 
 country, Dr. Goring, in 1824, suggested the idea of 
 working its concave ellipsoidal surface with a shorter 
 focus and a larger angle of aperture. Now, although 
 the working of a metal is in itself more difficult than the 
 forming a surface to a lens, yet the whole thing exhibited 
 so many facilities over the constructing of an achromatic 
 
INTRODUCTORY REMARKS. / 
 
 lens, that a skilful artist found but comparatively little 
 difficulty in succeeding. For, look at the loss of time 
 which must of necessity be incurred in making trial of 
 an achromatic. Every time each lens has to be tried, it 
 must be removed from its holder, the cement cleared off, 
 and then placed in its setting; whereas a metal is 
 always ready ; and there are no refractive or dispersive 
 powers to be ascertained, as with lenses, so that the 
 Amician reflecting microscopes were soon brought to 
 their most perfect condition. They have been made 
 with an angular aperture of 55 degrees, greatly surpassing 
 all the achromatics of that time. 
 
 The improved Amician microscope soon became a 
 decided favourite with amateurs, and had it not been that 
 considerable tact and experience were required in 
 bringing out " its mettle, and that its range of effective 
 powers was limited, it would never, perhaps, have been 
 driven out of the field by our present achromatic micro- 
 scopes. I was soon convinced that of the three instru- 
 mentsthe Jewel-doublet, the Amician reflector, and 
 the Achromatic the latter would ultimately come into 
 general use. And entertaining this view of the subject, 
 I published, in conjunction with Dr. Goring, a very fiill 
 account of the method by which an artist may test the 
 value of his productions, and know whether or not he is 
 proceeding in the right path 5 for I conceive that it is of 
 no trivial import to have the causes of defects pointed 
 out, when we desire to correct them. Since this publica- 
 tion in the Microscopic Cabinet, other artists besides the 
 
48 ; ICROSCOPIC ILLUSTRATIONS. 
 
 late Mr. Tulley have succeeded in producing achromatic 
 object-glasses. 
 
 In the same work, the amateur is made acquainted 
 with a class of curiously organized bodies, which re- 
 quire a certain degree of perfection in a microscope to 
 render their structure visible, and thus enabled to exercise 
 a salutary check over the workman. Since this work 
 appeared, the achromatic microscope has been elevated 
 to such a pitch of excellence in this country that it stands 
 unrivalled throughout the world. We can now construct 
 object-glasses with angles of aperture of great extent, 
 and with the aberrations so accurately neutralized, that 
 hardly any thing remains but the colouring from the se- 
 condary spectrum, or that which is occasioned by the 
 irrationality of the dispersive powers of the different 
 media employed in their construction. It is not to be 
 expected that much more can be done ; unless, indeed, 
 other substances could be found for the making of lenses, 
 such as will obviate those defects, which it is impossible 
 for the workman to remedy with his present materials. 
 
 Many valuable and interesting additions to microscopic 
 science have been made from time to time by Sir David 
 Brewster; these should not be passed over without 
 proper notice; but since Sir David has so ably described 
 them himself, it is far more becoming in me to refer the 
 reader to his works, than to enter upon an explanation of 
 their merits in this place. 
 
 Our observations have been hitherto directed almost 
 exclusively to the optical portion of microscopes, and 
 
INTRODUCTORY REMARKS. 49 
 
 more especially to the object-glasses of compounds ; eye- 
 pieces not having been so much attended to of late, unless, 
 indeed, by Professor Airey and Mr Coddington. We must 
 not, however, omit to mention Dr. Goring's plan for con- 
 structing these of corrected lenses; an improvement 
 which has been carried into effect at Vienna. 
 
 A few words may be added on the subject of mounting 
 these instruments for use; a consideration of so much 
 importance, that, if it be not duly regarded, the most per- 
 fect object-glass will be comparatively of little value. 
 I have seen many simple microscopes on stands so suit- 
 ably formed, that a great deal more might be done with 
 them, than with fine achromatics badly mounted. The 
 necessity of having convenient stands for microscopes 
 seems not to be much attended to, as yet, on the conti- 
 nent, their's being seldom suited for more than one class 
 of observations. I would not be understood as disre- 
 garding the nicety of their workmanship ; on the contrary, 
 the beautiful finish they give to their productions is 
 hardly to be surpassed by us. As a large portion of this 
 work is devoted to the mechanical part of the microscope, 
 it is unnecessary for me to enlarge upon it here ; I may 
 observe, however, that since the publication of the first 
 edition, considerable attention has been bestowed upon 
 this subject, and that the principles herein laid down are 
 more or less adopted in the construction of all modern 
 instruments. 
 
50 
 
 CHAPTER II. 
 
 On the Larva and Pupa of a Straw-coloured Plumed 
 Culex or Gnat. 
 
 Tipula cristallina of De Geer. 
 Chironomus plumicornis of Fabricius. 
 Corethra plumicornis of Stephens. 
 
 TRANSPARENCY is a quality so essential to the display of 
 the internal organization of living objects, that many 
 devoid of it are often disregarded, as of comparatively 
 little interest. The object now before us, however, is so 
 remarkably pervious to light, that under the microscope 
 we are enabled to view every part of its interior structure 
 with facility. Indeed, it is diaphanous in such a degree 
 as to render it difficult to be discovered in the waters in 
 which it resides, for it assimilates nearly in colour to 
 them, during the infant part of its larva state. 
 
STRAW-COLOURED GNAT. 51 
 
 Had its pellucid nature been its only quality, as an 
 object for tbe microscope, it would have been wortby of 
 attention ; but it possesses others which render it a sub- 
 ject of the highest admiration to all who have seen it 
 under a good instrument. 
 
 This larva, which is produced from an egg deposited 
 by the perfect insect, inhabits still waters, and may be 
 found in canals and clear ponds which have a sandy or 
 gravelly bottom, but is seldom met with in places where 
 there is much herbage or water grass. In 1829, it was 
 found remarkably abundant in the small ponds in Epping 
 Forest ; and in the same year, a fine large variety was 
 met with near Killarney, in Ireland. 
 
 The most certain time to procure it is during the 
 months of May and June, though it may occasionally 
 be obtained on warm days in December and the other 
 winter months, provided the wind is not very high a 
 circumstance which it is important to attend to, and 
 which will be again adverted to in our remarks on the 
 weather best suited for procuring aquatic insects. 
 
 The best method of obtaining this creature is by means 
 of a collecting net, made of linen cloth, which, after 
 
 being immersed in the water, near the edge, must be 
 taken out, and its contents removed into a glass jar. 
 
52 MICROSCOPIC ILLUSTRATIONS. 
 
 These must then be carefully examined by removing 
 small quantities at a time into a white saucer, or small 
 wide-mouthed phial. When they have been examined in 
 this vessel with a hand-magnifier, the objects sought for 
 should be removed to a store-jar, and the refuse thrown 
 away ; then inspect another portion, as before, until 
 all the water taken at a draught has undergone 
 examination. 
 
 Although nearly a quarter of an inch long in its 
 nascent state, this larva can scarcely be observed without 
 a hand-magnifier of about two or three inches focus 
 (unless the collector be short-sighted), as all its parts are 
 nearly colourless, except two pairs of kidney shaped 
 bodies, which have a metallic appearance, and are nearly 
 opaque, as shewn in the plate at b, and d, in the mag- 
 nified side view, fig. 1. 
 
 The drawing, fig. 1, represents the creature in the 
 larva state. Fig. 2, shews the same being a day after- 
 wards, when it had changed to the pupa state. On com- 
 paring the two drawings, however, a very remarkable 
 and complete change of structure throughout will be 
 clearly perceived. 
 
 In the larva, fig. 1, the obvious and curious parts are 
 the kidney-shaped bodies b, and d, two of which are 
 situated near the head, and the other two in the third 
 division from the lower extremity. The first pair are 
 inclined towards each other, while the others lie in pa- 
 rallel planes, as represented in the plan, or bird's-eye 
 view, drawn of the natural size in fig. 3. Physiologists 
 have not ascertained what may be the functions performed 
 
STRAW-COLOURED GNAT. 53 
 
 by these singular organs : it is worthy of remark, 
 however, that a similar structure is observable in the 
 tadpole, and figured in Sir Everard Home's Lectures on 
 Comparative Anatomy*. The other parts of its structure, 
 which appear equally singular and curious, are a number 
 of globules, , which are situated near the first pair of 
 bodies, b. These globules have a slight oscillatory mo- 
 tion in different directions, and, like the reniform bo- 
 dies, seem to have a metallic lustre, but are not opaque. 
 From the exquisite polish of these globules, they reflect 
 the forms of surrounding objects, as window-bars, &c. 
 which are indicated in the drawing by small squares, 
 resembling the images formed by convex mirrors. 
 
 When the larva (as shewn of the full size in fig. 3) is 
 examined from above, it exhibits the position and decussa- 
 tion of the various muscles lying along the back, which 
 are observed to cross at the joints, and at points situated 
 midway between them. 
 
 The alimentary canal appears to contain some particles 
 of a pinkish coloured matter, and has a slight peristaltic 
 motion : but every part of the object, as seen beneath the 
 microscope, is so accurately noted in the drawing, that a 
 more minute description must be deemed superfluous. 
 
 If the insect have a sufficient supply of food, it only 
 continues for a few weeks in the larva state, when it ra- 
 pidly changes to the pupa, shewn in the drawing, fig. 2. 
 When it is desirable to preserve it for the microscope, 
 
 * Vol 6, Sup. Plate 56, fig. 4, D, 
 
54 MICROSCOPIC ILLUSTRATIONS. 
 
 this change may be retarded by keeping it in clear spring 
 or river water. The former seldom offers sustenance to 
 animalcules, and therefore effects this object; which is 
 often very desirable, on account of the scarcity of this 
 species. 
 
 The transformation of this animal from the larva to the 
 pupa is one of the most singular and wonderful changes 
 that can be conceived; and, under the microscope, pre- 
 sents to the admirer of nature a most curious and inte- 
 resting spectacle. Although the whole operation be 
 under the immediate inspection of the observer, yet so 
 complete is the change, that its former organization can 
 scarcely be recognized in its new state of existence. 
 
 If we now compare the different parts of the larva with 
 the pupa, we remark a very striking change in the tail, 
 which, in the previous state of being, was composed of 
 twenty-two beautifully plumed branches; while, in the 
 latter, it is converted into two fine membranous tissues, 
 ramified with numerous vessels. This change appears 
 the more remarkable, as not the slighest resemblance can 
 be discovered between them, nor are the vestiges of the 
 former tail readily found in the water. The partial dis- 
 appearance of the shell-like orreniform bodies is another 
 curious circumstance. The lower two, it may be con- 
 jectured, go to form the new tail ; for, if the number of 
 joints be counted from the head, the new tail will be 
 found appended to that joint which was nearest to them 
 in the larva state, as referred to by the dotted line d, 
 connecting^ figs. I and 2. The two small horns, c, c, 
 
STRAW-COLOURED GNAT. 55 
 
 which form the white-plumed antennae of this species of 
 gnat, when in its perfect state, are discernible in the 
 larva, folded up under the skin near the head, at c, in 
 fig. 1. The alimentary canal appeal's nearly to vanish rn 
 the pupa, as in that state there is no necessity for it, the 
 insect then entirely abstaining from food ; while, near 
 this canal, the two intertwined vessels, seen in the larva, 
 have now become more distinct, and are supplied with 
 several anastomosing branches. 
 
 During the latter part of the day on which the drawing, 
 fig. 2, was taken, the rudiments of the legs of the perfect 
 insect might be seen, folded within that part which ap- 
 pears to be the head of the pupa ; and several of the 
 globules had vanished, those remaining longest that were 
 situated nearest the head. It may be necessary to ob- 
 serve, that the head of the pupa floats just under the sur- 
 face of the water; and the insect, in this state, is nearly 
 upright in that fluid ; while the larva swims with its body 
 in a horizontal position, or rests on its belly or sides, at 
 the bottom of the pond or vessel in which it is kept, the 
 fringed tail being downwards. 
 
 The circuitous manner in which the Creator appears to 
 produce this species of gnat, and many other of His 
 smaller creatures, is truly wonderful. Other beings are 
 produced directly, either from the egg or the maternal 
 womb. As, however, the Deity does nothing in vain, it may 
 be presumed that He must have had in view some impor- 
 tant object in the preliminary steps through which these 
 beings have to pass from the egg to the larva, crysalis. 
 
56 MICROSCOPIC ILLUSTRATIONS. 
 
 and perfect insect; and, however low these minutiae of 
 nature may be held in the estimation of the unthinking 
 mass of mankind, this most elaborate proceeding renders 
 it not improbable that they may be deemed by Him choice 
 and exquisite productions. These mysterious operations 
 of nature, as detected and unravelled by microscopes, are 
 surely GRAND AND CAPITAL SUBJECTS FOR OBSERVATION. 
 I should pity the man who scorned to be amused by 
 inspecting these MARVELLOUS METAMORPHOSES, and 
 disdained to be informed of the manner in which they 
 are effected. What a magnificent spectacle must such 
 a transformation present in the solar or gas achromatic 
 microscope., exhibited to a whole company with all the accu- 
 racy and fidelity which the pictures formed by those instru- 
 ments are capable of displaying ! 
 
 The colour of the larva when young is a faint and 
 scarcely perceptible yellow ; but as it approaches the 
 change, it assumes a richer and deeper colouring, and all 
 its internal parts acquire their definite forms and tints, as 
 exhibited in the drawing. 
 
 The natural history of this identical species of culex is 
 unknown, not having been noticed by any British writer 
 on entomology. The intense interest, however, which, 
 under the microscope, it excites in the observer, will 
 always render it an object of value. Other species of the 
 gnat are well known ; and a description of the common 
 gnat, from the egg to the perfect insect, has been illus- 
 trated by Swammerdam, in his work entitled Biblia 
 Naturce, sive Historia Insectorum* ; in which are two 
 
 * Tom. II. Tab. xxxi. et xxxii., 1737 ; also Reaumur's Hist. Insect. 
 
STRAW- COLOURED GNAT. 57 
 
 folio plates entirely devoted to it. This, however, is very 
 different from the species exhibited in our drawings. 
 Indeed, the larva and pupa of the common gnat are too 
 opaque and uninteresting to be of much value as micro- 
 scopic objects, except in their transfiguration. 
 
 A curious circumstance attends the observation of this 
 insect : so rapid is its locomotion, that it torments the eye 
 while attempting to delineate it, presenting alternately 
 its head and tail to the observer. This it effects by bend- 
 ing itself laterally into a circular form, and suddenly 
 whisking round in the opposite direction to that in which 
 it had just bent itself. 
 
 In the dark ages of the world, when man considered it 
 beneath him to be watchful of, or attentive to, the work- 
 ings of Nature, and when superstition exercised an undue 
 bias over his mind, the new phases which a creature as- 
 sumed were conceived to be miraculous conversions, alto- 
 gether enveloped in mystery. Now-a-days, that his zeal 
 for investigation has led him on to a greatly extended 
 survey of Nature, and to a more minute acquaintance 
 with her operations than his ancestors were wont 
 to possess not but that the ultimate causes to which 
 those operations are subservient remain as much a mys- 
 tery as they ever were his pride and arrogance would 
 instigate him to get rid of every thing approaching to 
 difficulty in the whole routine of creation. He would 
 tell you that the metamorphoses we observe in insects, 
 like those we have been describing, by which a living 
 creature, an inhabitant of the water, whose life depends 
 
58 MICROSCOPIC ILLUSTRATIONS. 
 
 upon its being immersed in that fluid, becomes a free 
 denizen of the air, incapacitated for a longer existence in 
 the former element, are of easy explanation : he would 
 tell you that the different transitions which the being has 
 to pass through, from the larva state to the condition of 
 the perfect fly, are but the semblance of a change ; for 
 that if you examine it just previous to the change, you 
 will discover the future fly folded up in the case of the 
 larva, and ready to issue forth when the covering is re- 
 moved. How badly this will serve for an explanation 
 of the whole phenomenon, and how little it detracts from 
 the greatness and the mystery of the change that has 
 been wrought, is sufficiently evident to every reflecting 
 mind : for, had the larva been examined in the early 
 state, not the slightest indications of the future fly would 
 have been traced. A gradual development of new forms 
 and new organs has been carried on under this old 
 covering, and an absorption of old forms and organs ; 
 which, however much it may seem at variance with the 
 antiquated notions of our ancestors as to there having 
 been an actual conversion of old matter into new, is not 
 a whit the less marvellous. The mere bursting forth of 
 the creature from its envelope is but a very small part of 
 the vast transaction. 
 
 Many species of this genus of insects are, in their 
 perfect state, possessed of a sheathed proboscis, con- 
 taining instruments with which they are enabled to 
 pierce the skin of men and cattle, injecting at the same 
 time an acrimonious fluid into the wound. The species 
 
STRAW-COLOURED GNAT. 59 
 
 we are now describing, however, has not been examined 
 minutely enough to determine the form of these organs. 
 It is of a light straw colour, and has two beautiful an- 
 tennae or feelers. 
 
 The wings, also, of this gnat are of a delicate straw 
 colour, and make very beautiful objects, when mounted 
 under thin glass in sliders. Some species have their 
 wings marginated, and covered with fine scales. These, 
 as well as the feathers on the edges, are good objects for 
 the microscope, and exhibit five or six longitudinal lines 
 on each, which are so strongly marked as to be seen with 
 any kind of light, and do not require superior penetration 
 in the instrument, to shew them. The whole wing may 
 be seen very beautifully under Goring's improved Amician 
 Engiscope*, with its lowest power, which is equivalent to 
 a lens of a quarter of an inch focus. To the larva and 
 crysalis of insects this instrument cannot be applied satis- 
 factorily, as it does not take in the extent of field necessary 
 to shew the whole at once. On this account the achro- 
 matic microscope has the decided advantage, from the 
 IOW T power and large area which can be obtained by it. 
 In examining this larva with the microscope, it will be 
 best seen when the light is thrown obliquely on it ; though, 
 when low powers are used, much attention to this parti- 
 cular is not required. 
 
 These insects generate while hovering in the air, and 
 the female lays her eggs in the water, selecting an unfre- 
 
 * See Micrographia, Chap. I. 
 
60 MICROSCOPIC ILLUSTRATIONS. 
 
 quented spot, where she may deposit them free from 
 danger. This is probably the cause why this larva is. dis- 
 covered with so much difficulty ; the collector being 
 seldom able to procure it two seasons consecutively in the 
 same place. 
 
 The vignette at the head of this chapter was drawn 
 from a specimen bred in a vase. Whilst in the act of 
 producing the cluster of eggs figured on the right side, 
 I took and preserved it in gum. The eggs were removed 
 from the surface of the water on a slip of glass gently in- 
 troduced beneath them. These measure between the 
 one-fortieth and one-fiftieth of an inch in length. 
 
 The short line between the two figures indicates the 
 actual length of the body of the Tipuladian Gnat. 
 
CHAPTER III. 
 
 On the Larva and Crysalis* of a Day Fly. 
 Ephemera marginata of Stephens. 
 
 WHEN this larva issues from the egg, which it does in 
 the decline of summer, it has at intervals an unsteady 
 motion in a manner, as it would seem, involuntary. 
 During this stage it is scarcely visible to the unassisted 
 eye, and is very pellucid. As it increases in size, the 
 serpentine vessels attached to the sides of the animal 
 become more apparent, and the tail assumes that rich 
 feathered appearance, which, in conjunction with the 
 paddles, form its pre-eminent beauty. 
 
 While the insect is very young, it is a well-suited 
 object for the solar achromatic microscope, in which it 
 will afford an ample fund of amusement, when repre- 
 sented on a screen, and dilated to the extent of about 
 two feet in length. If the instrument be truly aplanatic, 
 every part of the insect's internal organization may be 
 clearly seen; and the peristaltic motion of the intes- 
 tines, the circulation of the blood, and the pulsation of 
 the dorsal vessel, can be observed without the least 
 'rouble, by any number of persons. In the latter circum- 
 stance the principal advantage of this instrument con- 
 
 * It may, perhaps, be termed either, according to its state of growth. 
 
62 MICROSCOPIC ILLUSTRATIONS. 
 
 sists; for it cannot be denied that an aplanatic compound 
 microscope will exhibit these appearances with more 
 distinctness, and without any risk of killing or injuring 
 the object by exposure to the heat of the sun's rays 
 condensed upon it. 
 
 As the creature continues growing, it assumes a va- 
 riety of colouring, and becomes more opaque as its 
 change approaches towards completion, which takes place 
 in a few months from the time it leaves the egg. The eyes 
 are reticulated, as represented in figure 4, plate 11, and 
 are of a citron colour 5 while the body exhibits a most 
 beautiful play of different tints, and finally assumes that 
 of a rich brown, with various shadings. The large 
 air-vessel (trachea)* which runs along each side of the 
 body, together with its numerous ramifications traversing 
 the different membranous leaf-shaped paddles, are now 
 of a dark colour, and the elytra, or wing-cases of the 
 future insect, become daily more and more apparent. 
 
 The magnified representation of the larva (fig. 4) was 
 taken just previous to its change to the perfect insect. 
 Fig 5. gives the real magnitude of the creature ; and 
 fig. 6 exhibits a view of the perfect insect of the natural 
 size. 
 
 During the infant state of this larva's existence it is 
 very transparent, displaying, under the microscope, in a 
 most surprising manner, the circulation of the blood. 
 
 The systems of respiration and circulation in insects 
 differ most materially from those of the vertebrated ani- 
 
 * The coat of this vessel differs from that of most larvae, a fact not 
 noticed by naturalists. 
 
KPHEMEEL* MARGINATA. 63 
 
 mals. In land animals the function of respiration is 
 performed by lungs ; in fish by gills : in both of which, 
 however, the blood is propelled to the respiratory organ 
 to be aerated. In insects, on the contrary, the air is con- 
 veyed through the body in vessels, called the tracheae, 
 and the blood is aerated by circulating around these 
 vessels ; whence originates the term, a diffused circula- 
 tion. In place of the heart is substituted a single pro- 
 pelling organ, which, from its peculiar situation, has been 
 named the dorsal vessel. 
 
 The circulation of the blood in this larva is shewn in 
 a magnificent manner in an achromatic microscope, with 
 a triple-cemented object-glass of an inch or half an inch 
 siderial focus. For this purpose I would recommend the 
 selecting of a young specimen about one or two-tenths 
 of an inch in length, when it has just shed its skin, at 
 which time the entire creature is exceedingly pellucid. 
 
 Around the large air-vessel which passes down each 
 side of the body, as also along the legs, antennae, and 
 three-forked tail, the oat-shaped globules of blood may 
 be seen to pass rapidly at every pulsation of the dorsal 
 vessel. This vessel, according to Mr. Bowerbank's ob- 
 servations*, " extends nearly the whole length of the 
 body, and is of great comparative magnitude. It is fur- 
 nished at regular intervals with double valves about 
 equal in number to the sections of the body." A portion 
 of this vessel, with its valves, is represented in the an- 
 nexed figure. " Both above and below each of these sets 
 
 * Entomological Magazine, vol. 1, page 240. 
 
64 MICROSCOPFC ILLUSTRATIONS. 
 
 of valves is a pair of singular- look ing appendages, , a. 
 They are probably nervous ganglions, auxiliary to the 
 motion of the vessel, but so exceedingly pellucid as 
 scarcely to be denned. The action of the valves is a 
 most interesting and beautiful sight. While in their 
 greatest state of collapse, the point of the lower valve is 
 seen closely compressed within the upper one. At the 
 commencement of the expansion of the artery, the blood 
 is seen flowing in from the lateral apertures, (as shewn 
 by the arrows) and at the same time the stream in the 
 artery commences its ascent. When it has nearly 
 attained its greatest state of expansion, the sides of the 
 lower valve are forced upwards by the increasing flow 
 of the blood from the section below the valve, the lateral 
 openings are closed, and the main current of the blood 
 is projected through the two valves." 
 
 A portion of the larva of the Ephemera, greatly magnified, displaying the 
 structure of the Circulating System. 
 
 " It is not easy to see this beautiful structure of the 
 valves of the great dorsal vessel, for it is only when the 
 insect is in a state of exhaustion, or has been just so 
 much compressed as to destroy voluntary motion without 
 entirely depriving it of life, that it is possible to subject it 
 
EPHEMERA MARGINATA. 65 
 
 to a power sufficiently high to discern these extremely 
 delicate and transparent tissues; and even then, to see 
 them to the greatest advantage, recourse should be had 
 only to such as are in the last three or four sections of 
 the body." 
 
 When the larva is dead, the particles of the blood, of 
 an oblong figure, may be distinctly discerned in its forked 
 tail ; and their motion is perceptible in the limbs for 
 nearly an hour after they are separated from the body. 
 These particles, like those of the human blood, assume a 
 spherical form when mixed with water. 
 
 The peristaltic motion of the alimentary canal will also 
 present, under a good instrument, a beautiful and inte- 
 resting appearance. 
 
 The three-pronged tail of this insect, in its advanced 
 age, is beautifully fringed with clusters of fine, straight, 
 smooth hairs or bristle?, several in each bunch, as re- 
 presented in fig. 4. As the time for its transformation 
 approaches, the central prong of the tail becomes more 
 transparent, and assumes the appearance of a jointed 
 tube, or shell-like case ; while the two exterior ones dis- 
 tinctly exhibit portions of the tail of the perfect insect 
 inclosed within them, as shewn in the drawing. The 
 same may be observed as to the legs, which are seen to 
 contain those of the perfect fly. 
 
 This larva is produced from an egg, deposited by the 
 perfect insect in the waters of pools or ditches, among 
 duck-weed and the water-grasses. Its disposition is mild 
 and inoffensive. It is incapable of destroying creatures 
 
66 MICROSCOPIC ILLUSTRATIONS. 
 
 of its own size, while it is itself the prey of all kinds of 
 water-beetles and the larger larvae. 
 
 It feeds on the larvae of the smaller kinds of tipulae, or 
 crane flies, as well as on aquatic vegetables. Short 
 pieces of grass, &c. are frequently seen in the alimentary 
 canal, when examined under the microscope during the 
 spring season. For this reason, and also on account of 
 the sharpness of its claws, it should not be kept with any 
 of the family of the Characese, intended to exhibit the cir- 
 culation (cyclosis) of the sap, inasmuch as it pierces the 
 cellules whilst creeping up the stems, and thus destroys 
 the plants. 
 
 When it is intended to collect this larva, a mild calm 
 day should be chosen ; for if there be any cold wind, it 
 retreats into the mud at the bottom of the water. It may 
 be easily taken with a collecting net, such as is described 
 in the preceding chapter. This must be introduced 
 among the grasses and water-plants to which they attach 
 themselves. Having carefully done this on that side the 
 pond which is most exposed to the sun's rays, and drawn 
 it among the floating duck-weed, many hundreds may 
 often be collected at a single draught. 
 
 These larvae may be kept alive, for many weeks to- 
 gether, in a glass vase of water, with a little duck-weed 
 floating on its surface ; and will be ready to be applied 
 to the microscope whenever required. 
 
 The rapidity with which it moves is truly astonishing. 
 Besides its six legs, it employs the six double paddles at- 
 tached diagonally to the serpentine vessels on each side of 
 
EPHEMERA MARC IN ATA. 67 
 
 its body, and its tail, for the purpose of rowing and ba- 
 lancing itself, and two other paddles for steering, making 
 in all fourteen. Even when the creature is at rest, if in 
 health, all except the lowest two, or steering paddles, are 
 in rapid motiona circumstance which renders those 
 beautiful ramifications of the air-vessels shewn in the 
 drawing, figure 4, difficult to be viewed while the larva 
 is alive. 
 
 Independent of its locomotion, by means of its legs, pad- 
 dles, and tail, it possesses a power of leaping or spring- 
 ing in the water, which it effects by incurvating its body 
 backwards, and then suddenly straightening it, by which 
 kind of motion it raises itself to the surface of the water 
 with great celerity. 
 
 When the crysalis approximates to its perfect state, it 
 swims more elegantly : its motions now appear entirely 
 subservient to its will, and at the same time it leaps with 
 greater velocity. Within a few days, however, of its 
 change, it becomes rather sluggish, and attaches itself to 
 the stalks of water-plants, on which it will remain for 
 hours together, if undisturbed, only moving its paddles 
 at intervals. 
 
 At the period of its transformation to the perfect state, 
 some parts of the insect assume a metallic lustre, as if 
 the space between the crysalis (which may now be called 
 the case of the animal, every part of the latter being 
 perceptible through it,) and the inclosed insect, were 
 partly filled with mercury. This appearance is ultimately 
 extended over the whole body, and is occasioned by a 
 
68 MICROSCOPIC ILLUSTRATIONS. 
 
 small quantity of air or gas probably evolved during tbe 
 change ; and which, by insinuating itself between the 
 case and the insect, may facilitate the process. After 
 remaining a few minutes in this state, and making occa- 
 sional etforts to disentangle itself, it bursts forth from its 
 watery dwelling, and wings its flight into the aerial 
 regions, either leaving the entire skin behind it in the 
 water, or carrying part of it away. 
 
 The exuviae, or skins, which are shed previous to the 
 creature assuming the perfect form, are very fine trans- 
 parent objects for the single microscope, with a lens of 
 the tenth of an inch focus. In this state they afford much 
 useful information, as to the structure, formation, &c. of 
 which the paddles, tail, &c. furnish no inconsiderable 
 portion. If it be thought desirable to expedite the casting 
 of the exuviae, this may be effected by removing the 
 insect from the water in which it is found into spring 
 water. This change is most probably produced by the 
 water mechanically altering the dimensions of the skin 
 or case which inclosed the insect, and thereby suffering 
 it to escape with great ease ; or, perhaps, from the same 
 cause which produces this effect in the common spider 
 when deprived of sustenance. These skins should all be 
 mounted in sliders, in the order in which the insect sheds 
 them, that, by comparing them together under the 
 microscope, we may ascertain with precision the progress 
 made during each change. 
 
 When the crysalis is divested of its envelope,.it remains 
 apparently inert on some neighbouring plant for a few 
 
EPHEMERA MARGINATA. 69 
 
 minutes, where it casts off from its wings the last pel- 
 licle, which is a thin and delicate membrane, formed 
 under the elytra of the crysalis. It then appears in the 
 imago state, with its biforked tail and wings, as repre- 
 sented of the full size in figure 6. 
 
 Having become a perfect Ephemera, it hovers about 
 in the air, the male and female generate, the latter drops 
 her eggs in the water, and both die, existing only a few 
 short hours, to perform all the offices destined for them 
 to fulfil in the economy of nature. 
 
 So beautiful and perfect are the arrangements of the 
 Creator, in order to preserve a regular succession of his 
 creatures, that if the Ephemerae be kept from sexual 
 intercourse, I have known them live for several days. 
 Nor do they seem enfeebled when confined separately for 
 a week, as I have seen them fly away with great celerity 
 after that interval ; while others, which were bred the 
 same season, and were not kept separate, died after a 
 few hours' existence. 
 
70 
 
 CHAPTER IV. 
 
 Fig. 6. Fig. 7. 
 
 On the Larva of a Species of British Hydrophilus. 
 
 Hy drophilus carab oides . Auc forum . 
 
 IN examining the peculiarities of the structure and habits 
 of this larva, what most strikingly attracts our attention 
 is its ferocious and savage disposition, and the fitness of 
 its organs for the exercise of its ravenous propensities. 
 It may be safely asserted that no species of larva is 
 known to be provided with weapons of destruction so 
 powerful, so numerous, and so well adapted to their end, 
 as those which this creature possesses. It is on this ac- 
 count that it has been popularly called the WATER DEVIL. 
 Its size is but little inferior to that of the larva of any of 
 
SPECIES OF BRITISH HYDROPHILUS. 
 
 the British Coleoptera, as it measures, when arrived at 
 maturity, an inch and a half in length, while the superior 
 strength and courage manifested in its attacks on small 
 fish, and other animals larger than itself, is truly sur- 
 prising. 
 
 About the latter end of April, and during the month of 
 May, small nests of these insects are often found floating 
 among weeds and water plants, in stagnant pools, and 
 are frequently taken in the nets of those who are search- 
 ing for the early kinds of larva?. They are in the form 
 of balls, of a dusky white colour, and a silky texture, and 
 have each a small stem of the same nature as the nest, 
 but more dense. By means of this filament they are 
 attached to the roots or stalks of weeds at the bottom of 
 the water. (See the right hand figure at the head of this 
 chapter.) In this situation they remain during the 
 winter, and are thus effectually preserved from the 
 effects of intense cold. Early in the spring, the stem or 
 cable to which we have referred is detached from the 
 weeds, 1^y the winds which at that time prevail, and the 
 nest rises to the surface of the water, and there floating, 
 receives the genial influence of the sun. These nests 
 may be taken and placed in a basin of water, and, as the 
 season advances, hatched by the heat of the sun. On the 
 larva leaving the nest, which it accomplishes by gnawing 
 a hole in the side, the infant immediately descends to the 
 bottom of the vessel, with its jaws extended in search of 
 prey, and eagerly devours all the small aquatic insects 
 that are within its reach ; if, however, there is a scarcity 
 
72 MICROSCOPIC ILLUSTRATIONS. 
 
 of food in the immediate neighbourhood of the nest, they 
 may be seen to attack and devour each other. 
 
 These larvae, as well as most of those of the larger 
 kind, are found at a greater depth of water than are the 
 smaller ones ; although, in their pursuit of small prey, 
 they may occasionally be taken in shallow places. They 
 require a larger net to collect them with. A net made of 
 stout Irish linen, about a yard deep, and affixed to a 
 hoop half a yard in diameter, will answer the purpose 
 very well ; and if it be attached to a long stick, the 
 collector will be enabled to use it with greater facility. 
 
 In its infant state, this larva is very transparent ; hence 
 its internal structure may be clearly distinguished. 
 
 When about a quarter of an inch in length, it swims 
 very nimbly. The colour of the head is a strong Indian 
 yellow, with darker shadings of a bright chestnut; the 
 eyes are now bright carmine. It is more sparingly 
 covered with hairs, and its swimming appendages are 
 shorter than at a more advanced period ; and the head is 
 larger, in proportionate the size of the body, than when 
 the creature has arrived at maturity. In this respect it 
 resembles many other creatures in its mode of growth, 
 the head seeming to be developed and perfected before 
 the rest of the system. 
 
 When young, it may be readily preserved in gum- 
 water, in the manner described in the " Microscopic 
 Cabinet," p. 229. I have several of them mounted in 
 this way, varying in size from one-tenth to half an inch 
 in length ; and although nearly eight years have elapsed 
 
SPECIES OF BRITISH HYDROPHILUS. 73 
 
 since they were done, they are as perfect as when they 
 were first prepared. This method, however, will not 
 retain them so permanently as when they are mounted in 
 Canada balsam, between two slips of glass, in which case 
 the specimens may also be of much larger dimensions. 
 
 The manner in which this larva treats its prey evinces 
 an extraordinary degree of instinct. Many of the crea- 
 tures on which it feeds are crustaceous about the head 
 and back ; hence they are most vulnerable at the under 
 part of the body. This part, therefore, the larva attacks ; 
 and, to accomplish its aim, swims underneath the in- 
 tended victim, and bending back its head, which is even 
 with the surface of its back, is enabled thus to reach its 
 prey by means of its jointed antennae. (See a, fig. 30, 
 which represents a magnified view of the larva taken 
 while young.) Its next operation is to pierce it with the 
 mandibles (b). Having thus secured its object, it im- 
 mediately ascends to the top of the water, and, holding it 
 above the surface, so as to prevent it struggling, shakes 
 it as a dog would a rat. The prey of this larva is often 
 larger than its destroyer. Its next operation is to insert 
 the piercer and sucker (d), which is capable of being 
 thrust out or withdrawn at pleasure. When the juices of 
 the victim are not easily procured by suction or exhaus- 
 tion, the serrated pair of forceps (c), is employed to tear 
 and masticate it, and thus cause the juices to be more 
 easily obtained. If its food be plentiful, this larva arrives 
 at its full growth in the course of three or four months, 
 and is then nearly opaque, and covered with hair. It can 
 
74 MICROSCOPIC ILLUSTRATIONS. 
 
 be kept several days without food, and by this exinani- 
 tion its structure becomes considerably more transpa- 
 rent*, while its natural ferocity is greatly increased, so 
 that it will attack and fight with creatures much larger 
 than itself, and even with its own species. It may be 
 remarked, that it studiously avoids any contest with the 
 Nepa, or water- scorpion. 
 
 On a fine sunny day the larvae rise to the surface of 
 the water, and delight to bask in the sun ; but if watched, 
 they remain motionless, with their claws extended. If a 
 stick, or any other substance, be presented to them, they 
 will immediately seize it, and will sometimes suffer them- 
 selves to be cut into pieces before they relinquish their 
 hold. Their bite has been considered poisonous by many 
 persons, as it takes a greater length of time to heal than 
 other wounds of the same extent : so that caution should 
 be used in taking them. 
 
 Touching the anatomy of this creature, it may be ob- 
 served, that the sucker, marked d, is contained in a crus- 
 taceous sheath, and may be considerably protruded or 
 completely withdrawn at the pleasure of the larva : in 
 the engraving it is shewn extended to about three 
 quarters of its length. The eyes are compound, but 
 of a peculiar formation, consisting of seven oval aper- 
 tures, arranged like leaves upon a branch : in the draw- 
 ing they are denoted by the letter e. The whole of the 
 
 * Dr. Goring kept the subject represented in the engraving some time 
 without food, in order to render its interior organization more clear : it 
 may be observed that its intestinal canal is quite empty. 
 
SPECIES OF BttlTISH HYDROPHILUS. *Jb 
 
 head and thorax are curiously marked with a number of 
 lines and spots. The legs are six in number ; they are 
 thickly set with rows of hair on their opposite sides, and 
 each is furnished with a sharp claw. The number of 
 swimmers on each side is seven ; they are covered with 
 hairs, and, to the specimen from which the drawing was 
 taken, a vast number of Vorticellce convallaria* or bell 
 polypi, were attached. These will be recognized in the 
 magnified drawing by their bell- shaped figure. They 
 sometimes infest this species of larva to such a degree, as 
 considerably to impede its motions in swimming. On 
 each side of the abdomen, which commences near the 
 origin of the first pair of swimmers, arise the great 
 tracheae, or air-vessels, distinguishable in the coloured 
 engraving by their light blue colour ; the two approach 
 each other near the tail, where an exceedingly curious 
 process is also distinctly exhibited. The whole surface of 
 the body is thickly covered with hairs, and several tufts 
 are disposed in clusters, with some regularity, down the 
 back and sides. The flexible pulsatory organ or dorsal 
 vessel, situated at the lower part of the body, is in perpe- 
 tual motion. Its form somewhat resembles the letter *S, 
 inverted : it however varies a little during its vibrating 
 motions. The use of the curious appendages at the lower 
 extremity of the body is unknown. Its tail is biforked 
 and crustaceous, and is marked as shewn in the plate. 
 In figure 31 is given a representation of the larva, of its 
 
 * Described and figured in " Natural History of Animalcules," p. 149. 
 
76 MICROSCOPIC ILLUSTRATIONS. 
 
 natural size and proportions, taken at the same period 
 of its growth as that at which the drawing was made. As 
 it approaches maturity, it casts its skin several times, 
 from each of which it escapes by a rent formed down 
 the back. 
 
 The large tracheae, or air- vessels, which run along each 
 side of the body, with the numerous branches emanating 
 from them at various intervals, in their course, are com- 
 posed of delicate transparent membranes, distended by 
 means of fibres attached to, and wound about them in a 
 spiral form, like the thread of a screw or the spring 
 of a bell. The diameter of these vessels, one of which I 
 dissected from a larva nearly an inch and a half in length, 
 is one-sixteenth of an inch. When mounted between two 
 pieces of glass, and submitted to moderate amplification, 
 they exhibit the most beautiful and varied specimen of 
 line-work that it is possible to imagine. The fibres of 
 the upper and under sides intersecting each other at dif- 
 ferent angles, produce an effect which no engine-turner 
 could surpass; it would, therefore, be useless attempting 
 to illustrate it by a. wood engraving. A branch of this 
 vessel may also be observed running along each of the 
 legs. 
 
 Respiration by this creature is not performed as with 
 the larva of the ephemera, although, like it, it is an inha- 
 bitant of the water. The tracheae in the latter is supplied 
 with air from the membranous paddles on each side of 
 the body, which imbibe it from the circumambient fluid. 
 In the larva under consideration, the air is supplied by 
 
SPECIES OF BRITISH HYDROPHILUS. 77 
 
 proper orifices situated at its tail; and the creature is 
 obliged to ascend to the surface of the water, and elevate 
 its tail out of it, at every inspiration. When the air thus 
 inhaled has become unfit for respiration, it is expelled 
 at the same orifices, and a small bubble may frequently 
 be seen issuing from the tail, and ascending in the water. 
 On a careful examination of the skin, which by the by 
 affords an excellent object for the microscope, there will 
 not appear any spiraculas along the sides, as in perfect 
 insects. 
 
 The respiratory system of this larva, being, when dis- 
 sected out, more opaque than that of most similar crea- 
 tures, is seen to great advantage when mounted in 
 Canada balsam ; and if it be accompanied with the cor- 
 responding organs of a caterpillar, the comparison 
 affords a beautiful example of adaptation to the different 
 elements suited to life. In the terrestrial animal, every 
 part being exposed to the atmosphere, mouths or orifices 
 for inhaling the air are arranged along both sides of the 
 body ; in the aquatic larva, this system could not be made 
 available, without the creature being compelled to quit 
 its natural element, at every moment it had occasion to 
 respire. To obviate this, it is furnished with two large 
 apertures at the tail, instead of those at the sides. 
 
 I may mention that a preparation of this organ illus- 
 trates very beautifully one of the physical properties of 
 matter, namely, the colours produced by interference, 
 when light is made to pass between a series of fine lines. 
 
 After this creature has remained for a considerable 
 
78 MICROSCOPIC ILLUSTRATIONS. 
 
 time in the larva state, it buries itself in a hole, which it 
 forms for that purpose near the edge of the water, and 
 after passing through the crysalis state, it emerges in the 
 form of a perfect beetle. This is effected in two years 
 after issuing from the nest egg. 
 
 On examining the older books on Entomology, I found, 
 in Roesal's work, (a book not noticed in Stephens' ela- 
 borate Catalogue of British Insects,) a small drawing of 
 this larva, from which it appears to be the Hydrophilus 
 caraboides. It is much to be regretted that modern 
 entomologists attend so little to the aquatic larvae of 
 insects,- for surely it will not be pretended that a 
 description of the creature in its perfect state constitutes 
 its history. I have often made inquiries of the first 
 entomologists of the day as to what insect a particular 
 larva would become, and have as often been unable to 
 obtain the slightest information. In some cases they 
 have doubted as to the order to which it belonged. It is 
 true that had I been unable to procure its generic and 
 specific names, that circumstance would not have ren- 
 dered it a whit less valuable as a microscopic object ; 
 and I do not pretend to treat of them with entomological 
 technicality. 
 
 The engraving of the perfect Beetle at the commence- 
 ment of this chapter is taken, by permission, from Mr. 
 Curtis's British Entomology, vol. iv., plate 159. 
 
71 
 
 CHAPTER V. 
 
 On the Terms employed in Microscopic Science. 
 
 IT has been said that every science has its own language, 
 and we often find that words which in one science ex- 
 press certain ideas, do, when transferred to another, 
 convey to our minds a sense entirely different. 
 
 Although the study of the microscope does not require 
 a very extensive vocabulary, yet it is rendered more 
 easy by a previous acquaintance with those terms which 
 continually occur to the microscopist. 
 
 The following chapters are mechanical descriptions of 
 the microscope, together with a discussion of the question 
 as to whether there is a best possible way of constructing 
 its mounting, or mechanical part. The importance of 
 the latter must be evident, when we consider that 
 although we may possess the most perfect and finest 
 corrected and adjusted lenses, yet if their mounting or 
 stand be defective in its construction, we shall be 
 deprived of a great portion of their value, by being 
 unable to make them show in an efficient manner those 
 subjects which are within the scope of the instrument, 
 while many classes of objects cannot be viewed by them 
 in any way whatever. 
 
 In the present chapter, I shall merely give a succinct 
 
80 
 
 MICROSCOPIC ILLUSTRATIONS. 
 
 explanation of the different terms used in this and other 
 works respecting- the microscope, omitting Aplanatic ; 
 Achromatic ; Chromatic Aberration, or Dispersion ; 
 Spherical Aberration; Engiscope and Microscope, as 
 being explained already in the (< Cabinet/' p. 102. 
 
 Fig. 8. 
 
 1 
 
 N .|. 
 
 } 11 
 
 I/ 
 * 1 
 
 
 
 ^ ..fj_ 
 4 tt 
 
 Fig. 9. 
 
 First, then, we will commence with the optical part 
 of a compound microscope, or engiscope, the external 
 appearance of which let fig. 8 represent. This is gene- 
 rally called the body of the microscope, and is either 
 composed of a conical or a parallel tube, as here shewn. 
 The interior of the body should be as black and as sombre 
 as possible, and furnished with diaphragms or stops (as 
 at e), to prevent any false light entering the eye: of 
 course these stops must be so arranged, and of such 
 apertures, at not to cut off or impede any portion of the 
 rays passing through the various lenses. 
 
 The larger end of the body, a, is called the eye-end, into 
 which the eye-pieces fit, either by a screw or simply 
 by sliding them within it. The latter plan is the better, 
 
TERMS EMPLOYED. 8i 
 
 though it has been objected to, from the liability of their 
 becoming loose by wear ; the reverse of which, however, 
 occurs in practice, for as the tubes become oxidized they 
 tighten. 
 
 The eye-pieces usually consist of two lenses, as shown 
 at a and b, fig. 9. The first, or anterior lens, is called the 
 eye-glass ; the second, b, the Jield-glass. As I have made 
 some eye-pieces of quartz, it would be more appn>- 
 priate to call them eye-lens, or field -lens. 
 
 The diaphragm) stop, or field-bar, is shown at c, and 
 should be in the focus of the eye-lens, a. The lens 
 which screws into the body at d, is called the object- 
 glass; it may be composed of more than one lens-: 
 they are termed collectively objectives or object-glasses, 
 if they consist of three pairs, which is the usual 
 form for the best deep achromatic?, as shown at fig. 
 Fig. 10. 10, where o is the place for the object (that 
 being their combined acting focus) d is 
 
 e c o the anterior or first pair of object-glasses 
 e, the middle pair and /, the third or posterior pair 
 of objectives. As the inner surfaces of each pair are 
 cemented together, to an ordinary observer they appear 
 as one lens, and consequently the objective appears to 
 be composed of only three lenses. 
 
 Over the object-lenses is often fitted a concave silver 
 reflector, for throwing condensed lighten opaque objects ; 
 this is termed the Leiberkuhn, silver cup, or speculum, and 
 is shown at Plate 4, fig. 17- 
 
 The power of eye- pieces is determined by the focal 
 
82 MICROSCOPIC ILLUSTRATIONS. 
 
 length and distance asunder of their component lenses, and 
 as they usually bear a certain relation to each other when 
 their construction is alike, their power is sufficiently dis- 
 tinguished by simply expressing the focus of the eye-lens. 
 Thus we say this is an inch eye- piece, meaning that the 
 sidereal focal length of the eye-lens is one inch. The 
 acting length of the body is the distance from the object- 
 glass d, to the diaphragm c (fig. 9), though it is usually 
 expressed by the length of the tube (fig. 8.) 
 . I have been thus explicit, as the magnifying power 
 of an instrument is dependent on the focal length of 
 the objective, the power of the eye-piece, and length 
 of the body, taken together. 
 
 Fig. 11. 
 
TERMS EMPLOYED. 83 
 
 The mechanical parts of a microscope I shall illustrate 
 by a reference to fig. 11, which exhibits the simplest 
 construction of a microscope with which 1 am acquainted. 
 It will be seen in the sequel to be the same as my achro- 
 matic engiscope, described in Chapter VI., but without 
 its apparatus and stand, which can be added at any time 
 by its possessor, and thus avoid the disadvantage usual 
 in the purchase of a cheap microscope, viz. that when 
 a better is required, the cheap one is too slight and 
 defective in its construction to form the basis of a supe.- 
 rior instrument, and is, therefore, of necessity thrown 
 aside as useless. 
 
 The Vertical Microscope, fig. 4, consists of a double 
 arm, n, a; the former to receive the screwed end of the 
 compound body, d, fig. 8 (when employed as an engi- 
 scope), and the latter, a, to receive the single, doublet, 
 or triplet magnifiers, when used only as a microscope. 
 b is a triangular bar, with a rack cut on its posterior 
 edge, into which a pinion works, which is actuated by 
 turning the large milled head, s, which raises or depresses 
 it and the arm, a, n, and thus the adjustment of the 
 focus is effected, j is the stage on which the objects to 
 be viewed are placed, and these latter are kept down by 
 the forked piece, ov finger -springs, t. The mirror-socket, 
 m, slides upon the stem, c, and carries the mirror, o, 
 with its frame, p. Above the arm, a, n, is a milled head, 
 g, which serves to tighten it in any position in which it 
 may be placed : e is the circular foot or stand. When 
 intended as a travelling instrument, for lightness and 
 
84 MICROSCOPIC ILLUSTRATIONS. 
 
 portability, the stem, c, screws into the top of the case, 
 and the round foot is not required. 
 
 As the above will be sufficient to explain the terms 
 used in the following chapter, I shall, to avoid repetition, 
 explain the remaining terms, when describing Dr. 
 Goring's and my own Engiscopes. 
 
85 
 
 CHAPTER VI. 
 
 A description of the Achromatic Microscope, together with 
 its Apparatus and the mode of using it. 
 
 IN a country like England, preeminently distinguished 
 for mechanical inventions, it would be strange, indeed, 
 if the constructing of stands for microscopes, their 
 mountings, apparatus, &c. had not attracted the atten- 
 tion of some of her artists. Since the publication of the 
 first edition of this work, numerous contrivances of a 
 highly ingenious description have been devised for their 
 improvement ; and though many of them are too complex 
 ever to become of general utility, yet are they such on 
 the whole as have tended to advance this instrument to 
 its present effective condition. Several artists in France 
 and Germany have also especially directed their thoughts 
 to the microscope, and have produced some which, as 
 specimens of workmanship, claim, no doubt, very con- 
 siderable merit ; but they do not appear to have had 
 sufficiently in view the principles upon which they ought 
 to be constructed. Those which have fallen under my 
 notice are wholly unsuited for general purposes of ob- 
 servation a circumstance which is the more surprising, 
 because there being exhibited so much talent in the op- 
 
86 MICROSCOPIC ILLUSTRATIONS. 
 
 tical department, particularly in the eye-pieces, one 
 could hardly imagine that their usefulness would be 
 abridged through their inefficiency in other respects. 
 
 By a microscope suited for general observations, is to 
 be understood one of such construction as will allow us 
 to examine both opaque and transparent bodies with 
 such facility that we may ascertain their true forms and 
 structure. In order to compass this, both the instru- 
 ment and the illumination must admit of being turned 
 in all directions. The instrument must be further ca- 
 pable of carrying a wide range of magnifying powers, 
 so that whether the object to be investigated be of mo- 
 derate dimensions or a mere atom, it may come fully 
 within its scope. It is not an uncommon case to find a 
 microscope excellently constructed for a given power, 
 and totally unfitted for all others. For instance, that 
 beautiful little instrument for carrying Wollaston's 
 doublets, can only be employed with effect for viewing 
 very minute transparent tissues, as the scales of the 
 Podura, &c. It is not suited for examining the living 
 objects described in the previous chapters, nor most 
 animal or vegetable structures; and that splendid and 
 extensive class of bodies, opaque objects which require 
 strong reflected light and moderate shallow powers to 
 render their beautiful structures appreciable is entirely 
 beyond its reach. In either of the two microscopes I 
 shall describe in this work, every class of bodies that 
 I am acquainted with can be correctly judged of, whilst 
 they command a very extensive range of powers. Dr. 
 
DESCRIPTION OF TttE ACHROMATIC MICROSCOPE. 87 
 
 Goring's engiscope, described in Chap. VIII., being on a 
 much larger scale than that treated of in the present 
 one, is adapted for carrying the shallowest magnifiers 
 which can be required. 
 
 As these instruments have now become the standards 
 by which all the better kinds are made, modified more 
 or less so as to conform to the different views of the 
 artists who make them, I have judged it advisable to 
 treat of their several parts in detail ; in doing which 
 many of the remarks I shall have occasion to make, will 
 be found to have a general application. My own instru- 
 ment, with its various modifications and additions, to- 
 gether with their uses, will form the subject of this 
 chapter. In Chap. VIII. I shall give a mere explana- 
 tion of Dr. Goring's aplanatic engiscope ; the method 
 of making observations with, and managing it, being 
 penned by himself in the chapter following it. 
 
 The simplest modification of my achromatic micro- 
 scope is represented at page 82, fig. 11. This can be 
 used only in a vertical position ; but it is admitted, I be- 
 lieve, to be superior to any instrument of that kind, in 
 steadiness, simplicity, and range of powers. It will be 
 readily conceived, that when the circular foot is un- 
 screwed, and the stem c, fitted into a jointed stand, it 
 becomes the second modification, shewn in the following 
 engraving, fig. 12. 
 
 This microscope is represented with its stem and axis 
 in an inclined position, which is commonly the most 
 convenient one for the observer. Those parts of it which 
 
88 
 
 MICROSCOPIC 1 LLUSTRATIONS. 
 
 have been already described as belonging to the vertical 
 instrument, will be very briefly referred to here. Let 
 
 Fig. 12. 
 
 Fig- 13. 
 
 Fig. 14. 
 
 e, n, represent its compound body, which may be either 
 achromatic or periscopic ; its lower extremity screws 
 into the bent arm, n, whilst the other arm, , is suited to 
 receive single or compound magnifiers. The stage, t, 
 on which the object- sliders are to be placed, is remova- 
 ble at pleasure. It is shewn separate by fig. 14. The 
 protruding ears on each side, shewn at, 4, serve to hold 
 by, when putting it on or removing it from the instru- 
 ment. These being in a line with the two stout pins 
 which fit into the block on the top of the stem, c, prevent 
 any strain being occasioned by the operation. This 
 
DESCRIPTION OF THfc ACHROMATIC MICROSCOPE. 89 
 
 being attended to, and the triangular bar, b, sufficiently 
 raised, we may proceed with perfect safety. On the stage 
 is fitted, in a similar manner, by means of two smaller 
 pins, a fork-shaped plate of brass, shewn apart at fig. 13. 
 This plate is designed to retain the slider firmly on the 
 stage when the instrument is inclined. The focus is ad- 
 justed by means of a rack and pinion; the letter, #, 
 denoting the large milled head of the latter. The 
 mirror-frame slides up and down the stem, c, thus allow- 
 ing the angle of the pencil of light to be increased or 
 diminished. 
 
 The stand of this instrument appears not to be different 
 from those of the old-fashioned compound microscope, ex- 
 cept in having a simple round foot, which is here adopted 
 to save room and expense. Like them it has a joint for in- 
 clining the instrument at any required angle ; but it has, 
 besides, this novelty, which, although simple, is of great 
 importance, viz. a capability of permitting the stem, c, 
 to be turned round on its axis, so as to allow a glass 
 vessel, or slider accommodated for animated creatures 
 or plants, to be retained in an upright position, and 
 their contents investigated without having recourse to 
 stoppers or corks. When any of these vessels are used 
 in ordinarily-constructed microscopes, they require to 
 be placed on their sides, which, independent of disturb-? 
 ing the objects, admits not unfrequently into the field of 
 view, small portions of the air included in the vessels, so 
 as greatly to interrupt our view. When observations of 
 this nature are to be made, the stem, c, should be turned 
 
90 MICROSCOPIC ILLUSTRATIONS. 
 
 a quarter round to the left, which brings the milled 
 head #, upwards, and the arm, n, a, horizontal, where 
 the stem, c, is fixed by the pinching screw, z. In this 
 position a candle may be placed close behind the stage in 
 the axis of the instrument. This will be found of great 
 advantage in verification. Again, if the stage be re- 
 moved, objects in glass vessels, which are too large to be 
 held in the microscope, may be examined without re- 
 moving them for the purpose; as also minerals, blocks 
 of fossil woods, &c. 
 
 The universal or ball-and-socket joint being strongly 
 recommended in Chap. VII., it may with propriety be 
 asked, why I have introduced a different one into my own 
 instrument. The reason is, that while all the requisite 
 motions are obtained by means of the joint I have 
 adopted, it allows, at the same time, the stem of the 
 microscope to be over the centre of support, by which 
 it has no tendency to fall, when the pinching screw is 
 loosened. In the universal joint, there is always a 
 tendency to drop (except when it is on its side), and 
 a strain upon it. In practice, therefore, these are very 
 liable to get out of order, where the weight of the in- 
 strument is considerable. When formed on a small 
 scale, this objection vanishes; but when of the size 
 adopted by Dr. Goring, until a joint can be constructed 
 similar to that in the human arm, with a ligament in 
 the centre, and sufficient friction to support the weight 
 in any position, no person, I conceive, will give them the 
 preference : at least, so it has been 5 for where one 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPK. 91 
 
 microscope has been made with Dr. Goring's joint, 
 fifty have with mine. 
 
 Fig. 16. 
 
 Fig. 15 represents a pair of forceps and a pin- holder, 
 attached to a ball-and socket joint ; the stem, e, fits into 
 holes formed in the stage to receive it. Opaque objects, 
 when mounted on a circular disc with a pin, are held 
 more firmly by the pin- holder (as seen by the figure) than 
 by the forceps. It is preferable, also, when the instru- 
 ment is on a small scale, to that ingenious contrivance in 
 Dr. Goring's Engiscope (see plate, fig. 23), as it admits 
 of the forceps being made smaller. 
 
 The arm, fig. 16, is used for carrying the forceps when 
 the stage is removed. The pin, b, fits into one of the holes 
 in the block of the instrument; and the pin, c, fig. 15, 
 into the hole, a. This arrangement for viewing opaque 
 objects allows large silver cups to be used, for conden- 
 sing the light upon them. This could not be done, if 
 the stage did not take off, unless the aperture in it were 
 immoderately large. 
 
 The method of viewing opaque objects by means of 
 silver concave reflectors, is by far the simplest and the 
 readiest; and when these are of considerable diameter 
 and of proper curvature, we not only obtain the central 
 illumination, but also that beautiful play of light and 
 shade afforded by the oblique pencils. 
 
 For investigating insects, or other objects not mounted 
 
92 
 
 MICROSCOPIC ILLUSTRATIONS. 
 
 on discs in this manner, an additional arm for carrying a 
 small black cup, to be placed behind the object, in lieu 
 of the disc, is extremely convenient. 
 
 Description of the Solid Tripod-stand Achromatic 
 Microscope and Enaiscope. This instrument, on its 
 simplest mounting, is represented by the accompanying 
 engraving, fig. 1J. It is in all respects similar to the 
 preceding one, except in its stand. On this account 
 I have placed it in the position in which it is used for 
 
 Fig. 17 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 93 
 
 viewing opaque objects by silver cups, where a is the 
 concave speculum, orLeiberkuhn. 
 
 This stand differs from the other in the following par- 
 ticulars. The pillar is composed of a couple of tubes, 
 the one sliding within the other. The triple-milled 
 ring, q, screws upon the top of the outer one, and 
 serves to fix them together at any required elevation. 
 The base, u, u, u, consists of one solid piece of metal, 
 the form of which is that which several years' experience 
 has proved to be the best. Dr. Goring, in Chap. VII., 
 clearly demonstrates that a tripod bearing is indispen- 
 sable to insure firmness. In his instrument, however, 
 the base, instead of being solid, is composed of three 
 pieces (see plate, fig. 7)? the objection to which is, that 
 notwithstanding a screw is used to counteract their 
 springing, the stand is yet liable to rock. I have lately 
 made some of his instruments with a solid base, similar 
 to my own, which are decidedly preferable. 
 
 My tripod form has been objected to, although the 
 solid base is now adopted in all the best instruments. 
 It has been said to occupy more room than those having 
 folding legs, and, therefore, to require a larger case ; and 
 again, that it should be equilateral, having its three feet 
 of equal length. The answer to this is, first, that no 
 folding stand can be rendered perfectly firm, and that 
 there is some trouble and loss of time in arranging it for 
 use j and should it, through any hurry or mistake, be 
 set up without properly unfolding the feet, the instrument 
 is very liable to be injured. And, secondly, the base 
 
94 MICROSCOPIC ILLUSTRATIONS. 
 
 here selected can be inclosed in a case no larger than is 
 necessary to contain all the other parts of the micro- 
 scope; and, with a given weight and dimensions, the 
 form is more steady than when the feet are of equal 
 length. Of course, to obtain the greatest advantage, the 
 stem, c, should always be over the shortest foot. 
 
 Having alluded to the packing of microscopes, it may 
 be mentioned, that upright cases or cabinets are greatly 
 superior to boxes for the purpose, because, the instant 
 the door is unlocked, the instrument is ready for use ; 
 besides, sufficient space is afforded for a nest of drawers, 
 to contain the apparatus and objects, although the latter, 
 perhaps, are better kept in a separate cabinet. 
 
 On the selection of proper Magnifying Powers. In a 
 microscope constructed for single lenses, doublets, or 
 triplets, the plain split end of the arm, , is the situation 
 designed to receive them. Upon examining the lenses, 
 such as possess the largest aperture are of the lowest 
 power. With these our observations should generally 
 begin. There should be marked, on them all, their 
 respective sidereal focal lengths. This measure will 
 indicate very nearly the distance at which they require 
 to be adjusted from the object. When the object to be 
 examined is a seed or large body, commence with either 
 of the three lowest powers. If the object is hardly 
 perceptible with the naked eye, take those of 1-1 Oth of 
 an inch focus, and so proceed, step by step, to the higher 
 powers. 
 
DESCRIPTION OF THK ACHROMATIC MICROSCOPE. 95 
 
 Table of Magnifying Powers. 
 
 , , Magnifying powers, estimated by a 
 
 Focal Lengths. * lo'.inch standard of sight. ' 
 
 Inches and Parts. Diameters. Surface. 
 
 1 , 10 100 
 
 3-4ths 15 225 
 
 l-2nd 20 400 
 
 l-3rd 30 900 
 
 l-4th 40 1,600 
 
 l-6th 60 3,600 
 
 l-10th 100 10,000 
 
 l-15th 150 22,500 
 
 l-20th 200 40,000 
 
 l-30th 300 90,000 
 
 l_40th 400 160,000 
 
 l-50th 500 250,000 
 
 l-60th 600 360,000 
 
 l-80th 800 640,000 
 
 l-100th 1,000 1,000,000 
 
 Previous to placing the magnifier into the arm just 
 mentioned, the triangular bar should be raised, the 
 object put upon the stage, and the light reflected from 
 the mirror made to pass through it. All this is more 
 readily effected before the lens is fixed in its position. 
 
 Some persons prefer having stops or diaphragms under 
 the stage, when making observations with single lenses. 
 In this case, the aperture of the magnifier maybe greatly 
 increased ; for the stops, so situated, will have the same 
 effect of reducing the diameter of the emergent pencil of 
 light as a restricted aperture in the mounting of the lens 
 would do, whilst the vision for deep powers is rendered 
 by it more agreeable. 
 
96 MICROSCOPIC ILLUSTRATIONS. 
 
 Whenever high powers are used, a safety slider- 
 holder, similar to that shevvn by figs. 25 and 26 in 
 the plate, or fig. 20, page 106, should be chosen for 
 holding the object slide. 
 
 In the case of Engiscopes, or Compound Microscopes, the 
 arrangement for making observations will be different. 
 First, the triangular bar is to be raised, and the bent 
 screwed arm brought over to the centre of the stage. 
 Then screw into it the body or tube. To the upper extre- 
 mity fit an eye- piece, of which, in a complete microscope, 
 there are several of different powers : the longest is 
 always the lowest power, and is marked A. Its angular 
 aperture, which determines the size of the field of view, is 
 generally less than that of the others (if constructed on the 
 Huyghean plan), being limited by the diameter of the 
 body. It is usually a little under 30 degrees. The next 
 eye-piece or middle power, marked B, and the deepest, 
 C, have more than 30 degrees of angular aperture. Their 
 amplifying powers are commonly in the proportions given 
 given below. It is not unusual, however, to have eye- 
 pieces of intermediate powers, to suit any particular 
 class of objects we desire to investigate. 
 
 Proportional Powers of Eye-pieces. 
 
 Names. Diameters. Surfaces. 
 
 A - - 1 ! 
 
 B - - - - 2 - ... 4 
 
 C - - - - 4 to 6 - - - 16 to 36 
 
 For viewing thin sections of recent or fossil woods, 
 
DESCRIPTION OF THE .ACHROMATIC MICROSCOPE. 97 
 
 coal 7 jet ; the fructification of ferns and mosses ; fossil 
 shells; seeds; small insects, or parts of large ones; molus- 
 cans, or entomostraceans; the circulation in the frog, &c. 
 the eye-piece, A, is best adapted. 
 
 Again, for examining the details of any of the above 
 objects, the eye-piece, B, should be substituted ; as also 
 for observing the colours of crystals illuminated by pola- 
 rized light ; the pollen of flowers ; the dissections of 
 insects ; the vascular and cellular tissues of plants ; ani- 
 malcules; Haver's canals in sections of recent, and 
 the corpuscules, &c. in fossil bones ; the cyclosis in 
 plants ; the formation of shell ; the moveable fluids in 
 quartz and topaz ; the organic remains in thin slices of 
 flint, &c. ; the serrated lamina of the crystalline lenses 
 of the eyes of fishes and birds, &c. &c. 
 
 And lastly, when it is requisite to investigate either the 
 minutiae of opaque objects, or the minute structures of 
 delicate tissues, active molecules, polygastric infusoria, 
 and the raphides in the milky juice of plants, &c. the 
 eye-piece, (7, comes into use ; it being oftentimes incon- 
 venient, when a higher power is needful, to obtain it by 
 means of a deeper object-glass, which occasions a fresh 
 arrangement to be made of the illumination and focus. 
 
 It must, however, always be borne in mind, that the 
 more powerful the eye-piece is, the more will the imper- 
 fections in the object-glass become apparent, because 
 they will be the more dilated. Hence we should observe, 
 that less confidence ought to be placed in the observa- 
 tions made under a powerful eye-piece, than when the 
 
 H 
 
98 MICROSCOPIC ILLUSTRATIONS. 
 
 same amplification is obtained with a shallow one, and a 
 deeper object-glass : the preference, therefore, when 
 great accuracy is required, should always be given to the 
 latter arrangement wherever it is admissible. 
 
 An engiscope may be furnished with one or more sets 
 of object-glasses. They all have similar screws for fixing 
 them into the lower end of the body. When a set is 
 composed of only two or three lenses (the usual con- 
 struction of shallow ones), it can give but one magnify- 
 ing power. In like manner, when a set is constructed so 
 as to take up a very large pencil of rays with the utmost 
 definition, although composed of several pairs of lenses, it 
 can give but one perfect power. Such sets should never 
 be separated. In many cases, where a maximum angle 
 of aperture, or a perfect correction, can be dispensed 
 with, one set composed of three pairs of cemented lenses 
 may give by separation three different magnifying powers. 
 These sets are generally made so that their amplifica- 
 tions, in diameters, are in the ratio of one, two, and 
 three ; and are preferred by some observers. 
 
 Achromatic object-glasses for microscopes are of various 
 foci, differing from two inches to 1- 16th of an inch. As a 
 table of all the varieties would occupy more room than its 
 worth would justify my giving, I shall specify only those 
 which are mostly used in the table-microscope. They 
 afford a very wide range of powers, and one or the other of 
 them is suited for every class of objects which can be sub- 
 mitted for examination. It is seldom that an instrument 
 has the whole series attached to it; and since many per- 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 99 
 
 sons confine their attention to a particular class of objects, 
 some little consideration is necessary for selecting those 
 sets which are best adapted to the purpose. It is by no 
 means an unfrequent occurrence, that applications are 
 made to me, by those who have purchased expensive 
 instruments, for some particular object-glass, which they 
 do not possess, whilst they have others which to them 
 are of little or no value. 
 
 The first column in the following table gives the side- 
 real focal length in inches and parts. This focal distance, 
 in shallow object-glasses, is less than the distance an 
 object should be placed at from the lens when adjusted 
 for distinct vision ; whereas, with the higher powers, the 
 sidereal focal distance is greater than that between the 
 object and the lens. It varies also with the length of the 
 body. The second column shews the distance at which 
 the object should be placed from the object-glass. The 
 third column gives the mean angle of aperture, which 
 varies a few degrees either way. The last column gives 
 the magnifying powers in diameters, in combination 
 with different eye-pieces. It is calculated by a 10-inch 
 standard for sight. 
 
 Sidereal Focal Distance between the Mean Angle of Magnifying 
 Length. Object and Lens. Aperture. Powers. 
 
 Inches. Inchet. Degrees. Diameters. 
 
 2. 2,l-3rd 10 20to 50 
 
 1 1, l-8th 15 30 to 120 
 
 l_2d 3-8ths 22 65 to 280 
 
 l-4th l-8th . . 40 150 to 500 
 
 l-8th l-20th 50 .... 260 to 1100 
 
 M6th . , 1.40th. .70.. , 450 to 3000 
 
100 MICROSCOPIC ILLUSTRATIONS. 
 
 The above sets may be divided into three classes the 
 shallow, medium, and deep ; which, like the eye-pieces, 
 I shall particularize by the letters A, B t C. 
 
 The shallow object-glasses. A, are mainly serviceable 
 for opaque objects, which, when exhibited by means of 
 silver specula of proper dimensions and curvature, yield 
 a vast fund of amusement and instruction. The class of 
 objects, enumerated under the eye-piece, A, is suitable 
 for them. They display to great advantage, also, the 
 aquatic larvae of insects, polypi, &c. They are service- 
 able as a finder for minute objects ; for the general 
 examination and selection of insects ; minute fossils ; 
 coal, &c. ; the objects thus selected being afterwards sub- 
 mitted to the medium object-glasses, B. 
 
 Those denominated B, possessing considerable pene- 
 tration, bring out the striae and markings on the scales of 
 insects, the various forms of the elementary tissues and 
 fibres of vegetables, the internal structure of polypife- 
 rous zoophites ; whilst a general view of those minute 
 and delicate living creatures, the animalcula infusoria, 
 as also the circulation in the young of the lizard, aquatic 
 plants, &c., is obtained by the intermediate sets. The 
 medium-power object-glasses, when constructed with a 
 sufficient space in front, admit of a minute examination of 
 portions of insects preserved in Canada balsam between 
 plates of glass of the usual thickness. This class of 
 transparent objects, when mounted in the permanent 
 manner I have mentioned, is certainly the most instruc- 
 tive and amusing of any that we possess ; for although 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 101 
 
 we cannot enjoy the sight of their motions and habits as 
 with living specimens, yet are we in a measure recom- 
 pensed for it, hy the advantage of having them always 
 at hand. 
 
 The object-glasses, C, are best adapted for investi- 
 gating objects of the most minute and delicate descrip- 
 tion ; such, for instance, as the markings on the scales of 
 the podura 5 the forms and structure of minute fungi* ; 
 the globules of human blood, &c. 
 
 A variation of magnifying power is attainable also by the 
 body of the instrument being composed of two or more 
 tubes (see fig. 17,) the one sliding within the other, so as 
 to admit of an alteration of distance between the eye-piece 
 and object-glass : the power, in this case, will vary in the 
 direct proportion of the interval between the eye-piece 
 and object-glass. I am fully aware that this alteration 
 in the conjugate foci of the object-glass will affect its 
 corrections ; still, in some cases, the advantages to be 
 derived from it will counterbalance this objection. 
 
 Having now given a description of the instrument iu 
 its simplest mounting, together with examples for select- 
 ing its object glasses and eye- pieces, I shall proceed to 
 consider, separately, the several additions which can be 
 made to it, inasmuch as each one of them can be em- 
 
 * See Rev. M. T. Berkeley's admirable paper on Fungi, Annals fiat. 
 Hist. vol. i. p. 82. 
 
102 MICROSCOPIC ILLUSTRATIONS. 
 
 ployed in the microscope without reference being had to 
 any of the rest. By this means, persons engaged in parti- 
 cular investigations may easily judge of the peculiar appa- 
 ratus which is suited for them, and select accordingly. 
 
 SECT. 1. Candle-holder, Shade, and Condenser for the 
 Microscope. The first addition to the mechanical portion 
 of a microscope we may consider to be the arm, k, repre- 
 sented at fig. 21, for carrying a candle or lamp, to 
 supply the requisite illumination. Those persons who 
 are in the habit of using an artificial light for the micro- 
 scope, are fully aware of the difficulties there are in con- 
 ducting their observations satisfactorily with it. Upon 
 all accounts, it is advisable that the candle or lamp- 
 holder should be attached to the stem of the microscope ; 
 for by this arrangement the light may be easily kept at 
 the required elevation, and the inconvenience arising 
 from the foot of the candlestick coming in contact with 
 the stand of the instrument, will be wholly avoided. 
 Besides, you will not experience the annoyance of being 
 continually obliged to alter the direction of the light, 
 through unconsciously moving about the microscope. 
 
 Having given in the " Micrographia," page 219, a 
 description of the above-mentioned apparatus, together 
 with the shade, and shewn it by several engravings, in 
 the following pages, at the positions, k y I, it is unneces- 
 sary for me to remark further upon it here. I may men- 
 tion, however, that the same rod which is used for the 
 purpose of regulating the illuminations, is made to 
 
DESCRIPTION OF THET ACHROMATIC MICROSCOPE. 103 
 
 carry the condenser also, and in a manner the most effi- 
 cient for concentrating the light upon both opaque and 
 transparent objects. The condenser is represented at 
 m, in the same figures. 
 
 SECT. 2. Spring-double- Stage. This stage, which is re- 
 presented at a, b, c, c, in fig. 24, is a vast improvement 
 upon the ordinary stages shewn in the previous en- 
 gravings, figs. 11, 12, and 17- It answers the purpose of 
 a safety-stage for large objects, mounted on a slip of glass, 
 or contained in a brass trough, or large live-box, which 
 must be placed between the stage-plate, a, b } and the 
 circular plate beneath it. This lower plate being kept 
 up by the two springs, c, c, allows the slider, trough, or 
 aquatic live-box, to be moved about in the manner re- 
 commended by Dr. Goring, in Chap. IX. The upper 
 plate, 0, by is furnished with a forked piece similar to that 
 shewn in figs. 13, 14, and has likewise a circular ring 
 for reducing its aperture when necessary. This stage 
 was contrived by me for making experiments with po- 
 larized light ; for which I consider that it, or something 
 like it, is indispensably requisite. 
 
 It fits on to the microscope in the same way as do the 
 other stages, and like them may be furnished with dia- 
 phragms under it. 
 
 SECT. 3. Moveable Stage. Various contrivances have 
 been made use of to give the stage of a microscope, when 
 an object is placed upon it for examination, a convenient 
 
104 MICROSCOPIC ILLUSTRATIONS. 
 
 lateral motion. On Dr. Goring's principle, as described 
 in the next chapter, a corresponding effect, is produced 
 by moving the optical portion of the instrument; a 
 system which doubtless has its advantages, when either 
 the aquatic larvae of insects, or other large living objects, 
 are examined ; as also when any sizeable bodies are 
 being dissected under the regular microscopes employed 
 for that purpose. The moveable stage, however, for 
 small objects, and for general purposes, is usually pre- 
 ferred. 
 
 The ordinary method of adjusting a moveable or tra- 
 versing stage is by means of a couple of screws working 
 at right angles to each other ; by turning one or both of 
 which, a plate affixed to them, and carrying with it the 
 slider-holder containing the object, is moved in any di- 
 rection. Sometimes a screw and a rack and pinion are 
 substituted, by means of which all the movements are 
 effected on one side; at others, levers are made to 
 answer the purpose. But by far the most elegant in- 
 vention is that of the late Mr. Tyrrel, the celebrated 
 mechanical engraver, which, together with some im- 
 provements 1 have made upon it, I propose to describe, 
 as being the one most likely to be adopted generally, and 
 preferred to all others. Its advantage consists in that 
 with one hand and without removing it, you can give to 
 the object a lateral movement in all directions. This is 
 done by attaching a screw to the lower fixed plate of the 
 stage, within whose axis passes the axis of a smttll 
 pinion, the outer extremity of each terminating in a 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 105 
 
 small milled head. These milled heads T place on the 
 same side as the large milled head, x, which works in the 
 rack of the triangular bar, so that all the motions are 
 obtained by one hand, whilst the other is left quite at 
 liberty. 
 
 Fig. 18. 
 
 In the annexed figure, 18, a plan of this stage is given. 
 It fits into the block of my instrument by means of two 
 pins, just in the same manner as the other stages, &c. are 
 made to do. 
 
 e, and/, are the two milled heads for effecting the 
 lateral motion. The former of these is connected with 
 the screw, and will move the stage to the right or left, 
 the latter to the pinion, and will move the stage to and 
 fro. By turning both the milled heads, therefore, at the 
 same time, a diagonal motion (by the composition of 
 forces) will be obtained, by which the object for inspec- 
 tion can be brought and retained in the axis of the in- 
 strument, y, y, are two studs for the purpose of carrying 
 the arm d, at the further extremity of which a pair of 
 forceps (fig. 15) may be inserted for holding an insect, or 
 any other object which may be required. The studs 
 will support also a plate of glass when the instrument is 
 
106 
 
 MICROSCOPIC ILLUSTRATIONS. 
 
 in an inclined position, or the finger-springs, fig. 19 
 which are suited to hold large aquatic live -boxes, &c. &c. 
 
 Fig. 19. 
 
 C o 
 
 o 
 
 An arm, c, is attached to the lower fixed plate, for car- 
 rying a small condensing lens, or reflector, for throwing 
 condensed light upon opaque bodies. 
 
 In Mr. Tyrrel's arrangement two sockets were fixed 
 in the front at x, x : the one for holding the condensing 
 lens, and the other the forceps, instead of the studs ?/, y, 
 and the arm c. The objections to this plan were, that 
 the condensing lens was moved together with the object, 
 so that the illuminated spot did not remain under the 
 centre of the instrument. The necessary consequence 
 was, that a re-adjustment was to be made at every move- 
 ment. Also, the situation of the sockets was such as to 
 obstruct large plates of glass containing thin sections of 
 fossil wood, &c. &c. from being laid upon the stage. 
 These inconveniences are entirely obviated in the im- 
 proved stage. 
 
 Fig. 20. 
 
 The upper plate or moveable part of the stage has a 
 circular aperture, b, furnished with two angular notches, 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 10? 
 
 into which is fitted by a bayonet-joint the spring safety 
 slider- holder, fig. 20, or the finely-adjusting one de- 
 scribed in the Micrographia, page 217. 
 
 The utility of these safety-slider-holders is, that if by 
 any accident the object-glass should come in contact 
 with the slider containing the object when it is placed 
 between the plates, the object-glass will not be damaged; 
 for the spring which acts upon the lower plate will yield, 
 I most strongly recommend that a good object-glass of 
 deep power should never be used without this or a 
 similar contrivance. The numbers which are injured or 
 destroyed for want of this precaution are very great. 
 It is often quite impossible to restore them to their pri- 
 mitive excellence, even though the anterior lens of the 
 combination should not be broken. It may be remarked, 
 also, that in half the accidents of this kind which occur, 
 the observer is wholly unconscious of what has happened, 
 his attention being riveted to the object he is engaged 
 with. 
 
 SECT. 4. The Spring -Phial-Holder. This apparatus is 
 designed to afford us all needful assistance towards inves- 
 tigating a certain class of objects without removing them 
 from their natural element fluids. The circulation or 
 cyclosis in plants, the circulation in the tail of a fish, the 
 structure and functions of various marine and fresh- 
 water zoophites, the habits of such entomostraceans and 
 infusoria as attach themselves to the sides of vessels, the 
 development of the young from the ova of creatures 
 
108 MICROSCOPIC ILLUSTRATIONS. 
 
 which deposit them on the sides of vessels, &c. &c., fall 
 within the above description. 
 
 In examining any of these, this apparatus will be found 
 to be exceedingly convenient and useful ; for, by its aid, 
 we can with perfect facility carry our researches into the 
 contents of any open vessels, or phials, of a size propor- 
 tionable to the range of the holder. In the ordinary way, 
 a phial or vessel, unless it be very shallow indeed, must 
 be turned upon its side, and stopped with a cork to pre- 
 vent the water from running out ; and even then it often 
 happens that a portion of the air included in the vessel 
 presents itself in the field of view, so as greatly to dis- 
 concert our plans. In using the spring-phial-holder, 
 however, this inconvenience is avoided : and there is, 
 besides, a more considerable advantage to be derived 
 from it, viz. that of not disturbing the objects, by turn- 
 ing them about, so as to accommodate them to the micro- 
 scope. Again, as open vessels can be put in the holder 
 with as much readiness as an ordinary slider can be placed 
 on a common stage, a series of observations on the con- 
 tents of different vesels may be made in a very short 
 space of time. In viewing by common means the cyclosis 
 in aquatic plants, such as those of the order Characece, 
 where every cell throughout the plant has its distinct cir- 
 culation, it is necessary that a small portion of the plant 
 should be severed from the rest, placed in a slider, and 
 covered with a plate of mica or glass. This section will 
 not of course remain alive for any great length of time, 
 unless it be carefully replaced in a large vessel. With 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 109 
 
 the phial-bolder the plant may be of a size sufficient to 
 live for months, during which it is ever ready for exami- 
 nation without any preparation whatever. 
 
 In a microscope of the common construction, if the 
 magnifier or object-glass should be accidentally brought 
 into contact with the object, when it is placed on the slip 
 of glass used for the purpose, it is very liable to be de- 
 stroyed. In this we perceive another advantage of the 
 phial-holder; for the spring, in this case, behind the 
 vessel, would yield, and thus prevent the injury. 
 
 Fig. 21. 
 
 Fig. 21 represents the microscope with the body e, n 9 
 in a horizontal position. The stage is removed, and the 
 
110 
 
 MICROSCOPIC ILLUSTRATIONS. 
 
 stem c, turned a quarter round, where it is firmly clasped 
 by the screw z. The body and arm, a, g, is drawn out 
 by turning the pinion-head s, so as to admit of the phial- 
 holder Ti, being placed in its situation. This piece of 
 apparatus is shewn separately on a larger scale at fig. 22, 
 where p, p, are meant to represent the pins by which it 
 is attached to the microscope. Within the tube o, slides 
 another, s, which is propelled by a helical spring inserted 
 within it. This latter, or sliding-tube, has a thin con- 
 cave plate in front of it. When a phial, or other glass 
 vessel, similar to that shewn at fig. 23, is to be inserted 
 into the phial-holder, the tube s must be drawn back 
 towards o ; when liberated, the vessel will be firmly held 
 in any position. 
 
 Fig. 23. 
 
 Fig. 22. 
 
 Fig. 23 illustrates the method by which an aquatic 
 plant is mounted, to exhibit the circulation. The plant 
 is carefully laid along the inside of the vessel, which is 
 empty. A slip of glass, , is then covered over it, with suffi- 
 cient care to prevent it being injured. A small piece of 
 cork, b, is fitted behind it, and the vessel, then placed in 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. Ill 
 
 an upright manner, is filled with water. By this simple 
 contrivance, for which we stand indebted to Mr. Varley, 
 the plant is retained close against the interior surface of 
 the vessel, so as to admit of a magnifier of short focus 
 approaching it ; whilst in the space between the flat slip 
 of glass and the concave surface of the vessel, it will live 
 some months, without requiring any other attention than 
 merely adding a little water to replace that which shall 
 evaporate. 
 
 In reflecting upon what has been stated, it will be 
 evident that there are two points in the constructing of 
 our microscopes well worthy of notice, as tending to 
 facilitate the whole of this operation, viz. that the stage 
 be removeable, and the stem, c, capable of being turned 
 round. 
 
 When the phial-holder is mounted on a simple stand 
 for carrying only single magnifiers, it becomes a very 
 useful instrument for the above purposes; as also for 
 the examination of the progress of crystallizations and 
 other chemical actions. In this mounting it is named 
 the Phial- Microscope ; and as a mirror is not attached to 
 it, a lighted candle must be placed close to the end of 
 the tube, o, fig. 22. 
 
 SECT. 5. On the polarizing Microscope, or Apparatus 
 made use of for viewing objects by polarized light under 
 the Microscope. Yew subjects in experimental science 
 have roused the attention of philosophers of the present 
 age more than that of the polarization of light. Its 
 
112 MICROSCOPIC ILLUSTRATIONS* 
 
 curious action on natural and artificial bodies, eliciting, 
 as it does, a brilliancy of colours past all conception, has 
 created so deep an interest in the minds not only of men 
 of science, but of all who have ever witnessed it, that a 
 treatise upon an instrument so intimately connected 
 with it as the microscope is, would, I am sure, be deemed 
 altogether incomplete, if it did not embrace some par- 
 ticulars respecting this great phenomenon. The trans- 
 mission of light through two thin plates of crystal is an 
 operation which can be readily enough understood ; but 
 that the mere turning of one of them a quarter round 
 should occasion a total stoppage of light, although the 
 thickness of the plates and their inclination to each other 
 remain the same, is so contrary to the general laws of 
 optics and to our daily experience, that we should be at 
 once disposed to question the fact, if we were not prac- 
 tically convinced of its truth. Again, supposing the 
 plates to be in the position last named, by which, as we 
 have said, the light is intercepted in its passage through 
 them, and it was suggested to us to interpose between 
 them an additional plate of a crystal, the natural conclu- 
 sion would be, that this arrangement would operate to 
 occasion a greater exclusion, if possible, than did the 
 other, which, under ordinary circumstances, would doubt- 
 less be the case. Having done this, however, we find, 
 to our great astonishment, that the light is now freely 
 transmitted, and often accompanied with the most in- 
 tense and splendid colouring. To arrange plates of 
 crystals in chaste and pretty designs, so as to make these 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 113 
 
 beautiful effects visible to the natural sight, requires both 
 the talent of an artist and the experience of a workman. 
 Enlist, however, hut the microscope into this interesting 
 service, and the result will be that you will be supplied 
 at once, without a shade of difficulty, with an almost 
 endless variety of natural forms clothed in the most bril- 
 liant tints, and that too by the ordinary crystallization of 
 common salts. Thus, the application of a polarizing 
 apparatus to a microscope must be considered as being 
 highly desirable. 
 
 In a small pamphlet, entitled " A List of 2000 Micro- 
 scopic Objects," I have given a brief description of a 
 polarizer of my own contrivance, suggested to me 
 through being informed that Mr. FoxTalbot had applied 
 one to a compound microscope. To this gentleman is 
 due the merit of having first brought this arrangement 
 before the public ; as also the application to the engi- 
 scope of that beautiful invention of Mr. Nicol the 
 single-image calc prism. It appears, however, that 
 Sir D. Brewster had for years attached tourmalines to 
 single magnifiers for the purpose of making experiments 
 on polarization by crystals. He did this by cement- 
 ing the tourmaline to a plano-convex lens, and then illu- 
 minating the object by polarized light reflected from a 
 glass or japanned surface*. Professor Amici, also, has 
 introduced the ordinary rhombs of Iceland spar into the 
 compound microscope for a similar purpose. He has, 
 
 * Vide treatise on Microscopes, Encyclopaedia Britannica, p. 95. t 
 I 
 
114 
 
 MICROSCOPIC ILLUSTRATION*. 
 
 indeed, prosecuted the investigation of this subject by 
 means of the microscope, to a much greater extent than 
 any one else, by making it available towards exhibiting 
 the rings in plates of crystals cut across their axes, and 
 the exquisite tints in unannealed glass, &c. 
 
 Fig. 25. 
 
 p 
 
 In these experiments I find that achromatic object- 
 glasses are almost essential j for with common ones the 
 aperture is necessarily so reduced, that a sufficient 
 quantity of light will not be transmitted to give 
 intensity to the colouring. The object-glasses classed 
 under A, will be found most suitable; and the eye-piece, 
 B 9 best constructed to take in the largest field admissible 
 by the prism. 
 
 Let , fig. 25, represent the body of a compound micro- 
 scope, carefully placed over the centre of the spring- 
 double-stage, a, b. Let a slider of crystals be placed 
 upon the stage, a y b ; adjust to the focus, and illuminate 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPK. 115 
 
 with as intense a light as can be made to pass through 
 them. Now, insert into the socket under the stage, at 
 e, a single-image prism, and then examine the crystals. 
 They will still retain their usual colour, if any, though 
 the light passed through them is polarized. This prism is 
 termed the polarizer. 
 
 Next, unscrew the cap of the eye- piece, P, and apply 
 another cap, containing a tourmaline, or single-image 
 prism. This latter is shewn separately at P'. By 
 turning one of the prisms gently round, it will be 
 noticed that in certain positions no more light will be 
 transmitted than just so much as passes directly through 
 the crystal ; whilst the crystal, which, before the upper 
 prism or analyzer was added, was colourless, now dis- 
 plays the most brilliant colours. If a crystal be se- 
 lected exhibiting a deep orange tint, and the eye be fixed 
 upon it as the prism is turned a quarter round, the 
 colour will gradually change to green, and the light 
 will, at the same time, be freely transmitted through the 
 microscope ; in other words, the field of view will be- 
 come illuminated in the same manner as when a trans- 
 parent object is viewed in the ordinary way. Continue 
 turning the prism another quarter of a revolution, and 
 the light will again be stopped, and the crystal will 
 resume its rich orange tint. The two prisms are mounted 
 in brass tubes, with caps. The longer one is to be used 
 under the stage, as polarizer, and the shorter above, as 
 analyzer. 
 
 When the analyzer is a plate of tourmaline, instead of 
 
116 MICROSCOPIC ILLUSTRATIONS. 
 
 Mr. Nicol's single- image calcareous prism, a more 
 extended field of view will be obtained ; notwithstanding 
 which, the latter possesses many advantages over the 
 former, most especially that of its being colourless. 
 
 In cases where the ingenious invention of Mr. Nicol 
 cannot be procured, a rhomb of Iceland spar may be 
 well enough substituted, provided only that it has a 
 small aperture at the under surface, and that the upper 
 one is furnished with a sliding plate, with a correspond- 
 ing aperture, so that one or other of the images can be 
 seen at pleasure. Sir D. Brewster, who adopts this 
 method, cements a thin plate of glass upon both surfaces 
 of the rhomb, to take away their imperfections, and to 
 prevent accidents occurring, or deterioration of the 
 polish by cleaning. 
 
 The peculiar advantages resulting from the application 
 of the rhomb of Iceland spar, I shall give in Sir David's 
 own words. "These rhombs maybe made even out of 
 rhombs crossed with veins, which multiply the images, 
 because the multiplied images are at too great a distance 
 from the principal ones to be visible. This is a pecu- 
 liar advantage, as it is often very difficult to get good 
 .pieces of spar free from this composite structure. 
 
 "This method of constructing a polarizing rhomb 
 enables us to take advantage of the two lateral images 
 which accompany the two principal images in crystals 
 .crossed by on evein. These lateral images, shewn at 
 m, n, fig. 26, are distant from one another, and from the 
 principal images, b> c; and as each of them consists of 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 117 
 
 light wholly polarized in one plane, we have only to- 
 bring one of them under the aperture of the object-glass, 
 to have an admirable polarizer, without being at the 
 trouble of stopping out any of the other pencils. 
 
 "The images, m, n, are much less bright than the 
 principal ones, b, c ; but this is really of no consequence, 
 as we can obtain any degree of light we choose in tiie 
 microscope, either by the condensation of artificial, or 
 the use of solar light. 
 
 Fig 26. 
 
 ' 7 ~7 
 
 O O 
 
 "When the vein by which these lateral images are 
 formed is above a certain thickness, their light is white ; 
 but they are most frequently coloured ; and the observer 
 who understands the cause of these colours may make 
 tins coloured pencil of great service in microscopical 
 observations. If he uses a rhomb, which gives to m a 
 green of the second order, it will contain none of the 
 extreme violet and blue rays, and none of the extreme 
 red ; so that it affords a more homogeneous pencil than 
 if it were white light, and thus improves the performance 
 of a microscope that is not achromatic. 
 
 " He may, in like manner, use tints which give the 
 red extremity or the blue extremity of the spectrum, or 
 even, when the tint is divisible by the prism into perio- 
 dical bands, he may absorb the least luminous of these 
 bands, and create a homogeneous pencil of polarized 
 
118 MICROSCOPIC ILLUSTRATIONS. 
 
 light, of inestimable value in particular researches, and 
 with particular microscopes. 
 
 " But, independent of these advantages, the method of 
 using a lateral pencil, m, has the great advantage of not 
 requiring any thickness in the rhomb. A Nicol's prism, 
 and a rhomb in which the two principal images, b, c, are 
 used, must be about an inch thick, in order to be effica- 
 cious ; but the distances, m, n, or m, b, are the same at 
 all thicknesses, so that we can use rhombs for this pur- 
 pose which are quite useless for any other. 
 
 " It is scarcely necessary to add, that similar rhombs in 
 which either the principal images, b, c, or the lateral ones, 
 m, n, are used, may be employed for the analyzer. For 
 this purpose, a thin plate, in which m, or n, is white, is 
 peculiarly applicable, as it enables us to see at once the 
 whole field of the microscope*." 
 
 In all the effects produced by the foregoing arrange- 
 ments, the field of view is either black or white. By 
 employing, however, the stage represented by fig. 25, 
 and inserting between the upper plate and the circular 
 one a plate of crystal or unannealed glass, the back 
 ground may, at pleasure, be made to assume a variety of 
 colours, so that the crystals shall appear like so many 
 coloured gems set in different tinted mountings ; whilst, 
 without this addition, they all seem to be set on black 
 velvet. 
 
 Considerable advantage is derived, also, from my con- 
 trivance of the double stage, in the examination of Mr. 
 
 * Treatise on Microscopes, p. 101. 
 
DESCRIPTION OP THK ACHROMATIC MICROSCOPE. 119 
 
 Talbot's " Analytic Crystals." As these experiments, 
 however, are full of interest, I have judged it advisable 
 to extract from the Philosophical Transactions, and give 
 entire, in the Appendix, Mr. Talbot's own papers upon 
 the subject. 
 
 If, in place of the upper prism, or analyzer, a double- 
 image prism be fixed at P, another and very beautiful 
 series of effects will result from the exchange. With this 
 modification there will at the same instant be seen two 
 images of the object arrayed in their complementary 
 colours ; and if we turn this prism round in the manner 
 before described, an interchange of colours will take 
 place between the images, the one assuming that of the 
 other. If long needle-shaped crystals are procured for 
 the purpose, the two images will be so far separated as 
 to appear quite distinct. They will vary in their positions 
 also, as well as in their colours and intensity, while the 
 prism is being turned. A moon-shaped or crescent form 
 of complementary colours w T ill be discernible on each 
 side of the field of view, while the latter will be white, 
 from the union of the two complementary colours. The 
 most gorgeous effect of all, however, is produced by this 
 double-image prism when a plate of quartz is inserted 
 below the crystals (say crystals of chlorate of potass) 
 placed at a, b. In short, the whole experiment is so 
 beautiful, that it is in vain to attempt any thing like a 
 description of it. 
 
 Some crystals possess the peculiar property of exhi- 
 biting two colours, when viewed by polarized light; 
 
120 MICROSCOPIC ILLUSTIIATIONS. 
 
 the one being elicited by the ordinary, and the other by the 
 extraordinary ray. Hence they are termed dichromatic. 
 To display dichroism in crystals, the lower prism, or pola- 
 rizer, should alone be used ; the eye-piece being allowed 
 to remain as when objects are viewed by common light. 
 If, whilst the dichromatic body is under examination, the 
 polarizing prism be turned a quarter round, the colour 
 will also be changed. In conducting these experiments, 
 the illumination need not be so intense. In large crystals, 
 dichroism may be readily seen without the aid of a micro- 
 scope, simply by placing them between the eye and a 
 japanned reflector inclined at the polarizig angle. 
 
 In order to convert the microscope into a complete 
 polariscope, I employ, instead of the mirror n, a pola- 
 rizing reflector, formed of several parallel plates of 
 thin glass fixed at the angle of polarization. A stage 
 must be constructed with a large condensing lens, and 
 made to slide upon the same stem, c, as is used for the 
 mirror. A double object-glass must be attached to the 
 body for obtaining a proper magnifying power, and the 
 analyzer should be made of tourmaline, in order to pro- 
 cure an expansive field of view. With this arrangement, 
 the phenomenon of polarized light, as exhibited in unan- 
 nealed glass, plates of amethyst, as also the coloured 
 rings and black crosses in crystals cut across their optical 
 axes, are seen in the most perfect manner. Professor 
 Amici was the first to exhibit these curious effects by 
 means of the microscope, although I do not know the 
 exact construction of his apparatus. 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 121 
 
 SECT. 6. Apparatus for dissecting. Since the days of 
 Lyonet and Swamraerdaii), until of late, very little pro- 
 gress has been made towards producing a microscope of 
 a more complete description than their's, for the purpose 
 of dissecting. It is true that two or three of this sort 
 have been constructed, but then they have been suited 
 for only one particular class of objects, such as the ele- 
 mentary tissues of plants, &c. Dr. Goring's engiscope, 
 with the additions hereafter mentioned as belonging to 
 myown microscope, probably approaches the nearest of. 
 any to au efficient dissecting microscope ; nevertheless, 
 a still further advantage may be derived from having a 
 separate stand of proper dimensions made expressly for 
 the purpose. As one set of magnifiers will of course 
 suffice for both stands, the expense will not be consi- 
 derable. The microscope described in this chapter, with 
 the dissecting apparatus attached to it, is, I conceive, as 
 complete as any one of that size can be rendered ; its 
 dimensions however, are barely sufficient for the dis- 
 secting of large objects. 
 
 Having alluded to Swammerdam's microscope, it may 
 be advisable, I think, before describing my own appa- 
 ratus, to refer the reader to an account of his method of 
 proceeding (as given in the Appendix) ; containing, as 
 it does, many useful remarks. After the perusal of this, 
 and a brief description of Lyonet's dissecting micro- 
 scope, the reader will be fully prepared to understand 
 the construction I have adopted. 
 
 Lyonet's dissecting microscope may be thus de- 
 
122 MICROSCOPIC ILLUSTRATIONS. 
 
 scribed : It consisted of a circular brass table, in which 
 was cut a circular aperture half way between the centre 
 and the circumference. This table and aperture served 
 the purposes of an ordinary stage, and held a watch- 
 glass, trough, &c. to contain the object for dissection. 
 Under the aperture was placed the mirror ; and on the 
 opposite side to that where the aperture was made, was 
 inserted a socket, to which a number of universal joints, 
 strung together like a row of beads, were attached. On 
 the terminal one was placed the magnifier. Now fig. 27 
 
 Fig. 27. 
 
 is a drawing of my instrument, placed in a vertical 
 position, as if intended for dissecting. Nos. 2, 3, represent 
 a wooden rest for supporting the hands ; this fits on to 
 the joint on the top of the pillar of the microscope, where 
 it is firmly retained. Near to each end, on the upper 
 surface of the rest, is a deepened or hollow place to pre- 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 123 
 
 vent the hands from slipping. It will be observed, that 
 the rest here given is perfectly straight ; notwithstand- 
 ing, in some cases, it will be more convenient to have it 
 of a semicircular or curved form. There is an arm, 0, 
 which slides in a socket attached to an universal joint. 
 This arm has a spring ring for carrying the magnifiers, 
 which are inserted into it in the same manner as into 
 the plain end of the arm, shewn in several previous 
 figures. The dissecting trough, #, is placed in the 
 moveable stage in the way described in " Micrographia," 
 page 215. This trough, which contains the object to be 
 operated upon, is capable of being turned about in any 
 direction. When the object is opaque, the condensing 
 lens, m, shewn in figs. 29, 30, must be brought into the 
 position, A, to concentrate the light upon it. 
 
 Single or doublet magnifiers are commonly preferred 
 to the compound microscope for dissecting, because the 
 latter instrument inverts the image of the object. The 
 amplifying powers, which are most generally service- 
 able, are those from an inch-and-a-half focus to the one- 
 tenth of an inch. When higher powers are requisite, 
 the lenses should be made of jewels, as they allow Oi 
 more space between the object and the lens than do 
 glass ones. 
 
 The advantages which I conceive this arrangement 
 possesses over others, are as follow : In the first place, 
 the rest or support, being made of wood, does not con- 
 duct the heat away from the hands so freely as if it were 
 of metal. 2dly. It being situated over the central sup- 
 
124 ;-> MICROSCOPIC ILLUSTRATIONS. 
 
 port of the instrument, and being independant of it, all 
 straining upon the microscope is avoided, and the stage 
 may be inclined without disturbing the hands. This 
 will be found to have a two-fold advantage ; for, by in- 
 clining the instrument, the operator is placed more at 
 his ease ; and the superfluous matter can be easily drained 
 away, which is necessary when large insects are being 
 dissected. For dissecting these objects, thin pieces of 
 soft wood, having brass rings cemented to their under 
 sides, and being fitted into the moveable or other stage, 
 will be found extremely convenient. 
 
 The integuments of these creatures, as soon as they 
 are laid open, should be pinned down upon the wood, 
 while the nervous, respiratory, or alimentary organs 
 are being removed. The fatty matter, so very abundant 
 in insects, must be washed away with a camel-hair 
 pencil. By forming a ledge round the wood with a 
 little wax, the creature may be dissected under water 
 or diluted spirits. 
 
 The arm, a, having an universal joint, may be turned 
 about in any direction ; and should any remarkable 
 feature present itself whilst dissecting, which it is de- 
 sirable to examine without removing it, this may be 
 readily effected by taking off the screw- nut, #, and substi- 
 tuting the usual arm of the microscope, with the achro- 
 matic body. 
 
 Sometimes the achromatic body is employed in making 
 dissections ; in this case, the image being inverted, it 
 requires some practice in using the dissecting instru- 
 
DESCRIPTION OF THB ACHROMATIC MICROSCOPE. 125 
 
 merits, unless an erecting eye-piece is applied, as men- 
 tioned in the following chapters. 
 
 SECT. 7 Single Lenses mounted in imitation of Achro- 
 matic Object-glasses. A series of single lenses mounted 
 as object-glasses, and of the same foci and angular 
 aperture as achromatics, will prove an excellent 
 method of instituting a comparison between their relative 
 values. The object-glasses are the most costly portion 
 of an achromatic microscope, notwithstanding to ordi- 
 nary observers they appear not to differ exteriorly from 
 common lenses mounted in the manner described. Some 
 persons are very apt to imagine that the additional price 
 of the one above the other is owing to the achromatics 
 superior magnifying power ; but as this does not 
 necessarily follow, these single lenses will manifest at once 
 wherein the superiority consists. 
 
 By way of trial, screw to the body of the microscope 
 one of the single lenses ; place an object under it, and 
 examine it with attention : then remove the single 
 lens, and substitute an achromatic object-glass in its 
 stead, allowing the object, eye-piece, and other portions 
 of the instrument, to remain as before. The difference 
 will be apparent enough in a few minutes to leave a full 
 and lasting impression upon the mind, of the extensive 
 improvement which has of late years taken place in this 
 respect. I know of no better mode than this by which 
 persons unacquainted with optical science can judge 
 fairly of what has been effected for the microscope. 
 
126 MICROSCOPIC ILLUSTRATIONS. 
 
 The achromatic will be distinguishable from the single 
 lens by its exceeding penetration, and by the total free- 
 dom from spherical and chromatic aberration with 
 which the light is transmitted through it. It is true 
 that if the common lens have a diaphragm to limit its 
 aperture, the vision will be more distinct, and the light 
 less coloured ; but then, compare the two again, and in 
 the case of the single lens there will be a perceptible 
 diminution of light, and the eye will soon grow weary in 
 endeavouring to trace out the minutire of an object. 
 
 SECT. 8. Garden Clamp for the Microscope. This piece 
 of apparatus enables us to employ the microscope for 
 the examination of flowers and plants in the garden or 
 greenhouse. The clamp is represented by the annexed 
 engraving, fig. 28. The end, c, may be screwed to a 
 
 Fig. 28. 
 
 tree, door-frame, or any other convenient place. The 
 body of the microscope is then inserted into it at a, and 
 fixed there by the pinching screw, b. To the lower end 
 of the body is screwed a small arm, constructed for the 
 purpose, arid carrying the forceps for holding the object. 
 The object-glasses, A, with their silver specula, are the 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 127 
 
 most serviceable for this occupation, as the adjustment is 
 very readily made with them. The microscope, suitably 
 equipped, may be used for examining flowers without 
 either its stand and apparatus, or the clamp : the body, 
 with the arm above mentioned, being merely held 
 in the hand. A variation in the magnifying power is 
 obtained by sliding the body in and out, and thus avoids 
 the necessity of changing the object-glasses, which, 
 out of doors, would be attended with some risk of loosing 
 them. 
 
 SECT. 9. Steadying Rods. When we have recourse to 
 very high powers for making microscopic observations, 
 it is of consequence that the microscope be kept as 
 steady as possible, all motion, or tremor, being magnified 
 to the same extent as the object itself. Under such cir- 
 cumstances, it is often expedient to have one or two 
 sliding rods, whose upper extremities may be attached to* 
 the eye- end of the body, and their lower to the feet or 
 pillar of the stand. When these rods are employed, all 
 their tightening screws must remain loose until the 
 instrument is set in the proper position the moveable 
 stage, with its fine adjustment, put on, and the object- 
 glass, selected and adjusted to distinct vision by mean& 
 of the rack and pinion, in the usual way. When this is 
 effected, the steadying rods may be clamped tight with 
 the pinching screws, and all subsequent adjustment to 
 distinct vision made by the fine adjustment, the rack and 
 pinion being now considered immoveable. 
 
128 MICROSCOPIC ILLUSTRATIONS. 
 
 Although this contrivance sufficiently obviates the an- 
 noyance occasioned by tremor, yet I strongly recommend 
 a small stout stand being had expressly for the purpose ; 
 the additional expense not being great, whilst, from 
 its simplicity and convenience, you will be amply 
 recompensed. 
 
 Another purpose to which the steadying rods may be 
 applied, is that of drawing with the camera lucida. In 
 this case, the body of the microscope should first be 
 ranged horizontally, the steadying rods tightened with 
 the clamping screws, and then the requisite adjustments 
 made, the body remaining a fixture. 
 
 SECT. 10. Camera Lucida for drawing with the Micro- 
 scope. This instrument is constructed in a variety of 
 ways, but the most serviceable is that shewn in the plate, 
 fig. 12. As there is only one reflection, the image of the 
 object is presented to you with its sides reversed, re- 
 sembling a copper-plate engraving from which the im- 
 pression is taken. The method of using this instrument 
 is as follows : With my microscope, the cap, or cover, 
 which limits the pencil emerging from the eye-piece, 
 must be unscrewed, and the ring, to which the camera is 
 attached, substituted in its stead. This ring is furnished 
 with a pin for the camera prism to slide upon, which 
 may thus be made to approach or recede from the eye- 
 lens to suit the emergent pencils of light. The inclina- 
 tion is regulated by turning it on its axis. 
 
 For making drawings of microscopic objects, the eye- 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 129 
 
 piece may be conveniently removed from the body of the 
 microscope, and the end covered with a cap, having a 
 plate of finely ground or greyed glass fitted into it. On 
 this plate the image of the object will be formed, and 
 distinct vision obtained by adjusting the focus in the 
 usual manner. In this case, the eye-end of the micro- 
 scope, in order to produce the desired effect, must be 
 kept darker than the other extremity. If, instead of the 
 plate of glass, a piece of tracing paper be employed, the 
 image may be delineated at once. Also, the glass may 
 have divisions marked upon it similar to those in the 
 plate, fig. 13, and be used with drawing paper traversed 
 with squares, on Mr. Bauer's plan, described in the 
 Micrographia. 
 
 One great advantage which the camera lucida derives, 
 from its connexion with the telescope or microscope, is, 
 that the reflected image is thrown on the drawing paper, 
 so as to appear in the same plane with it. When it is 
 used without either of these instruments, the image 
 is formed at the same distance below it as the object is 
 distant from it in front ; thus occasioning an error by 
 parallax with the image and pencil, upon the slightest 
 motion of the eye. 
 
 Beginners in drawing experience some difficulty in 
 obtaining a distinct view of the point of the pencil simul- 
 taneously with the magnified image. The Rev. J. B. 
 Reade informs me, that by illuminating the object with 
 strong lamp-light in the day-time, and the paper on 
 
130 MICROSCOPIC ILLUSTRATIONS. 
 
 which the tracing is made with day-light, this inconve- 
 nience is entirely remedied. 
 
 SECT. 11. Micrometer Eye-pieces. The micrometer eye- 
 piece is made to slide into the body of the microscope, 
 like the ordinary eye-pieces. This eye-piece is designed 
 either for drawing or measurement. When we employ 
 it for drawing, a plate of glass, with divisions similar to 
 those represented in the plate, fig. 13, must be used, in 
 the manner described by Mr. Bauer. When for mea- 
 surement, a slip of pearl, with divisions of 100 or 200 
 to an inch, answers best for all practical purposes. It 
 must be remarked, however, that the same divisions re- 
 present different measurements according to the power 
 of the object-glass and length of the body. When, there- 
 fore, they are once ascertained by the rules laid down 
 in the Micrographia, page 48, they must not be altered. 
 
 For either of the above purposes, the micrometer must 
 be placed at c, page 80 5 in the focus of the eye-lens, a. 
 Now as the distance of distinct vision differs in different 
 persons, and even in the same person, according to his 
 state of health, the adjustment must be made so as to vary 
 the distance between a and c. This is ordinarily done 
 by moving the lens a, with a screw. On consideration, 
 however, it is evident that by this means the achromatism 
 of the eye-piece is disturbed. I have obviated this defect 
 with my eye-pieces by giving the adjustment to the 
 field-bar, c, which carries the micrometer. 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 131 
 
 SECT. 12. Method of viewing moveable Fluids in the Ca- 
 vities of Topaz, Quartz, and other Crystals. The existence 
 of small cavities in crystals is of such frequent occur- 
 rence, that few persons who are in the habit of seeing 
 crystals at all, can be unacquainted with the fact. 
 That some of these cavities, however, contain moveable 
 fluids, is a recent discovery, for which we stand indebted 
 to Sir D. Brewster. As I am not aware that the method 
 of observing these under the microscope has ever been 
 explained, a few words upon this subject may not be 
 considered superfluous. 
 
 Let your microscope be placed in a vertical position ; 
 apply a power of 100 or 200 diameters, and put the 
 crystal for examination upon the stage. Bring one of 
 these cavitie's into the centre of the field, and adjust to 
 distinct vision, when the fluid will be discerned within 
 the cavity, resembling an air-bubble. The proof that 
 this appearance is occasioned by a fluid contained within 
 the crystal is very readily ascertained. Take a metal- 
 heater of a proper size, having a hole in the middle to 
 allow the light to pass through ; warm it, and hold it by 
 its handle under the aperture in the stage, close behind 
 the glass plate upon which the crystal is situated. 
 During this operation you will observe the fluid in 
 the cavity expand, and, in some cases, become vola- 
 tilized j and then, upon cooling again, return to its fluid 
 condition. 
 
 Many of the cavities in gems, particularly in the 
 
132 MICROSCOPIC ILLUSTRATIONS. 
 
 diamond and garnet, are perfectly spherical ; and, owing 
 to the great refractive power of these stones, they have 
 the appearance of black and opaque globules with a 
 small bright spot of light in their centres. The propor- 
 tion between the diameters of the dark annulus and the 
 luminous spot, Sir D. Brewster has ingeniously proposed 
 as a measure of the refractive powers of both the solid 
 and the fluid. He has suggested, also, that these cavities 
 being permanent, are preferable to Dr. Goring's* air 
 bubbles in Canada balsam, for determining the aberra- 
 tions of object-glasses ; but a little consideration, 1 
 think, will suffice to shew that both the one and the 
 other of these tests must give erroneous results for 
 ordinary objects. And for this simple reason, because 
 the light coming from the cavity to the object-glass 
 through a portion of the gem, will be refracted by it, 
 and enter the object-glass in a different state to what 
 it would have done had no such medium intervened 
 between the cavity and the lens. 
 
 In examining the organic remains embedded in flint, 
 the object-glass requires to be corrected by this test, as 
 the rays of light proceeding from the creatures dis- 
 coverable in thin plates of that substance, have to pass 
 through it before they enter the object-glass. The 
 recent discovery of the crystatella f and other infusoria 
 in flint, as also the sporules of ferns, has opened an en- 
 
 * See Micrograpbia, page 107. 
 f Zanthydium furcata of E. 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 133 
 
 tirely new field to the geologist, and one which promises 
 some very important results in throwing light upon the 
 original formation of that stone. Some beds of flints are 
 crowded with these infusoria, whilst others are wholly 
 without them. In viewing these creatures (crystatella) 
 a powerful object-glass is requisite ; and, as their 
 multi-pronged spines protrude from all parts of the 
 surface of their bodies, each part being in a different 
 plane, there must be a new adjustment of the focus. 
 
 Some of the cavities in minerals contain " two fluids 
 unknown to the chemist : groups of crystals, floating 
 balls, &c." The forms of these cavities are various. 
 Those containing two fluids, which do not mix though 
 in the same cavity, appear globular. 
 
 SECT. 13. Concave Specula, Silver Cups, or Leiberkuhns, 
 for illuminating opaque objects. In viewing opaque 
 bodies by means of concave specula, it is generally 
 advisable to remove the stage of the instrument, and to 
 insert the arm, fig. 16, with the forceps, fig. 15. An 
 object-glass of a right focus must then be screwed into 
 the body of the microscope, and the silver cup, properly 
 fitted, made to slide over it. If candle-light, w r hich is 
 the simplest artificial illumination, be used, although a 
 small argand lamp is often preferable, the light may be 
 placed from about 3 to 6 inches behind the object. 
 
 When we have day-light, the concave mirror must 
 be brought into the rear of the object. In this case it 
 
134 MICROSCOPIC ILLUSTRATIONS. 
 
 should be placed as near to it as it conveniently can 
 be, in order that the cone of rays reflected from the 
 mirror may spread over the whole surface of the silver 
 cup ; otherwise, the maximum illumination will not be 
 obtained. 
 
 Having brought the object into focus, the silver cup 
 must be adjusted, without altering the adjustment of the 
 focus of the instrument. This is done by sliding the tube 
 belonging to the silver cup over the tube which is screwed 
 before the object-glass. In order to obtain the maximum 
 quantity of light, the cup must be placed so that the 
 object is a little within its focus. As the length of this 
 focus, however, varies with the distance at which the 
 candle, lamp, or other illuminating source is placed from 
 the cup (the rays being thus either parallel, diverging, 
 or converging,) the adjustment must be made after the 
 position of the illuminator is determined upon. Thus, 
 when the cup is used with a candle placed at 3 or 4 
 inches distance from it, the rays being divergent, the 
 focus will be elongated, in which case the two tubes 
 must be thrust in. Again, when we illumine by day- 
 light reflected from a plane mirror, the focus is shortened, 
 and the outer tube requires to be drawn out a little. 
 
 In the old constructed microscopes, the concave spe- 
 culum, or silver cup, was attached to the stage of the 
 instrument, so that a fresh adjustment was always re- 
 quired for every change of object. By the above men- 
 tioned plan, as soon as the cup and light are adjusted, 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 135 
 
 nothing further is necessary in this respect; for when 
 the object is brought into the focus of the microscope, the 
 illumination is sure to be correct. 
 
 Persons who have never viewed opaque objects with 
 shallow or medium power object-glasses, and silver 
 cups of proper diameters and curvature, are not aware 
 of half the beautiful effects resulting from the achromatic 
 construction. And as for objects that are suited to this 
 method of illumination, they are almost infinite in num- 
 ber ; and, consequently, a greater fund of instruction and 
 pleasure is derivable from it than can be obtained with 
 deeper powers, where the objects which can afford 
 amusement are much more limited. 
 
 SECT. 14. Stops or Diaphragms are used for the purpose 
 of limiting the illuminating pencil of light, and may be 
 described as being small tubes with apertures at each of 
 their extremities. These are made to slide into sockets, 
 which screw into orifices formed to receive them at the 
 under side of each of the stages of the microscope. 
 They are sometimes furnished with a wheel of apertures, 
 similar to that shewn in the plate, fig. 24 ; and some- 
 times these sliding tubes have a small condensing lens, 
 such as that exhibited in the Microscopic Cabinet, page 
 170. By having the stops or diaphragms in the form of 
 tubes, the effect they are designed to produce may be 
 varied, if necessary, by sliding them up and down within 
 their sockets. Those possessing the lens, and intended 
 to act on Dr. Wollaston's principle, require this adjust- 
 
136 MICROSCOPIC ILLUSTRATIONS. 
 
 ment, in order that a fine definition may be obtained with 
 them. "The centre of the aperture must be in the 
 common axis of the lenses" of the microscope, " and the 
 image of the perforation formed by the condensing 
 lens must be brought by such adjustment into the 
 same plane as the object to be examined*." 
 
 Stops or diaphragms serve to increase the definition of 
 the outline of delicate objects. Thus, the cilia and inter- 
 nal structure of animalcules become much more visible 
 by their aid than without it, especially when we have 
 single magnifiers only. They enable us, also, to reduce 
 the quantity of light, when using achromatic microscopes 
 with object-glasses of long foci ; by which reduction our 
 observations on thin transverse sections of wood, as well 
 as on cylindrical and spherical bodies, may be more 
 accurately made. 
 
 With high magnifying powers it is often desirable to 
 have a lateral adjustment for the Wollaston illumination, 
 to effect which, a movement to and fro must be given to 
 the screw which holds the socket, on the under side of 
 the moveable stage (page 105) ; whilst the body admits 
 of a lateral adjustment, to the right or left in the other 
 direction. By these means we may obtain either a cen- 
 tral or eccentric pencil of light. 
 
 The reader must bear in mind, that stops have the 
 effect of decreasing the angle of aperture of the object- 
 glass ; arid therefore, in testing it, he must make proper 
 allowance for them. 
 
 * Philosophical Transactions, p. 9, 1830. 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 137 
 
 SECT. 15. On viewing Transparent Objects by Day-light. 
 The best position of the instrument by day- light is 
 that represented at fig. 29. When the object-glass is of 
 long focus, a screen must be interposed to stop the light 
 from falling upon the object. If this precaution be not 
 taken, we could not obtain a correct knowledge of it a on 
 account of the confusion occasioned by the transmitted 
 and reflected light. They must not, therefore, be used 
 together. A small piece of black card placed upon its 
 edge, on the stage, will prevent the light from falling 
 upon a transparent body. 
 
 SECT. 16. On viewing Transparent Bodies by Intermitting 
 Light. It has been ingeniously suggested, in order to 
 determine the number and arrangement of the rotatory 
 organs of Infusoria (such as the wheel-animalcule), to 
 illuminate them by a succession of galvanic or electric 
 sparks. By this means, an effect is produced similar to 
 that occasioned by the revolution of a cogged wheel when 
 viewed in a mirror. The reflection through the cogs 
 makes the wheel, although in rapid motion, appear 
 stationary. 
 
 As there are several methods by which this effect can 
 be produced, it is unnecessary to enter upon the details 
 here, some persons preferring one way and some another. 
 In my own experiments, I have used the sparks emitted 
 by the electric magnetism of a wheel dipping in mer- 
 cury. 
 
138 
 
 MICROSCOPIC ILLUSTRATIONS. 
 
 SECT. 17. The Black Ground Illumination for Trans- 
 parent Objects. The Rev. Mr. Reade, in the Appendix 
 to the JMicrographia, has described a method of illumi- 
 nation for transparent objects, differing from any at 
 present in use. It consists in illuminating the object by 
 a very strong light placed at such an obliquity to the 
 axis of the microscope that none of the rays can enter it, 
 except those which impinge directly upon the object, and 
 are thereby bent so as to pass through the object into 
 the instrument. As the arrangement for viewing objects 
 in this way may not readily suggest itself from a verbal 
 description only, it has been thought advisable to illus- 
 
 . 2y. 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 139 
 
 trate it more fully by means of the accompanying en- 
 gravings, so that any one may make trial of it, and 
 thence form a correct judgment of its merits. I should 
 mention, that the effects produced are not so striking in 
 some objects as in others, and therefore that it is worth 
 while to make trial of several kinds. 
 
 Fig. 29 represents one of my instruments : its body, 
 eye-piece, and object-glass, n, i, e; the spring double- 
 stage, t ; the mirror, o, which is turned aside ; the candle- 
 holder, k, /; and jointed condenser, m. In this arrange- 
 ment it is proposed to illuminate the object by an arti- 
 ficial condensed light. Let the object-slider be placed 
 between the stage and the circular plate ; adjust to the 
 focus, and place the candle, /, and condenser, m y in the 
 positions shewn in the drawing, so that the pencil of rays, 
 r, r, may form the required angle with the axis of the 
 instrument, i, i. This angle will be dependent upon the 
 angle of aperture of the object-glass; but in all cases it 
 must exceed half that angle, in order to produce the 
 desired effect, otherwise the field of view will be illumi- 
 nated. But if a small transparent object, such as the 
 pollen of a flower, the scale of an insect, &c. be placed 
 within the field of view, the rays impinging upon it will 
 be refracted, so as to enter the microscope, and they 
 will form an illuminated image of it upon the dark 
 ground. 
 
 When this experiment is conducted by day-light, the 
 instrument should be placed in the position shewn by 
 
140 
 
 MICROSCOPIC ILLUSTRATIONS. 
 
 fig. 30, with the condenser, m, to condense the light in 
 the manner there represented. 
 
 Fig. 30. 
 
 v. 
 
 If the rays, r, r, make an angle with i, i, similar to 
 that in the former case, the result will be the same. The 
 most convenient plan of managing this mode of illumi- 
 nating by day-light, is by means of a large concave 
 reflector fitted upon the stem of the candle-holder ; or 
 upon a separate stand, so placed that the reflected light 
 shall make the proper angle with the axis of the micro- 
 scope. The instrument, by this means, may be placed 
 in a vertical position, if so preferred. 
 
 The medium power object-glasses, usually marked 
 jB, are best suited for this method of investigation. 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 141 
 
 SECT. 18. Method of viewing Opaque Bodies by oblique 
 reflected Light condensed upon them by Mirrors. It is often 
 necessary to examine an object under object-glasses, 
 which differ widely in their amplifying powers, or focal 
 lengths, when great difficulty is experienced in mounting 
 
 Fig. 31. 
 
 it, so as to accommodate it to them. For instance 
 suppose the object to be mounted on a circular disc, 
 suitably for an object-glass of one-inch focus, it would 
 be barely possible to illuminate it with a silver cup, 
 adapted to an object-glass of l-4th of an inch. Deep 
 object-glasses necessarily require the silver cups to be 
 small, by which means it is impracticable to give an 
 oblique direction to the light. The method usually pur- 
 sued, is to employ a silver concave reflector of the form 
 shewn at t, fig. 31 ; by the aid of which light may be con- 
 densed upon the object, so as to exhibit it under object- 
 glasses of different focal lenghs. 
 
 In order to obtain a greater condensation of light, Mr. 
 
142 MICROSCOPIC ILLUSTRATIONS. 
 
 Ross, who first proposed the adoption of this reflector, 
 places a condensing lens in front of the stage. The dis- 
 persion occasioned by this lens, however, is apt to give 
 a false colouring to the object; so that I prefer 
 making use of a large concave mirror in the manner 
 represented in the above (fig. 31), which obviates this 
 defect, whilst it admits of the light being placed in a 
 more convenient position. 
 
 SECT. 19. A Method for determining the Forms of certain 
 minute Bodies. In the investigation of long stick-shaped 
 bodies, it is sometimes difficult to ascertain whether a 
 section of these would be of a circular, or of a flattened 
 oval form ; since, in either case, the object, when at rest, 
 presents the same appearance to the eye of the observer. 
 It occurred to me, therefore, that by immersing a body 
 such as these in a fluid, which evaporates rapidly, the 
 currents occasioned by the evaporation would raise and 
 turn it upon its edges, so as to afford a view of its different 
 sides. This plan I found to answer my expectation fully. 
 As an example illustrative of this mode of verification 
 may not be without its use, I have thought it worth while 
 to present the reader with the following one : 
 
 Take on the point of a penknife a specimen of fossil 
 infusoria (or more correctly speaking, the siliciotis cover- 
 ing of these creatures), place it on a slip of glass, or what 
 is perhaps more convenient, the tablet of an aquatic live- 
 box, the cover being removed. Having equipped your 
 microscope with a moderate power, and adjusted it to 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 143 
 
 distinct vision, let a drop of spirits of wine, or ether, 
 fall upon the infusoria, and observe the effect. As the 
 evaporation proceeds, the shells of the creatures will be 
 lifted up, presenting to the eye alternately their edges 
 and sides, whilst they are kept in motion. When all 
 the spirit has evaporated, excepting so much as is con- 
 tained within the shells, they will lie on their sides, and 
 the fluid within them will flow towards the opening. 
 Thus a correct idea is obtained, not only of their external 
 but also of their internal forms. 
 
 The evaporation will be retarded by the addition of a 
 little water to the spirits of wine, and accelerated by the 
 use of ether. 
 
 SECT. 20. On viewing the Currents in Fluids during their 
 Evaporation.}? conducting this experiment, the fol- 
 lowing method is to be pursued. Place the microscope 
 in a vertical position ; select shallow or medium-power 
 object-glasses, which with the body shall magnify from 
 about 40 to 100 diameters; or single lenses from a 
 quarter to the tenth of an inch focus. Take an aquatic 
 live-box of moderate size, and place upon the centre of 
 it a drop of the oil of turpentine, or spirits of wine, &c. 
 Then cover it with the thin glass cover, taking care that 
 the thickness of the fluid be not more than about I -15th 
 of an inch. 
 
 If this fluid be quite pure, the motion of its particles 
 will not be perceptible : but Mr. Varley, who introduced 
 the experiment to the Society of Arts, grinds a small 
 
144 MICROSCOPIC ILLUSTRATIONS. 
 
 portion of coal into it, which occasions the currents pro- 
 duced by the evaporation to be distinctly seen. The 
 specific gravity of coal not being great, these minute 
 portions are held in suspension during the whole expe- 
 riment. The following observations on this subject are 
 given in the Society's Transactions, vol. 50 : 
 
 1. A drop of spirit of wine, or of naphtha, exhibits two, 
 three, or four vortices, or centres of circulation, accord- 
 ing to the size of the drop ; and if these vortices are 
 viewed laterally, the lines of particles will be seen 
 forming oblique curves from top to bottom of the drop. 
 
 2. Oil of turpentine shews a rapid circulation in two 
 continuous spirals, one to the right, the other to the 
 left, around the drop. These meet in the opposite 
 diameter, from which the particles are slowly carried 
 across the diameter to the place of starting, and this 
 continues while there is fluid enough to let it be seen. 
 
 3. If, however, the drop does not exceed one-tenth of 
 an inch in diameter, it presents the appearance of par- 
 ticles continually rising up in the middle, and radiating 
 in gentle curves to the circumference. 
 
 4. If the liquid be put into a very small phial, similar 
 motions are perceived, the particles, when they have 
 reached the side of the phial, going down to rise up after- 
 wards in the centre or axis. 
 
 5. If a bubble of air be inclosed in the liquid, motions, 
 similar to those described in No. 2, are observed in the 
 part immediately in contact with the bubble. 
 
DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 145 
 
 6. In a flat drop of new wine laid on the tablet or disc 
 of the aquatic live box, but not compressed by the cover, 
 the motion was a regular uniform circulation, the par- 
 ticles rising from below at one end of the drop, then 
 passing straight across on the surface, and descending at 
 the other end. 
 
140 
 
 CHAPTER VII. 
 
 Practical Remarks on Microscopes for viewing and drawing 
 Aquatic Larva, fyc. 
 
 BY C. R. GORING, M. D. 
 
 THE instrument which I used in executing the drawings 
 was a single microscope mounted with three achromatic 
 lenses of the 9-10th, 6-lOth, and 2-10th of an inch sidereal 
 focus. Had I not possessed these, I should probably 
 have used sapphires, or even equivalent common ones ; 
 for I greatly prefer a single microscope as a drawing tool, 
 from its being so very handy and manageable, and takin g 
 up so little room on the table. All the necessary mo- 
 tions were given to the optical part, and riot to the stage, 
 in order that living objects should be disturbed as little 
 as possible. I always found it make a great difference 
 for the better when they were allowed to remain un- 
 moved, for their natural restlessness, when exasperated 
 by motion, renders them completely intractable ; when 
 undisturbed, they will sometimes remain quiet for half a 
 minute together. An aquatic live-box*, of a little larger 
 
 See Plate, fig. 21. 
 
DRAWING WITH THE MICROSCOPE. 147 
 
 diameter than the length of the insect to be drawn, 
 answers best to confine them : they may be kept quite 
 closed up, for they do not appear to me to require air to 
 support their existence* I was in the habit also of observing 
 them with a compound aplanatic microscope, having a 
 boot to slide over the objective end, so that it could be 
 introduced into the large transparent vessel filled with 
 water, which was their usual abode, and in which they 
 would remain pretty quiescent, especially when well 
 supplied with food. I must remark that Thames water 
 is utterly poisonous to nearly the whole race of aquatic 
 insects. I presume it is not necessary to say that every 
 species of camera lucida, and all the contrivances which 
 may be used for tracing inanimate objects, are altogether 
 useless in drawing living ones. The colours of every 
 object are given as seen with the 9-10th focus lens, having 
 its full aperture, as they appear in mere day-light, with- 
 out any artificial illumination whatever. I utterly dis- 
 approve of lamp-light for larvae \ it gives, indeed, a strong 
 outline, but confuses and more than half obliterates the 
 viscera, while it makes the colours very dingy and dull. 
 The tints of these objects become fainter and fainter, and 
 the shades darker and darker, as the magnifying power 
 is increased. I did not find it necessary to employ any 
 power beyond that of the simple 2-10th focus, in order 
 to develop the whole of their structure : superior powers 
 seem to me merely to magnify, without shewing any 
 thing about them (except the circulation) more satisfac- 
 torily, and do not take in a sufficient portion of the object 
 
148 MICROSCOPIC ILLUSTRATIONS. 
 
 for a pleasing view. When we want to examine these 
 objects merely for the purpose of amusement, it will be 
 advisable to use compound instruments, on account of 
 their large field of view, luxurious accommodations, and 
 arrangements : their powers must be made equivalent to 
 the single lenses I employed, namely, one inch, half an 
 inch, and a quarter of an inch focus, which require their 
 objectives to be about four, two, and one inch focus re- 
 spectively, (as the weakest compound body which can 
 be applied usually quadruples the power of the object- 
 glass.) For the reasons stated above, all the requisite 
 motions and adjustments must here also be given to the 
 optical part, and not to the stage. It will also be found 
 a very great convenience if the stage is so constructed as 
 to admit of being altogether removed, so that any large 
 body, such as a square vase on a large flat slider, con- 
 taining the objects, can be substituted in its place, in 
 order that there shall be no necessity to remove the 
 insects from their usual places of abode in the said 
 vessels, &c. as they are very apt to receive injury and 
 become restless by such removal. In order to effect this 
 very desirable arrangement successfully, it will be neces- 
 sary that the bar of the microscope, instead of working 
 upon the stand with a cradle joint , should possess a ca- 
 pability of turning round by moving in a ball and socket, 
 or some other similar contrivance, so that the space 
 occupied by the stage may fold down on one side, and 
 allow of the introduction of the jar, &c. into it on its ap- 
 propriate stand; when, all the necessary adjustments 
 
DRAWING WITH THE MICROSCOPE. 149 
 
 being possessed by the optical part, the instrument is 
 just as perfect and manageable without its stage as with it, 
 and may be used upon any large body whatever, which will 
 be found a great convenience. This construction also 
 gives great facilities for demonstrating all sorts of test 
 objects, as well as for verifying and proving the nature of 
 bodies inspected (a subject which is as yet understood 
 by few observers.) 
 
 With respect to the exhibition of larvae, &c. in the 
 solar microscope, it may be observed generally, that what- 
 ever object-glass shews them well in a compound aplanatic, 
 with the assistance of the body and eye-glasses, will shew 
 them unassisted in the solar instrument, because the dis- 
 tance to which the rays are suffered to diverge does the 
 work of the compound body in giving the necessary 
 amplification, which it is the peculiar property of this 
 instrument to effect to a vast extent, without altering the 
 size of the field of view. A solar microscope may in fact 
 be defined iu its optical principle as a mere object-glass, 
 forming an image on a skreen, instead of the space in- 
 cluded in the field bar of a compound body. 
 
 C. R. G. 
 
150 
 
 CHAPTER VIII. 
 
 Whether there is a best possible way of constructing the 
 stand, or mounting, fyc. of Microscopes (the specific pur- 
 pose or purposes to which they are to be applied being first 
 determined) ? 
 
 BY C. R. GORING, M. D. 
 
 I APPREHEND that the construction of all kinds of me- 
 chanical implements, tools, utensils, musical, philo- 
 sophical and mathematical instruments, &c. may be 
 reduced to fixed principles, and that one best possible 
 way of making them, may, and can be discovered, (when 
 the specific and particular end and object of their fabri- 
 cation is duly settled). It is otherwise with all those 
 manufactures which are of an ornamental nature, and 
 therefore subject to the caprice of taste and opinion ; and 
 also with every thing made for the gratification of any 
 of our senses, or for the ease and accommodation of our 
 personal wants. Thus it would be very absurd to pre- 
 tend that there is a best possible system of cooking, or 
 making caps and bonnets, unless we choose to assume 
 the position, that the specific end of cooking and mantua- 
 making being the gratification of the whims and fancies 
 of some particular individual, even these arts are reducible 
 
ON CONSTRUCTING STANDS OF MICROSCOPES. 151 
 
 to fixed principles (such as they are). Every hing 
 connected with tjie fine arts always resolves itself into 
 a matter of opinion, about which it is perfectly useless to 
 contend, because every man considers his own private 
 judgment, however outre or singular, as of equal value 
 at least with that of his neighbour's. The standard of 
 taste is, then, a mere chimera, because it differs like 
 the face, or form of the limbs, in every particular spe- 
 cimen of the human race ; unless, indeed, it might be 
 ascertained by taking the average of that of the whole 
 species. 
 
 However, mankind seem to have come to a common 
 decision concerning the utility, excellence, and supe- 
 riority of many things, and to consider them as incapable, 
 or nearly so, of any real improvement. This is the case 
 with regard to many points, even determined by the 
 judgment of the senses; but more so with regard to 
 hose whose value is supposed to be ascertained by the 
 test of experience. Thus a variety of mechanical tools, 
 &c. may be considered as erected into standards, and not 
 at all likely to undergo modification in future. Even 
 musical instruments, as regards the quality of their tone 
 and sound, seem to have arrived at a regular and deter- 
 mined mode of construction. 
 
 Now, if so many other things have been perfected and 
 erected into standards, by the common consent of the 
 human race, why should not microscopes also ? Why 
 should we not bend the whole force and power of our 
 invention and reflection towards the fabrication of every 
 
152 MICROSCOPIC ILLUSTRATIONS. 
 
 thing connected with them, till every point is effected in 
 the best possible manner ? These instruments have under- 
 gone a complete revolution in their optical parts, and 
 become truly dignified and respectable : assuredly their 
 mechanical structure should correspond with the advance 
 which has taken place in their optical constitution. 
 
 However, like other things, they are varied in their 
 construction to serve particular purposes. Thus, if made 
 merely for commerce, or to look at, or to wear in the 
 waistcoat pocket, like a snuff-box, they must evidently 
 be made on a plan quite different from that required for 
 scientific observations. 
 
 I pretend not to meddle with matters concerning com- 
 merce; the trade understand them far better than I do, 
 and shall therefore merely observe, that several indivi- 
 duals experienced in business have assured me, that the 
 first thing to be considered in the construction of a mi- 
 croscope is its price ; and the second, the size of its case, 
 and how it is to look when packed in it, with all its little 
 eye-traps and trinkets about it. It is frequently sold in 
 two minutes, long before the merits of its construction 
 can be known; therefore one kind is just as good as 
 another, the public at large being no judges or con- 
 noisseurs in such matters. 
 
 Again, instruments may be made expressly for trans- 
 parent objects, or opaque ones, or even some particular 
 class of those bodies, for lamp-light only, or for that of 
 the atmosphere, for drawing, for public exhibition, &c. 
 all of which circumstances will produce a variation in 
 
ON CONSTRUCTING STANDS OF MICROSCOPES. 153 
 
 their fabric, while they are correctly adapted to their in- 
 tended use, and, therefore, perfect in their kind. The 
 construction which I wish to discover myself, is that of 
 an approximation to a standard for general purposes ; and 
 I think the principles at least of such a structure may be 
 ascertained, though the best possible way of carrying them 
 into effect, perhaps, cannot at present at least by myself. 
 In the first chapter* I have already sketched an outline 
 of the kind of instrument which I here propose now to 
 fill up and reduce to a determinate form, but shall first 
 detail a few of the obstacles which all who attempt to 
 reform the construction of microscopes may be expected 
 to encounter. First, there is the inveterate hatred and 
 contempt of all innovation whatever, which seems to 
 form a part of our nature, when arrived at a certain 
 period of life. Men long used to some particular way of 
 doing any thing, cannot endure the idea of going to 
 school again, and learning to act upon a new system. 
 The very idea of the possibility of being taught any thing, 
 galls and wounds their self-love to the quick : if this 
 concerns a matter relative to their particular profession, 
 or something, perhaps, on which they peculiarly pique 
 themselves : they become altogether indignant and 
 furious. If by any possibility they are brought to recog- 
 nise the necessity of some change, they must make it 
 themselves ; what a disgraceful and lamentable poverty 
 
 * Chapter VII. in this edition. 
 
154 MICROSCOPIC ILLUSTRATIONS. 
 
 of genius would it not shew, to copy a good thing which 
 had been invented by another man ! 
 
 It cannot be denied, moreover, that there really is a great 
 deal of trouble in getting up new philosophical instru- 
 ments of all sorts. There are fresh patterns to be made, 
 and tried, and re-made ; workmen to instruct afresh, and 
 after all, particular parts of the instrument may not fulfil 
 the intended purpose, and must be remodelled; all of 
 which circumstances occasion a great deal of annoyance 
 and expense, and naturally render men averse from the 
 adoption of new constructions 3 more especially as the 
 public at large never can be imbued with a sense of the 
 difficulties, expense, and loss of time, occasioned by 
 what are called out of the way jobs, and consequently 
 can never be induced to pay for them so as to remunerate 
 the artists employed. 
 
 If it is indeed true, that conceit is given to man to 
 console him for want of talent, and that the less of one 
 of these qualities he may possess, the more he is likely 
 to have of the other, what must be the consequence, 
 should it happen that an individual, possessing seventy- 
 five parts of conceit and twenty-five of talent in the 
 hundred, comes in contact with something invented, or 
 recommended, by a man who has but twenty- five of the 
 former quality to seventy-five of the latter ? Is it likely 
 he will adopt it ? Never. He will imagine that he can 
 produce something infinitely better, and will be still 
 more strenuous in this opinion if he happens not to 
 
ON CONSTRUCTING STANDS OF MICROSCOPES. 155 
 
 possess a grain of experience on the subject under deli- 
 beration. 
 
 All these considerations have had so much weight 
 with me, that I have more than once determined never 
 to meddle with the stands and apparatus of microscopes 
 at all, but leave men to settle them according to their 
 own fancies ; but my friends have told me, that I have 
 as good a right to consider an ounce of my own wit equal 
 to a ton of my neighbour's as other men have, and 
 that I ought to make the experiment, of giving a con- 
 struction of my own, whether it may be adopted or not. 
 I shall therefore state my own views on the subject, and 
 for every particular structure recommended, shall give 
 a reason which every man may, of course, either admit 
 or disprove, together with the construction deduced 
 from it. First, then, I say that the stage should be mo- 
 tionless, and that the optical part only should possess the 
 requisite power of traversing and adjustment ; because 
 living objects are much more quiet when allowed to 
 remain at rest, and therefore more easily observed : if 
 some one who has been less plagued by them than I 
 have, chooses to say that it makes no difference whether 
 they are moved about or not, I will give up the point ; 
 but still insist on the propriety of making the instrument 
 perform all its adjustments as perfectly without the stage as 
 with it, for then we may remove the stage altogether if 
 we please, and substitute any thing we think proper in 
 its place, and all will go on as before. This arrange- 
 
156 MICROSCOPIC ILLUSTRATIONS. 
 
 ment gives us the power of examining small parts of 
 large bodies without detaching them from their wholes. 
 
 Thus we can examine the contents of a vase of polypi, 
 or aquatic insects, or a small part of a large specimen of 
 a mineral, or a nosegay of flowers, with the insects 
 which inhabit them ; or with an erecting eye-piece we 
 may apply the instrument to a turning-lathe or some 
 piece of delicate machinery, and work upon it much 
 more comfortably than by the help of single lenses. But 
 for these purposes the space occupied by the stage must be 
 left free and open; and to obtain this accommodation the 
 bar of the microscope must swing round on its stand in 
 any direction, on a ball and socket joint, which con- 
 joined with a rotatory motion of the arm on the top of 
 the said bar, will almost always enable us to gain the po- 
 sition required. An instrument which does not possess 
 all these properties will only do half the work it ought to 
 perform. It must be evident, also, that this construc- 
 tion gives the utmost facility for introducing a lamp or 
 candle, either close behind the stage for transparent 
 bodies, or before it for opaque ones ; and this arrange- 
 ment is a point of the last moment for demonstrating 
 and verifying a variety of objects, particularly proof 
 opaque ones, as I shall shew in due season, when I come 
 to treat on them and on the subject of verification. No 
 microscope which is from any cause lame or impotent, 
 either in its optical or mechanical construction, can be 
 fit for such purposes. 
 
ON CONSTRUCTING STANDS OF MICROSCOPES. 15J 
 
 The stage must also possess a capability of turning 
 round on a pivot, so that by tilting the microscope we 
 may be enabled to view the side or elevation of a body 
 as well as its plan ; for example, the curve of a small 
 lens, when laid flat on its face, the edge of a razor, the 
 point of a needle, &c. without using any particular ap- 
 paratus to preserve such bodies in the position in which 
 we wish to see them. The use of such a mode of ob- 
 serving, in cases of dissection, must be manifest ; for in 
 this way the side of an insect, &c. may be viewed and 
 worked upon without disturbing the parts laid open, in 
 the horizontal position, and thereby occasioning much 
 extra trouble and confusion. By turning the subject 
 round on the stage, every elevation or side view of it may, 
 of course, be successively obtained. 
 
 There is yet another advantage to be gained by a rota- 
 tory motion in the stage; for if we employ a diagonal 
 objective, and tilt both the stage and the bar into a hori- 
 zontal position, so that the body of the microscope shall 
 be under the stage, we can examine crystallizations of 
 salts without being annoyed by the steam which arises 
 from them during consolidation, and which is almost 
 certain to condense on the object glass when used in any 
 other position. In the present case this is impossible; 
 for nothing but the under surface of the glass on which 
 the salt is placed, is presented towards the object glass. 
 All sorts of fluids, and the bodies contained in them, may 
 in the same way be managed without being troublesome, 
 
158 MICROSCOPIC ILLUSTRATIONS. 
 
 and thus chemical actions of all sorts may be studied under 
 a magnifying power. 
 
 Moreover, the stage must be enabled to shift down to 
 the end of its bar, so that we may use object-glasses of 
 long foci, arid also place any large bulky body upon it 
 without building up the whole instrument to an extrava- 
 gant height, and thereby rendering it top-heavy and 
 rickety. It must also be of such strength as not to spring 
 under the weight of our hands when used for dissection ; 
 for if it does, it will be utterly impossible to preserve a 
 steady focus, which will be found a dreadful nuisance 
 with a high power, and indeed with any power. Its size 
 must also be such as to afford a sufficient support for the 
 hands. The distance of the hole in which the slider 
 holder is fixed from the bar, must also be such as to allow 
 a slider to turn completely round, otherwise we cannot 
 get test objects into that position relative to the light in 
 which only they can be demonstrated. (This is especially 
 the case with the markings on the scales of beetles and 
 butterflies.) 
 
 I do not much approve of attaching illuminators to the 
 stage, either for opaque or diaphanous objects. It will 
 be found much more commodious to cause the spectrum 
 for opaque objects to travel along with the body. Accord- 
 ingly, a condenser should be attached to it. Both the 
 mirror and condensing lens under the stage should be 
 caused to swing round by means of a spring socket and 
 pinching screw working on a round bar, so that they 
 
ON CONSTRUCTING STANDS OF MICROSCOPES. 159 
 
 may be instantly removed out of the way when not 
 wanted, without being detached from the bar. 
 
 The large illuminating mirror should (if not of very 
 considerable dimensions) be made oval, and should work 
 upon a swivel joint at the bottom instead of the side, 
 otherwise it will be rendered unserviceable, for the long 
 axis of the ellipse cannot otherwise be brought into the 
 position necessary to give a round spectrum when placed 
 at the requisite angle for illumination, say 45 degrees. 
 The reason for the ellipticity of the mirror is this : An 
 image of it is constantly formed in the optic or visual 
 pencil, at the eye-piece, as may be seen with a magnifier 
 whenever bodies are viewed by the intercepted or trans- 
 mitted light which it furnishes. Now unless this image 
 is of equal size with the said visual pencil, an effect is 
 produced not very dissimilar to that of mutilating the 
 pencil in some other way, and exactly the same as to the 
 quantity of light lost. Now a round mirror (unless it is 
 of such size as to allow for the cutting off occasioned by 
 its angular position,) gives an elliptic pencil. But I 
 presume it is always advisable not to increase the bulk 
 of any part of an optical instrument when it can be 
 avoided ; consequently the true form for the mirror is an 
 ellipse. This form does not easily admit of the use of a 
 concave reflector; but I disapprove of them, as they 
 always seem to me to occasion a certain indistinctness in 
 vision by intercepted light, and if wanted only for illu- 
 minating opaque objects, by the help of silver cups, may 
 be dispensed with 5 for a much better action is produced 
 
160 MICROSCOPIC ILLUSTRATIONS. 
 
 by a plane mirror, co-operating with a condensing lens. 
 Because an angular plane mirror and a condensing glass 
 produce a round spectrum ; whereas a concave mirror, 
 when tilted, always gives an oblong one ; however, in 
 the present construction, I always suppose that when 
 silver cups are employed, the condenser is to act by itself 
 without any mirror at all, by which arrangement the 
 maximum of illumination is given. 
 
 The size of the mirror is determined by its distance 
 from the object-glass, or single magnifier used, and by 
 the angle of aperture possessed by them. The longer 
 the bar, and the greater the aperture of the optical part, 
 the larger must be its dimensions to fill up the visual 
 pencil. 
 
 The reverse of the mirror will be best occupied by a 
 surface of plaster of Paris, to reflect the light of the sun, 
 which will be found of great use in ascertaining the true 
 tints of transparent objects, the colours of which are 
 shewn by this sort of light with the utmost truth and 
 delicacy, far better than when brought out by a piece of 
 ground glass interposed between them and the polished 
 mirror. The double action of the mirror will be com- 
 pletely commanded by the most inexperienced observer, 
 if a milled head is attached to its transverse axis, as will 
 be shewn in its proper place. 
 
 Now these are, I think, the main and capital points to 
 be attended to in the construction of a stand. As, how- 
 ever, every question is said to have two handles to it, I 
 shall endeavour to discuss the merits of some opposite 
 
ON CONSTRUCTING STANDS OF MICROSCOPES. 161 
 
 constructions. First, then, it has been said, that the 
 double motion of a ball and socket joint is of a most un- 
 manageable nature, and that a cradle joint is a true, 
 regular, and far superior movement : this I admit, and 
 should say, that if a telescope had, by means of a ball 
 and socket, to follow the motion of a star, no motion 
 could be more unappropriate and detestable ; but in a 
 microscope it happens always to be a fixture, and when 
 clamped tight by means of a pinching-screw, is as steady 
 as a rock. 
 
 This observation may appear very unnecessary to my 
 readers in general ; but I have met with individuals of 
 such obtuse understandings, that they cannot be brought 
 to comprehend that a telescope performs a part of its 
 motion by means of its cradle joint, whereas that of a 
 microscope is the result of a traversing motion in its arm, 
 combined with a rotatory one on the top of the bar, and 
 therefore has nothing to do with that on the head of the 
 stand, be it what it may, which is always a pro tempore 
 fixture. 
 
 Again, it has been said, that by giving the motions and 
 adjustments to the optical part, it is rendered much less 
 steady than it might be under other circumstances ; that 
 it is like mounting a telescope on a mop-stick : that 
 immobility is of most virtual importance for observa- 
 tion, &c. I answer, that by making the work sufficiently 
 strong and solid, and the optical part no larger or longer 
 than is necessary, the instrument is rendered abundantly 
 steady, and free from tremor even with its highest powers. 
 
 M 
 
162 MICROSCOPIC ILLUSTRATIONS. 
 
 At the same time I willingly admit, that if the body is 
 made very long or large it will be almost impossible to 
 combine the stability requisite for practical purposes 
 with a capability of traversing and adjustment. 
 
 It has been asserted, that if after we have adjusted our 
 light for viewing a transparent object satisfactorily, we 
 throw the axis of the optical part out of the position 
 which we first selected, we must begin our work of illu- 
 mination over again, or lose the effect of the original 
 one. This is true beyond certain limits, but the body of a 
 microscope may be moved very considerably, without in 
 the least affecting the vision of ordinary transparent 
 objects, and to opaque bodies, of course, the observation 
 cannot apply. Further, it is said that the employment 
 of cones and diaphragms below the stage is of vast use 
 in developing the nature of many transparent bodies, 
 and in tempering the light to the shade to produce a 
 maximum of distinctness ; and that if these are employed, 
 the most perverse and prejudiced antagonist cannot re- 
 fuse to admit that any diversion of the optical part from 
 the line of adjustment must utterly destroy, not only 
 distinctness, but nearly all vision whatever. 
 
 To obviate these objections let there be a large roomy 
 slider-holder, with a good steel spring, so as to pinch the 
 plates together pretty tight ; the object can then be moved 
 about by the hands in any direction, and the body may re- 
 main a fixture, save as to the adjustment of the focus. 
 I have always found that people like to move an object 
 about with their hands, even when they have screw mo- 
 
ON CONSTRUCTING^ STANDS OF MICROSCOPES. 163 
 
 tions for traversing, and indeed I prefer it myself, (pro- 
 vided the object does not yield too easily,) even with very 
 high powers. 
 
 Lastly, it has been remarked, with great justice, that 
 if we choose to employ any of the generation of camera 
 lucidas for drawing objects, they can only be used with 
 microscopes in which the body is altogether immoveable, 
 Nothing is more certain. At the same time I cannot re- 
 frain from expressing my hatred and contempt of the 
 whole tribe, as rather impediments than helps to a real 
 draughtsman. This much I will venture to say, that if 
 a man cannot draw without them, he cannot with them. 
 
 The slider-holder enables the present construction to 
 operate with these nuisances, for the adjustment of the 
 focus will not sensibly disturb that of the image on the 
 paper, and if it should, a very little motion, backwards 
 or forwards, given to the drawing, will re-arrange it. 
 The only aid which I can with confidence recommend*, 
 as of real utility, is a micrometer, composed of glass, 
 placed in the field bar of the microscope, and divided 
 into about 40 or 50 parts to the inch, with double divi- 
 sions, or very strong lines at every fifth space. These 
 should be rubbed over with black lead, after which they 
 must be burnished with the edge of a piece of hard 
 wood, so as to clean the surface of the glass without rubbing 
 the black lead out of the divisions, (a device of the late 
 Dr. Wollaston, and bearing the stamp of his original and 
 
 * See Micrographia, p. 221. 
 
164 MICROSCOPIC ILLUSTRATIONS. 
 
 penetrating genius) ; then let another piece of thin plane 
 glass be smeared with Canada balsam, and cemented 
 upon the divisions, which will preserve them for ever. 
 Now if corresponding black lines, of any scale required, 
 are placed under the paper on which we draw, we can 
 with great facility put in our outline in the most correct 
 proportions. Or if the paper is too opaque, a frame 
 with black silk threads stretched upon it may be laid 
 upon it till the sketch is completed. The contrivance 
 can be used with my microscope as well as with any 
 other. 
 
 Mr. Lister has constructed the stand of a microscope 
 with a particular view to the use of the camera lucida : 
 accordingly his body is motionless and of considerable 
 length, so as to clear a large space on the table for the 
 field of view. The ocular end is, moreover, firmly lashed 
 to the legs of the stand, (which is very solid,) by means 
 of steadying rods, forming altogether the best construc- 
 tion for the use of long heavy bodies, and camerse lu- 
 cidae, which I have seen. 
 
 It must be most evident that a single microscope re- 
 quires exactly the same powers, properties, and capa- 
 bilities as a compound one. I should, therefore, construct 
 its stand and apparatus precisely in the same manner as 
 that of a compound one, only it may be made on a much 
 smaller scale : as little strength is required for carrying 
 single magnifiers, triangular drawn tubes may be used 
 instead of the triangular gun-metal bar of the compound 
 (to be hereafter described). The drawing of triangular 
 
ON CONSTRUCTING STANDS OF MICROSCOPES. 165 
 
 tubes is a novelty produced by Mr. Pritchard expressly 
 for his diamond and sapphire instruments. 
 
 I therefore conclude that the principles, at least, of the 
 best possible construction for the mechanical part of 
 microscopes, may be defined. I have attempted to re- 
 duce them to practice in an instrument the patterns of 
 which 1 have executed with my own hands, and Mr. 
 Pritchard has made a mechanical drawing of it, which 
 he will describe in the next chapter, under the title of 
 my Operative Aplanatic Engiscope. Before, however, the 
 description proceeds, I must beg a boon of my readers 
 and the microscopic world at large, viz. to permit me 
 to supplant the term Compound Microscope by the word 
 EXGISCOPE, which seems to me more apposite. 
 
 It is derived from two Greek words, lyyCc, near, and 
 ffKovlw, to view, and therefore well expresses an instru- 
 ment for viewing close objects, or for viewing objects 
 closely, and is in strong contradistinction to the term 
 telescope, derived, as my readers well know, from reXoc, 
 an end or distant limit, and vKoirlta, and therefore denoting 
 an instrument for viewing distant objects. The term 
 microscope, derived from /m'poc, small, and O-KOTTCW, sig- 
 nifies an instrument for examining small objects, which 
 is perfectly correct also, but in bad contradistinction to 
 the telescope, which would have to be named megala- 
 scope, to oppose it (from ptyas, great, and o-jcoTn-'w,) which 
 expression would be perfectly accurate when applied to 
 the telescope, for it views very large objects, such as the 
 totality of a planet or the sun, &c. Now a compound 
 
166 MICROSCOPIC ILLUSTRATIONS. 
 
 microscope and a telescope pass into each other hy in- 
 sensible degrees : a species of telescope can always be 
 made of the former, and a kind of microscope of the 
 latter, but both of the most execrable kind. To this last, 
 namely, a short spy-glass, or perspective glass, with a 
 long pull-out tube, so as to permit it to adapt itself to 
 rays which are considerably divergent, the term Engi- 
 scope has already been-applied by Martin and others, but 
 I think most unwarrantably, for such an execrable piece 
 of trumpery is wholly unworthy of a separate existence, 
 or a separate name ; it is just as easy to make a piece of 
 velvet of an ass's hide as to make an object-glass which 
 shall answer both for a telescope and a compound mi- 
 croscope, be its focus what it may. The only veritable 
 engiscope is the latter instrument, with an objective- glass 
 truly adapted to act with divergent rays ; a short tele- 
 scope may be made of six inches focus, and a long com- 
 pound microscope with an object-glass also of six inches 
 focus, but the construction of each must be utterly 
 different, or they will only nominally be what they are 
 called. 
 
 I therefore hope that I shall be allowed (as I have 
 reformed and revolutionized both the optical and mechanical 
 structure of microscopes) to change their names also, and 
 shall therefore take the liberty of using the term engi- 
 scope, as aforesaid, instead of compound microscope, 
 and retaining the expressions microscope or simple mi- 
 croscope, for convenience sake, to denominate what is now 
 called a single microscope, in opposition to a compound. 
 
ON CONSTRUCTING STANDS OF MICROSCOPES. 16J 
 
 The term compound microscope will then be applied 
 appositely to all those instruments with which we view 
 real objects instead of images of them, constructed by com- 
 bining two or more lenses together; for the optical part of 
 what is commonly called a single microscope may be made of 
 as many as four glasses, whereas the optical part of a com- 
 pound (as it is usually termed) does not necessarily consist 
 of more than two, viz. an object and ocular glass. Thus 
 if we retain the old terms, how are we to define accu- 
 rately a magnifier composed of two double aplanatics ? 
 Are we to christen it a single quadruple, or double 
 doublet ? or a simple compound ? This will be as ludi- 
 crous as Mr. Callaghan O'Brallaghan's definition of the 
 first bit of bread he ate in England, which it seems was 
 a roasted potato, cooked in a brass saucepan. 
 
 It is presumed, therefore, with due deference to the 
 opinions of better men, that the term engiscope is lawful 
 and orthodox, and will, probably, be adopted by posterity, 
 if not by the present generation : as to the fuss which 
 has been made about my using an i instead of a y, in the 
 conversion of the Greek upsilon (both letters having 
 the same sound, and having been used indifferently by 
 ancient English writers), I beg to state that I did so on 
 purpose, to make a tub to amuse the whale with, and to 
 draw the attention of the learned to the term. 
 
 C. R. G. 
 
168 
 
 CHAPTER IX. 
 
 Description of Dr. Goring' s Operative Aplanatic* 
 Engiscope. 
 
 IT is necessary to premise that figures 7, 8, 10, 11, 12, 
 14, ]J, 18, 19, 20, and letter A, are drawn one-quarter of 
 the real size ; all the rest are one-sixth^ except figure 23, 
 which is of the true size. 
 
 Figure 7 represents a geometrical elevation of the in- 
 strument, just removed from its case, the body, a, being 
 screwed on, and the triangular bar, b } somewhat ele- 
 vated above the stage, which is fixed in situ. To begin 
 from the foundation, the legs are all of the same thick- 
 ness, but tapered in breadth towards their extremities, 
 in order to obtain strength where most required. In 
 the upper one is a hole to receive the projecting pin at 
 the end of the circular bar, c. Underneath the pillar, d, 
 is a screw nut, e, which lets down, to obviate the spring- 
 ing of the legs. The pillar, d, is constructed of two 
 pieces of very stout tube, moving stiffly in each other. 
 At the head of this pillar is a socket,/, to receive the ball 
 shewn in figure 8. This universal joint is made in the 
 
 * Aplanatic, derived from the priv. a, and -it\uvn, error, means free from 
 both kinds of aberration, or devoid of all errors. 
 
DESCRIPTION OF DR. CORING** ENGISCOPE. 169 
 
 customary manner, but the socket has a slit for the neck 
 of the ball, only through one-quarter of a circle, to avoid 
 weakening it unnecessarily. It is pinched by a screw, ^, 
 with a vice-handle. The ball, the socket of the circular 
 bar, h, together with the stage-pin, i, are all cast in one 
 piece, as shewn in plan, fig. 8. The stage, j, has a cir- 
 cular aperture, which has a sink and two nicks in it, 
 together with a square hole, k, (as seen in fig. 9). There 
 is likewise an arrow pointing to a nick in the socket, 
 h, to shew when it is truly at right angles to the bar, b. 
 It fixes on to the pin, i, which is clamped tight by a 
 screw, / : into the bottom of this screw is inserted a 
 piece of metal, the end of which is worked to correspond 
 to the curve of the pin, i 9 which causes it to clamp tight 
 with a very slight effort, and preserves the pin from iii- 
 jury. Within the socket, h, is soldered a stout tube, 
 forming the external bar, c. On this circular bar slide 
 the split sockets, m, m, m, carrying the condensing lens, n, 
 the inferior pin, i, for holding the stage, and the oval 
 mirror, o : all travel up and down at pleasure, and may 
 either be moved out of the way, by turning them one- 
 quarter round, or stripped off altogether, if thought more 
 convenient. Their clamping screws fix them tight in 
 any required position. A nick is made in front of each 
 of the sockets, and a line drawn on the front of the bar, 
 to shew when the condenser and mirror, &c. are truly in 
 the axis of the aperture of the stage. The oval mirror is 
 plane, but its reverse is formed of plaster of Paris. It re- 
 volves on a vertical axis, p, and on a horizontal one, q y 
 
170 MICROSCOPIC ILLUSTRATIONS. 
 
 the motions of both of which are governed by the milled 
 head, r. The triangular gun-metal bar, b, is truncated at 
 its edges, and is therefore strictly speaking a hexagon, 
 this form giving greater strength in proportion to the space 
 occupied by it than a perfect triangle would, while it still 
 possesses the property of being locked fast when pinched 
 on only one side : it is evident that even an equilateral 
 hexagon would, when pinched on one side, always cause 
 two others to bear close against a triangular containing 
 tube. This bar slides up and down in the external cir- 
 cular one, c, between two plates of metal, pierced with 
 triangular apertures to receive it : one is soldered in at 
 the summit of the socket, h, the other below the milled 
 head, s ; and springs are placed on the same side as the 
 rack, both above and below the pinion, to steady the 
 bar, and give the friction necessary to prevent it from 
 falling from its position by gravitation. If the present 
 stand was made to operate solely with an engiscopic refract- 
 ing body, it would be better to omit the rack- work of the 
 upright bar altogether, and substitute a pinching screw 
 for it, giving a fine movement to the body within the neck 
 of the arm, by a screw on Ramsden's principle. In 
 its anterior side is sunk the rack which adjusts the focus 
 by means of the milled head, s, which is repeated on the 
 opposite side. Rack-work is not perhaps the best sort 
 of movement which might be contrived, but it is good 
 enough, when well executed, for all practical purposes. 
 On the top of this triangular bar is fixed the gun-metal 
 wheel, t, within the teeth of which works an endless 
 
DESCRIPTION OF DR. GORINGs ENGISCOPE. 
 
 screw, the milled head of which, u, is seen in the draw- 
 ing. Above the wheel is shewn the triangular pinion 
 socket, v, which carries the arm, w, which is also of gun- 
 metal, truncated at its edges with a sunk rack on its 
 superior side, exactly on the principle of the other : this 
 bar is worked by a pinion, x, seen in the cross elevation 
 of this socket and bar, fig. 10. Diamond and sapphire 
 lenses, and all sorts of simple microscopes, are attached 
 to this arm, by being thrust into the hole at its other 
 extremity, as drawn in the engraving. 
 
 The superiority of sapphire lenses over those of glass 
 seems now universally recognised, and virtually, the su- 
 periority of diamonds over sapphires. (For whatever will 
 prove the former of these propositions will most assuredly 
 prove the latter also.) If these substances possessed no 
 other advantages over glass, save their invulnerability 
 and capability of being burnished into their brass settings, 
 these properties, coupled with their extreme thinness, the 
 natural result of their shallow curves, when properly 
 executed, (and which allows more room for the object 
 and for the illumination of it, if opaque, with any given 
 power and aperture,) and their superior magnifying 
 power with a given curve, also would fully justify the 
 patronage they have received*. 
 
 The pinion, x, fig. 10, has a projecting square at each 
 end, to receive the ivory milled lever, y, or milled head, 
 z, on either side. The whole of this socket revolves on 
 
 * See Microscopic Cabinet, chap. xiv. 
 
172 MICROSCOPIC ILLUSTRATIONS. 
 
 the upright triangular bar, b, and is kept in its place by 
 the gun- metal garter-piece, shewn at 'a, and has a saddle- 
 piece well screwed on, covering it over, to prevent the 
 loosening the pinion, by the action of the milled lever. 
 The endless screw may be detached from the wheel, t, 
 by slackening the pinching screw, b, which serves to keep 
 the pinion in gear, the said endless screw being affixed 
 to a separate piece, which turns round on the project- 
 ing and inferior part of the socket, by means of a pivot : 
 the pinching screw, 'b, operating in an arc, formed to 
 receive it. This wheel and endless screw are absolutely 
 necessary to enable us to command the motions of the 
 arm, when the axis of the body of the instrument is not 
 perpendicular, or nearly so. 
 
 The body, a, is constructed on the smallest scale on 
 which it can well be made without impairing its optical 
 properties. The only true way of preventing the genera- 
 tion of false light is to allow so much space in the body that 
 the rays proceeding from the object- glass shall not impinge 
 on any part of the tube ; for no sensible light is produced 
 until they flash against something. Into the superior part 
 of the eye-tube, 'c, are screwed the various inverting eye- 
 glasses, and into its inferior part, 'd, the erecting glasses, 
 A. Upon the neck of the object end, 'e, are screwed the 
 various object-glasses, one of which is represented attached 
 in the plate. Upon this neck also slides the illuminator, '/, 
 for opaque objects ; it has a split socket and clamping 
 screw, 'g, by which it is adjusted, and is composed of a 
 plain convex lens, having an inch and a half of aperture 
 
DESCRIPTION OF DR. GORINGs ENGISCOPE. 
 
 and the same focus operating by lamp-light, with another 
 isosceles con vex of same power and diameter screwed to it : 
 these curves have been found by experiment the best both 
 for refracting the maximum of light, and for preventing 
 any loss of it by reflection from the convex surfaces of 
 the lenses, 'h is the cap of one of the eye-pieces in dotted 
 lines ; it is supposed to be removed for the purpose of 
 applying the camera lucida, fig. 1 1, which is thrust on over 
 the eye-tube, a side view whereof is represented at fig. 12. 
 By far the best species of camera lucida is, I think, this 
 (which is the invention of Professor Amici), and is com- 
 posed of a simple piece of thick plate glass, with truly 
 parallel surfaces, placed at an angle of 45, and reflecting 
 an image while we see through it, for it allows the point of 
 the pencil, the drawing on the paper, and the image, to be 
 seen at the same identical moment, without any dodging 
 or effort. Care must be taken to have a sufficient thick- 
 ness of glass, to prevent the visual pencil from reach- 
 ing the inferior surface, otherwise there will be two 
 images. 
 
 Fig. 13 is a micrometer, to be placed in the field-bar 
 of one of the lowest eye-pieces. It is made of glass, with 
 divisions of 40 or 50 parts to the inch, which are filled 
 with black lead, well burnished in, and secured by 
 another piece of glass being cemented over them. 
 
 Figure 14 is a square bar, sliding up and down in the 
 square tube in the stage, k, fig. 9. It carries the illumi- 
 nator,/, when it is thought advisable not to load the body 
 with it. 
 
174 MICROSCOPIC ILLUSTRATIONS. 
 
 Fig. 15 represents the lengthening tube, 'i, with the 
 direct boot, 'k, drawn over it. The lengthening piece 
 screws on in place of the object-glass, which is again 
 attached to its extremity. This piece, from its narrow-- 
 ness, will generate false light; it must, therefore, be lined 
 with black velvet, which is, perhaps, the best substance 
 to stifle it when generated. 
 
 Figure 16 is the diagonal boot. It is constructed in the 
 same manner as the other, but has a plane metallic specu- 
 lum or rectangular prism of glass placed at 7, to reflect 
 the rays received at right angles. It is necessary that 
 the piece of plate-glass which is affixed to the extremity 
 of the cones of these boots should be very perfect, and 
 set in truly parallel. 
 
 SirD. Brewster first conceived the project of causing an 
 aplanatic objective to be so constructed that it should be 
 capable of acting in water instead of air (see his Treatise 
 on New Philosophical Instruments) . I am afraid it would 
 be very difficult to make a naked object-glass water-tight, 
 and at the same time to adjust it perfectly; and after all it 
 would only serve to act in water, and would be useless out 
 of it. The present arrangement really seems to answer 
 every practical purpose completely well, though cer- 
 tainly Sir David's plan is the more scientific way of doing 
 the thing. 
 
 Figure 17 is a cup or silver speculum, which slides 
 over the end of the object-glass. 
 
 Figure 18 is a shade to slide over the lengthening 
 piece, 'i, figures 15 and 16. 
 
DESCRIPTION OF DR. GORING's ENGISCOPE. 175 
 
 Figure 19 is a small cone, to slide over the object- 
 glass when acting by plain artificial light. 
 
 Figure 20 is the arm by which the optical part of the 
 AmicianCatadioptric Engiscope* is attached to the present 
 stand : it must be considerably longer than the other, 
 and, like it, racked to its extremity, to admit of its being 
 taken out of the socket. By means of its swivel and 
 pinching-screw, it allows the body to turn round, 
 and present itself either in front or at the sides of the 
 stage. 
 
 Figure 21 is the aquatic live-box, with an extra pierced 
 lid, 'm, for aerial insects and land objects, &c. There 
 should be about half a dozen of these, of different sizes, 
 varying from that given in the plate, to about 2-10ths of 
 an inch in diameter. It would be found a convenience 
 if the larger ones had watch-glasses fixed into them with 
 their curves applying to each other, and their concavi- 
 ties pointing outwards, instead of the plane glasses, as 
 given in the drawing ; for by this arrangement, when 
 used with very low powers for exhibiting a grand melee of 
 aquatic insects, &c., taking in nearly their whole area, 
 the edges of the field will be as distinct as its centre; 
 the foci of these two parts of the visible superficies being 
 always different with &flat object, and a low power, even 
 with the best aplanatics. 
 
 The glasses, whether flat or concave, must be well 
 cemented into their cells by painters' ground white lead 
 or opticians' cement, that the box may be completely 
 
 * MicrograpLia, chap. i. 
 
176 MICROSCOPIC ILLUSTRATIONS. 
 
 water-tight ; and the tubes themselves may be greased 
 for the same purpose if necessary. These boxes are as useful 
 pieces of microscopic apparatus as ever were invented. 
 
 Figure 22 represents the black ground box (an inven- 
 tion of Mr. Lister), which in the present instrument also 
 forms a stage for the single microscope, and for dis- 
 section. 
 
 At 'n is seen its lid, of which a side view is given at 'o ; 
 at 'p, its bottom with its side view also ; its interior 
 surface is lined with black silk velvet, and so is the 
 superior surface of rhe lid ; the sides of the cone, 'q, are 
 well blacked ; at 'r is seen a disk, also covered with black 
 silk velvet, which acts along with the silver specula. 
 Its arm revolves on the top of the cone, and can be 
 removed altogether, if necessary; at its extremity is a 
 hole, in which is inserted the cradle-joint, which carries 
 a brass needle; a small piece of brass tube, stuffed with 
 cork, is affixed to one of its extremities, and to the other 
 a pair of forceps, which are represented of their full size 
 at figure 23. These are constructed as usual, except 
 that a hole is drilled through them at 's, to enable them 
 forcibly to grasp the head of a common pin. 
 
 Figure 24 is the piece which carries the wheel of dia- 
 phragms ; 't is the plan of their under side. There should 
 be about five apertures, varying from half an inch to 
 l-10th in diameter. This apparatus screws on to any of 
 the aquatic boxes, also to the slider holder, figs. 25 and 
 26, and the false stage or black ground, figure 22, in 
 place of the bottom, 'p. It was invented by Monsieur 
 
DESCRIPTION OF DR. GORINGs RNGISCOPK. 
 
 Le Baillif, at Paris ; but we have long been in the habit 
 of using cones for the same purpose in England. 
 
 Figure 27 is a contractor, which drops into the sink 
 formed in the aperture of the stage, when too large for 
 carrying on dissections, or other operations of a similar 
 nature ; it has two bayonet-catches, which secure it by 
 being turned one quarter round : into the aperture of 
 this latter piece, a small slider-holder may be advan- 
 tageously fixed (to be used with the Amician Engiscope 
 or the simple microscope), which should likewise be 
 made to fix to the top of the false stage, figure 22, when 
 its lid is removed. 
 
 Figure 25 represents a plan of the four-pillar-slider- 
 holder, which is by far the best contrivance hitherto in- 
 vented, for attaching all sorts of objects in sliders and 
 between slips of glass, &c. to the stage : its elevation is 
 shewn at figure 26, with the two pins which secure it. 
 In the plan are drawn the two cross connecting bars, 
 'u y 'u, between which and the superior plate all large 
 opaque sliders, &c. are placed. There is another plate 
 below the first: between these two a common transpa- 
 rent slider is shewn. The helical spring is of steel of some 
 strength, so that the sliders shall not be permitted to 
 move about between the plates too easily. The holes at 
 the corners of the plates which slide up and down on the 
 pillars, must be sufficiently large to prevent any clinging 
 or sticking. 
 
 Figure 28 shows a plan and profile of an adapter, 
 which carries a small aquatic live box, to be used with 
 
 N 
 
MICROSCOPIC ILLUSTRATIONS. 
 
 the wheel of diaphragms ; it has a bayonet-catch to fix 
 the box securely, independent of its gravity. 
 
 Figure 29 is a frame and slip of glass for crystal- 
 lizations of salts. 
 
 Such is the description of an instrument which will be 
 found a regular working tool, either for amusement or 
 scientific investigation. It is not pretended (be it always 
 remembered) that it is a perfect or standard instrument, 
 but Dr. G, hopes it will be found an approximation to one. 
 
 A. P. 
 
 March 1829. 
 
 Most of the apparatus described in Chapter VI. as appli- 
 cable to my microscope can be applied to this. In the 
 instruments of this kind manufactured by me I have 
 made the socket at h to extend an inch or more above 
 the surface of the stage : this increases the steadiness of 
 the triangular gun- metal bar, #, which in practice is not 
 required to be depressed much lower than represented 
 in the engraving. The milled head, s, of the pinion is 
 made of much larger diameter than represented ; it 
 works against the posterior angle of the triangular bar, 
 and is placed at h. The fine adjustment for the focus, as 
 also the moveable stage described at page 105, can be at- 
 tached to this stage,./, and the foot of the stand made of a 
 solid tripod. 
 
 A. P. 
 
 May 183P. 
 
170 
 
 CHAPTER X. 
 
 Manner of Observing with and Managing the Operative 
 Aplanatic Engiscope. 
 
 BY C. R. GORING, M. D. 
 
 General Observations. Select an apartment on the ground 
 floor, if possible, which has a northern exposure, and the 
 casement of which (it should have only one) is not over- 
 shadowed by trees or buildings, but presents a clear view 
 of the open sky sky-lights are not proper for a micro- 
 scopic study. In such a room I would recommend you 
 to keep your instruments and apparatus, &c. open, and at 
 all times ready for observation ; but take especial care to 
 keep it locked, as if it was the case of your microscope, 
 which, in fact, it may be termed. Have the fear of the 
 cat before your eyes, and also of all those busy, inter- 
 meddling, officious housewives, who, under pretence of 
 dusting, cleaning, and setting to rights, will subvert and 
 revolutionize the whole economy of your observatory, 
 and perhaps throw half your tackle behind the fire. 
 Never allow the apartment to be cleaned until you 
 have secured your apparatus. 1 think it myself a great 
 convenience (and I am not singular in my opinion) to 
 have an apartment especially devoted to my microscopes, 
 as much as the observatory of the astronomer is to his 
 
180 MICROSCOPIC ILLUSTRATIONS. 
 
 transit and circles, for then there is no loss of time when 
 you wish to go to work, and you are sure to find every 
 thing just where you left it. I cannot be plagued by 
 eternally packing and unpacking my implements, &c. 
 but this is no rule for those who like such occupation. 
 Have caps to cover the objective and eye-glasses of your 
 instruments, and get a little nest of drawers to stand on 
 yoiir table to keep talc sliders, &c. in, with a glass cover 
 or two (like those of watchmakers) to secure the more 
 delicate parts of your apparatus ; the rest will suffer no 
 harm from the dust. Procure a tripod stand, like those 
 used for small telescopes, and of such height, that when 
 you sit down and turn the body of your instrument 
 against the sky, like a telescope, the eye-piece may 
 present itself in a commodious position for observation. 
 You will also, probably, want a tripod stool to rest your 
 engiscope upon, when placed in a vertical posture for 
 dissecting and similar uses, as tables generally run too 
 high for the purpose, and are always more or less un- 
 steady. In towns the tremor occasioned by the transition 
 of carriages is frequently a great nuisance in the upper 
 stories of a house, more particularly if your mountings 
 are rickety and unstable. 
 
 Now, courteous disciple, I will endeavour to instruct 
 thee how to manage thy tackle, and will, moreover, have 
 the extreme complaisance to suppose thee (in all micro- 
 scopic matters at least) one of the awkward squad as 
 stupid as an owl, and as ignorant as a cart-horse. I will 
 tell thee as well as I can all that thou art to do, and all 
 
OBSERVING AND MANAGING ENGISCOPB. 181 
 
 that thou art not to do. I will try to make thee know the 
 right end of thy instrument from the wrong one ; how 
 to put a fly's eye before the object-glass, and a fool's eye 
 before the eye-piece; with many other things equally 
 curious, important, and interesting ; and if perchance I 
 shall succeed in learning thee how to deal with the 
 instrument under consideration, the management of all 
 others constructed on similar principles must be per- 
 fectly obvious. 
 
 It will be necessary for me to premise somewhat con- 
 cerning the optical part; though it is not my intention 
 to treat upon it in a regular way in this chapter, as a full 
 and particular account thereof will be given hereafter, 
 I shall speak of it now only as an appendage to the me- 
 chanical fabric of the engiscope, and the directions re- 
 lative to its use will be only of a general nature, but stiR 
 sufficient for practical purposes. The optical part, then, 
 is divided into the objective and ocular; the objective 
 glass is situated at 'e in figure 7? and screws on to the 
 neck of the body ; it is always next the object of which it 
 forms an image or picture, which is viewed by the ocular 
 or eye-glass at 'c, always situated next the eye, (for be it 
 remembered, that in strict language we do not view the 
 object itself in an engiscope, but its image or picture.) 
 
 Several aplanatic object-glasses and eye-glasses ac- 
 company the instrument ; their powers or foci are 
 generally marked upon them ; but the high powers may 
 be known from the low ones by this simple rule, viz. 
 the largest object-glasses are the lowest in power, and 
 
182 MICROSCOPIC ILLUSTRATIONS. 
 
 the longest eye-pieces, having the largest glasses in them, 
 are also the lowest powers. Now the total power of the 
 body or optical part of the engiscope, is the result of that 
 of its object-glass multiplied by or combined with that 
 of the eye-piece ; therefore the highest power will be 
 given by the smallest object-glass used with the shortest 
 eye-piece, and the lowest by the largest object-glass with 
 the longest eye-piece. The eye-pieces mentioned above 
 are all of the inverting kind, like those belonging to 
 astronomical telescopes ; there is, however, a pair of 
 erecting glasses, A, to screw into the bottom of the pull- 
 out tube, which, with the former, make erecting eye-pieces ; 
 these are used for dissecting, &c., and also for giving 
 very loiv powers when wanted below the scale of the 
 weakest inverting ones. The use of all these I shall 
 shew in their proper places. Each object-glass combines 
 with each eye-piece, whatever their number may be, 
 according to the fancy of the observer, but the power 
 which results from the combination can only be known 
 by trial, and should be given by the maker of the instru- 
 ment, in a written paper. 
 
 The object-glasses are frequently made to combine 
 with each other. An increase of power is obtained by 
 the elongation of the body, effected by drawing out the 
 tube, 'c : whenever the length of the body is doubled, for 
 example, the power is also doubled (cceteris paribus], 
 
 In winter it will be proper to cause the instrument to 
 be slightly warmed at the fire before it is used, until it is 
 of about the same warmth with the human body, other- 
 
OBSERVING AND MANAGING ENGISCOPE. 183 
 
 wise the perspiration from the eye will be perpetually 
 condensing on the eye-glasses, &c. and greatly impede 
 vision. 
 
 Always begin to examine your object with the lowest 
 power you have, unless it is very minute ; it may be laid 
 down as a general rule, that large objects require low 
 powers, and small ones high powers ; that the low 
 powers show the whole or general view of an object, and 
 the high ones only its parts in succession. 
 
 Never use high powers unless absolutely necessary ; 
 for as the power increases, so does the difficulty of 
 finding the object, and of adjusting the focus. The 
 colours also grow fainter and more diluted, and the 
 shades darker and darker, until all ease and satisfaction 
 in observation, together with all certain vision, fade 
 away a and so very small a portion of the object is seen, 
 that it is difficult to know what we are looking at. Re- 
 collect that a really good aplanatic object-glass shews every 
 thing with very low powers ; in fact in this property its 
 goodness and beauty consist : do not plume yourself, 
 therefore, upon having an instrument which shews 
 objects with very high powers, but with very low ones. 
 
 Where, however, from the minuteness of an object, it 
 becomes really necessary to use a high power, always 
 select a small deep object-glass, and use it along with a 
 shallow or long eye-piece, in preference to using an object 
 glass of low power, with a short or powerful eye-piece, 
 for magnifying power is much more valuable and effec- 
 tive when derived from the object-glass than from the 
 
184 MICROSCOPIC ILLUSTRATIONS. 
 
 eye-piece (this is a subject which I shall recur to 
 hereafter.) 
 
 When you clean the eye-glasses (a point of great im- 
 portance to pure vision) do not remove more than one 
 at a time, and be sure to replace it before you begin 
 with another : by these means you will be sure to pre- 
 serve the component glasses in their proper places : 
 recollect that if they become intermingled, they will be 
 useless. Keep a piece of well-dusted chamois leather, 
 slightly impregnated with some of the finest putty or 
 crocus powder, in a little box to wipe them with (for it is 
 of consequence to preserve it from dust and damp), 
 the former will scratch the glasses, and the latter pre- 
 vent you from wiping them clean. As to the object- 
 glasses, endeavour to keep them as clean as possible 
 without wiping, and merely use a camel's hair pencil to 
 brush them with ; for wiping them hard with any thing 
 has always a tendency to destroy their adjustment, unless 
 they are firmly burnished into their cells. 
 
 Now I shall suppose that you want to view some trans- 
 parent inanimate object mounted in a slider. There are 
 several ways of doing this, according to the position in 
 which you choose to preserve your instrument, whether 
 directed upwards towards the sky, horizontally, or ver- 
 tically, looking downwards, &c. The first is the best 
 method for direct day-light ; the second, for lamp or 
 candle-light ; the last is the worst of all for either, and 
 should not be resorted to when the others can be used. 
 There are, however, intermediate positions between the 
 
OBSERVING AND MANAGING ENGISCOPE. 185 
 
 horizontal and vertical ones ; say angles of 40 and 45 
 degrees, which are comfortable and convenient for the 
 observer, though, 1 think, unfavourable to the action 
 of the illuminative part of the instrument. I shall pro- 
 ceed to give directions, on the supposition that you wish 
 to see in the simplest and best possible manner. 
 
 1st. Manner of mounting for viewing inanimate trans- 
 parent objects by pure intercepted day -light. When the 
 instrument is first removed from its case, the circular 
 bar, c, will be locked fast into the upper or longest leg, 
 by means of its projecting pin ; it is loosened by drawing 
 the internal tube of the pillar a little out, for which pur- 
 pose, grasp the ball and socket in one hand, and the legs 
 in the other : by pulling in opposite directions, the inner 
 tube will be detached, and the pillar heightened in any 
 degree requisite for observation, at the same time the 
 bar will be left at freedom to move in any direction by 
 means of the ball and socket : immediately underneath 
 the pillar is an adjusting screw, e, by means of which the 
 pillar gains the support of a fourth leg, and is rendered 
 incapable of springing ; it can be used when great stea- 
 diness is required : it should but barely touch the table 
 or stand on which the engiscope is placed. 
 
 Having then spread out the legs, and arranged the op- 
 tical part according to the preceding directions, attach 
 the slider-holder, fig. 25, to the stage. This is done by in- 
 serting the projecting catches into the nicks, and turning 
 it one quarter round : this fixes it fast, so that it- cannot 
 
186 MICROSCOPIC ILLUSTRATIONS. 
 
 fall out whatever may be the position in which it is 
 placed : pass the slider between the plates until the ob- 
 ject appears within the aperture; slacken the pinching 
 screws of the split sockets belonging to the mirror and 
 condensing lens (if necessary), and turn them one quar- 
 ter round, so as to be out of the way of the body, or they 
 may be removed altogether, by sliding them off the bar ; 
 then, by means of the double action of the ball and 
 socket, elevate the bar, until the instrument, when 
 placed on its tripod stand, comes into the position of a tele- 
 scope directed towards the heavens, at any angle you 
 please. In fine weather, it will not be improper to let 
 down the upper part of the casement ; some micro- 
 scopists have recommended us to select a white cloud, if 
 possible, to furnish our transmitted light ; I think myself 
 that the blue light of a clear atmosphere is preferable, 
 especially for test objects. The ball performs the office 
 of a cradle joint, by being moved up and down in the 
 quadrant slit of the socket. Thus, if you wish the bar to 
 point upwards, and the ocular end of the body down- 
 wards (as will be the case in the present instance), make 
 the following arrangement : Move the neck of the ball, 
 by grasping the bar until it is about half way between 
 the upper and lower end of the quadrant slit, (just as if it 
 was a common cradle joint) : the body thus forms an angle 
 of about 45 degrees, and will be in the required position 
 (but unless the motion now to be described has been 
 previously accomplished), in a direction directly opposite 
 to that now required, for you will only be able to observe 
 
OBSERVING AND MANAGING ENGISCOPE. 
 
 with it by looking downwards, whereas you are to look 
 upwards. To effect the object at present in view, turn 
 the ball completely round in the plane of the bar, until the 
 end of the bar comes into the place of the eye-piece. 
 The instrument is then fit for use (at least when placed 
 on its tripod stand and directed towards the sky) ; re- 
 member, that by the motion of the ball as a cradle joint, 
 conjoined with the rotatory movement natural to it, every 
 possible position is attained. All you have now to do 
 will be to adjust the focus, and to bring the body to bear 
 on the part of the object you wish to see. The adjust- 
 ment of the focus is accomplished by means of the milled 
 head, s, and the traversing motion of the body is go- 
 verned by the rack- work at v, coupled with the rotatory 
 movement on the top of the bar, which is regulated by 
 the milled-head at z ; the method of managing this part 
 of the instrument will be gained with the utmost facility 
 by a little practice. In the present case it will not be 
 very necessary, as you may move the object about at will 
 with your hands between the plates of the slider-holder. 
 
 When you look through the instrument, be sure to 
 place your eye quite close to the eye-piece, otherwise 
 the whole field of view will not be visible, and note, 
 moreover, if you see a round disk of light, at least when 
 the object is not in the slider-holder ; if you do not, it is 
 a sign that something is wrong; perhaps the body is not 
 placed directly before the aperture of the slider-holder, 
 or may not be truly directed towards the light, &c. It 
 only remains for me to observe, that the method of using 
 
188 MICROSCOPIC ILLUSTRATIONS. 
 
 the instrument just described, is by far the best and 
 simplest with which I am acquainted for all sorts of 
 transparent objects, and should, therefore, always be 
 used when practicable - } and this observation applies to 
 all sorts of microscopes and engiscopes. Occasionally, 
 however, the weather is so dull and gloomy, that we see 
 better by artificial light : I therefore now give the 
 
 2nd. Method of mounting for viewing transparent objects 
 by artificial light. Setting out on the supposition that 
 the instrument has been just removed from its case, and 
 is in the position given in the plate, proceed as follows : 
 Place it on a table ; detach the bar from the front leg, 
 as before \ move the mirror and condensing lens to one 
 side ; then turn the bar round till it assumes a hori- 
 zontal position, keeping the neck of the ball at the bottom 
 of the quadrant slit, in the position in which it is repre- 
 sented in the drawing : its rotatory action is the only one 
 at present required. This arrangement will leave the 
 space, both behind and before the stage, open for the in- 
 troduction of a lamp or candle, &c. The condensing 
 compound lens, 'f t attached to the body, together with the 
 plane-convex one, n, under the stage, and the mirror, 
 must remain inert and unemployed, in this as in the pre- 
 ceding mode of observation, and may be removed alto- 
 gether. Adjust the height of the instrument to suit that 
 of your light and eye, by drawing the tube of the pillar 
 in or out : place your light centrically behind the aper- 
 ture of the stage, at two, four, or six inches distant from 
 
OBSERVING AND MANAGING ENGISCOPE. 189 
 
 it, according to the strength of the illumination required, 
 observing always to preserve a perfect round disk of 
 light in your field of view, and every thing goes on as 
 before, the candle or lamp supplanting the day- light. 
 You may, if you please, tilt the instrument a little out of 
 the horizontal plane, for convenience of observation. 
 
 3rd. Mode of mounting for viewing diaphanous bodies 
 by reflected day -light, either in a horizontal or vertical 
 position. For the horizontal position every thing is ar- 
 ranged in the same way as when artificial light is em- 
 ployed ; only, now the oval mirror comes into use, which 
 must be placed truly in the axis of the body, and of the 
 aperture in the stage. For this purpose, cause the mark 
 on its split socket to tally with the line drawn on the bar, 
 and then clamp the split socket tight by the screw, m: 
 then, having first removed your instrument out of the 
 direct light proceeding from the window, by placing it 
 in the middle of the apartment, proceed to reflect the 
 light given by the said window into the body of your in- 
 strument, as follows. I must premise that it will be 
 necessary that the engiscope should so present itself to- 
 wards the window that its pillar and bar should not im- 
 pede the light : thus, supposing the window is on your 
 left hand, the pillar and bar must be on your right, and 
 vice versa, so that nothing shall be between the window and 
 the mirror. Observe, that a direct lateral position of the 
 window is the most convenient and manageable. Then, 
 by means of the milled head, draw the mirror round on 
 
190 MICROSCOPIC ILLUSTRATIONS. 
 
 its axis, at the bottom of the arm p, until it fronts the 
 casement; then all that is requisite is, to turn the milled 
 head, r, and along with it the mirror, round, until its 
 light fills your field of view ; which will be at an angle 
 of about forty-five degrees, if the window is directly op- 
 posite the mirror, and your bar parallel with it. You 
 may now, if you please, use the condensing lens, n, but 
 I do not recommend it, as, along with an increase of 
 brightness, it generates a degree of confusion and nebu- 
 losity. If employed, see that it is clamped tight into its 
 true position on the bar. Like the mirror, it should be 
 as close to the stage as possible. 
 
 If you wish to be exactly acquainted with the colour of 
 transparent bodies, use the plaster of Paris side of the 
 mirror to reflect the light of the sun, in an apartment 
 exposed to it. With this intent, place the instrument on 
 a table on which the sun falls, in such a manner, how- 
 ever, that it shall only impinge on the plaster, and leave 
 the rest of the engiscope in shade ; then proceed with it 
 as if it were day-light. This, in my opinion, is the only 
 way in which the light of the sun can be employed with 
 advantage for illumination, and it brings out colours 
 with the most perfect fidelity. 
 
 When the instrument is used in the VERTICAL POSI- 
 TION, it is, of course, in the altitude represented in the 
 plate, and is governed precisely in the manner just de- 
 scribed. I do not like this position at all : it should, I 
 think, only be used with bodies which must be confined 
 to the stage by their gravity alone, which case may some- 
 
OBSERVING AND MANAGING KNGISCOPE. 191 
 
 times occur ; but I have made most ample provision for 
 presenting all sorts of objects in any position, without re- 
 gard to their gravitation, by means of the aquatic wet and 
 dry glass boxes, and the slider-holder, &c. I think that 
 the practice of poring doivmvards (the ordinary way of 
 observation with microscopists of the old school,) is pe- 
 culiarly detrimental to the head and eyes, having a great 
 tendency to determine the blood to them. We never 
 hear of astronomers, who are perpetually looking upwards, 
 having their sight injured, like observers with microscopes, 
 though they have to look, if possible, still more stead- 
 fastly and intently than the latter, with the highest 
 powers. I had nearly forgot to mention a circumstance 
 very necessary to be attended to in the management of 
 the mirror; it is this, to keep the milled head, r, inva- 
 riably outwards. It is not made to travel completely 
 round, nor is it necessary; for every possible position 
 required for practical purposes can be attained without 
 its traversing more than the half of a circle, by reversing 
 the superior or inferior end of the ellipse, according to 
 circumstances. Thus, supposing there were two win- 
 dows in an apartment, one on the right, and the other 
 on the left of the instrument, it would command either, 
 without revolving on its inferior axis, merely by being 
 tilted on the right or left side. 
 
 Way of viewing Crystallizations of Salts and various 
 Chemical Actions. Use the mounting (No. 2) ; screw on 
 the lengthening piece, 'i, and attach the diagonal boot 
 
192 MICROSCOPIC ILLUSTRATIONS. 
 
 (fig 1 . 16) to it. By means of the wheel and endless screw 
 let down the arm of the instrument till the body shall be 
 considerably below the aperture of the stage ; then take 
 off the lid of one of the aquatic boxes, and fix the other 
 part, first warmed at the fire, upon the stage; which 
 turn round on its pivot until its surface with the box 
 shall point downwards ; then insert a drop of the salt to 
 be viewed upon the inferior glass of the box (now, by 
 virtue of its position, the superior one) ; turn the boot 
 round on the lengthening piece until its projecting cone 
 shall look upwards, and adjust the focus by means of the 
 wheel and endless screw, which now performs this office, 
 while the rack- work used under other circumstances, for 
 the same purpose now only gives a traversing movement. 
 Many chemical actions, between various bodies, both 
 fluid and solid, may also be viewed in this way. More- 
 over, crystallizations, &c. can be seen under any of the 
 usual modes of mounting, by placing the salts or other 
 bodies, between the glasses of the aquatic boxes, with 
 the closed or pierced lids, as the occasion may demand 5 
 only, whenever there is evaporation or evolution of gas, 
 remember to employ the pierced lids. 
 
 I think, nevertheless, that the best way of viewing salts 
 is to use the glass slip, with its frame (fig. 29), and 
 merely to rub the liquid salt over its surface, allowing it 
 to drip a little at an angle of 45, before it is placed in 
 the slider- holder. The glass must not be warmed in this 
 method; and the salt must be allowed to crystallize 
 very slowly, in its own natural way. By this process, 
 
OBSERVING WITH AND MANAGING ENGISCOPE. 193 
 
 though somewhat tedious, none but the veritable rmcra- 
 scopic crystals are obtained. Deliquescent salts, in damp 
 weather, can, however, hardly be brought to consolidate 
 in this way. 
 
 I hope the description of the method of management 
 is now tolerably complete, as concerns transparent inani- 
 mate objects ; but the method of dealing with living ones 
 requires some illustration. 
 
 4th. Way of Mounting for Transparent Living Objects* 
 These are not seen well by artificial light, as I have 
 already observed, their internal machinery being more 
 or less confused by it, whether its rays come diverging, 
 converging to a focus, or parallel. 
 
 It will be rather difficult to get a power low enough, 
 with the present instrument, to obtain a general view of 
 a number of the larger aquatic larvae ; for I do not think 
 the present engiscope will carry an object- glass of longer 
 focus than two inches ; which, with the lowest inverting 
 eye-piece, will be fully twice too high (for, be it ob- 
 served, the lower the power is the more of an object it 
 will include in the field of view, and the higher the less); 
 but, with the erecting eye- piece thrust pretty far into 
 the body, a power will be obtained that will easily in- 
 clude a space of one and a half, two, or even three 
 inches ; therefore use the erecting glasses A, screwed 
 on to the bottom of the internal tube of the body, and 
 draw it in or out, until it just takes in the area of your 
 
 o 
 
194 MICROSCOPIC ILLUSTRATIONS. 
 
 largest aquatic live- box, and use the first or third way of 
 mounting. 
 
 The method of using the aquatic live-box is as follows : 
 Take off the lid, applying your finger to the hole in it, or 
 stop it up, pro tempore, with a bit of wax ; then pour 
 in some water to the depth required, and insert your 
 larvae, and apply the inner part to it, observing still to 
 keep the lid undermost : having slid it on far enough to 
 keep the water from escaping, place the box in such a 
 position that the hole in the lid shall point upwards ; then, 
 by squeezing the lid on further (having, of course, first 
 removed your finger orbit of wax from the hole), you 
 may expel all the air, and as much of the water as may 
 be superfluous. It is now fit for use (for the pressure of 
 the atmosphere prevents the water from coming out of 
 the hole) by being placed in the aperture of the stage, 
 and turned one quarter round. A single drop of water, 
 containing animalcules, may be placed between the 
 glasses of the smaller boxes ; and, if the tubes are made 
 air-tight by a little grease, and the hole stopped up, may 
 be kept for weeks together without evaporating. 
 
 If very delicate and exceedingly transparent animal- 
 cules are to be examined, use the adapter (fig. 28), with 
 the smallest aquatic live-box mounted upon it, and se- 
 cured by the bayonet-catch : let it be placed between the 
 connecting cross-bars, 'u 'u, and the superior plate of the 
 slider-holder, which arrangement permits the adapter 
 to be moved about: screw the wheel of diaphragms 
 (fig. 24) on to the bottom of the slider- holder, and see 
 
OBSERVING WITH AND MANAGING ENGISCOPE. 195 
 
 that the body of the engiscope is truly concentric with 
 the hole or diaphragm ; otherwise you will have no light. 
 Recollect, that the body of the instrument must now be 
 stationary, and that you must move the slip about with 
 your fingers. Whenever the diaphragms are employed, the 
 body must be a fixture , or nearly so. 
 
 A very useful way of observing aquatic insects and 
 larvae is by means of the direct or diagonal boot drawn 
 over the lengthening piece, 'i, of your object-glass, which 
 may then be inserted into the reservoir in which they 
 are kept. The method of mounting is as follows : Un- 
 screw the body from the arm, w, and attach fig. 15 or 16 
 to it, instead of its usual neck : see that the plane-glass at 
 their ends is as far removed from the object-glass as pos- 
 sible (for the less water you have to look through the more 
 distinct will be the vision} ; detach the stage from its pivot 
 by slackening the screw, /, and clamp it tight on the 
 lower pivot, i, having previously removed the con- 
 denser, w, altogether : then place the glass vessel on the 
 stage, which must be truly horizontal, and clamped tight 
 by the pinching screw, I : insert the body into the reser- 
 voir, by rescrewing it, with its new appendage, on to the 
 arm, w, again. If you want to observe laterally, or to 
 take a periscopic view of the contents of the vessel, use 
 the diagonal boot in place of the direct one ; and, by 
 turning it round on the lengthening piece, any object 
 situated on the sides of the vessel, such as polypi or vor- 
 ticellag, may be observed without disturbing them. The 
 object-glasses apply either to the ordinary neck of the 
 
196 MICROSCOPIC ILLUSTRATIONS. 
 
 body, or to the lengthening piece over which the boots 
 slide. Dissections may frequently be carried on under 
 the surface of fluids by means of these boots, with great 
 effect, which can hardly be accomplished in the common 
 way. 
 
 Whenever the instrument is in a vertical position (as will, 
 of course, be the case in the present instance,) or when it 
 is not far off from it, the lever y may be used with great 
 advantage. In observing live subjects it is peculiarly use- 
 ful, for it enables us to follow their movements much more 
 rapidly than can be done by means of the wheel and end- 
 less screw 5 and the double motion of the arm being, by its 
 assistance, governed entirely by one hand, the other is left 
 at liberty to adjust the focus (which is, of course, perpe- 
 tually changing, according to the motion of the insects 
 observed) ; it is attached to the pinion on either side of 
 the rack-work, according to circumstances, by being 
 pushed on the square projecting pin. The wheel must 
 then be detached from the endless screw, by slackening 
 the clamping screw, 'b, and drawing the piece which 
 carries the said endless screw backwards away from the 
 wheel, causing it to traverse over a portion of the arc 
 which it forms on its pivot, and fixing it afresh, so that 
 it shall not catch the teeth of the wheel : the arm then 
 moves freely round on the top of the bar, and is subservient 
 to the action of the lever, the milled head of which operates 
 upon the pinion in place of the ordinary one, and is 
 governed by the thumb and index finger ; while the others 
 
OBSERVING WITH AND MANAGING ENGISCOPE. 197 
 
 act upon its extremity, and give the rotatory movement 
 which was before performed by the wheel, &c. More- 
 over, whenever the lengthening piece is in use for trans- 
 parent objects not contained in fluids, the shade (fig. 18) 
 may be employed. It slides down completely over the 
 object observed, and guards it effectually from all rays of 
 light not transmitted through it, so that it is observed purely 
 as a diaphanous body, and not partially also as an opaque 
 one (as is frequently the case with transparent objects 
 treated in the common way). This piece of apparatus is 
 of much use in verification. 
 
 The stage may, moreover, be removed altogether; and 
 the bar being in the horizontal position, the vase may be 
 placed on a proper support in its stead, and observed 
 from without as well as from within. 
 
 5th. Method of mounting the Diamond and Sapphire 
 Microscopes for Transparent Objects. When the use of 
 the engiscopic part of the instrument is thoroughly 
 attained, nothing can be more easy than the management 
 of these. All that is requisite is to unscrew the body 
 from the arm ; and by detaching the teeth of the wheel 
 from the endless screw, according to the directions already 
 given, to reverse the ends of the arm 9 so that the micro- 
 scope shall come into play; whent he rotatory movement 
 must be again secured, by bringing the endless screw iu 
 contact with the wheel, and clamping it tight : thus the 
 instrument becomes converted into a simple microscope 
 of the first quality, which is to be used exactly as the 
 
198 MICROSCOPIC ILLUSTRATIONS. 
 
 engiscope was: I can give no better directions concerning 
 it. The sapphire and diamond lenses have their own 
 peculiar settings in a species of dish or cup, which is 
 attached to the arm by being thrust into the hole at its 
 extremity. The superior cohesion of the precious stones 
 admits of their being burnished into a cell, which is again 
 fixed into the setting. This arrangement allows the 
 lenses to be shifted about without difficulty, or risk of 
 losing them. If they are plano-convex, as they should 
 be, another advantage results, which is, that they may 
 be used either with their flat or convex side towards the 
 object, according as great distinctness, or a large field of 
 view, may be required (for you cannot have these quali- 
 ties combined). Remember, therefore, if you want a 
 large field of view, and do not, at the same time, require a 
 large aperture, united with great distinctness, to place the 
 lens with its flat side next the eye. If, on the contrary, you 
 must have a very large aperture, conjoined with the utmost 
 possible degree of distinctness, then place the convex side 
 next your eye. The aplanatic lenses should also have set- 
 tings on which they can be screwed at pleasure, to be used 
 as simple microscopes. If, at any time, when bodies are 
 merely laid upon the stage, and confined to it only by their 
 gravity (the stand being used vertically, as in dissections, 
 for example), the face should come into too close contact 
 with the stage, the apparatus (fig. 21) must be used, 
 which is inserted into the aperture, like the live-boxes and 
 slider-holder : the object being laid upon it, due space will 
 be afforded for the nose, and the breath will be less likely 
 
OBSERVING WITH AND MANAGING ENGISCOPE. 199 
 
 to steam the lenses. The arm, iv, is made to turn round 
 in any position most convenient to the observer. The 
 adapter (fig. 28), which is to be used between the bars of 
 the slider-holder, to carry a small aquatic live-box, or the 
 like, affords another species of convenience of a similar 
 nature, and serves to elevate the face of the observer 
 above the stage. 
 
 Opaque Objects. Opaque objects seldom require such 
 high powers, or such large apertures, as transparent ones. 
 Their colours are best brought out by daylight, natural 
 or condensed ; but artificial light, perhaps, shews them 
 best in all other respects. 
 
 They should always be viewed upon a black ground of 
 some description. The more sombre, and dingy, and 
 faint, their tints are, the greater need there is of a black 
 ground to stifle all heterogeneous light. 
 
 As they only reflect or radiate light, a much stronger 
 illumination is requisite for them than for diaphanous 
 bodies, which almost always transmit a great deal more 
 light than opaque ones are capable of reflecting. More- 
 over, they seldom present flat surfaces, as most pellucid 
 bodies can be made to do. This, when the power is con- 
 siderable, causes one part of the object to be indistinct, 
 while another is seen accurately : this must always be 
 allowed for. A very perfect instrument can only have 
 one point in focus at once, and, consequently, can only 
 shew one point distinctly at once, all the rest not on the 
 plane of the focus being confused. 
 
200 MICROSCOPIC ILLUSTRATIONS. 
 
 The direct light of the sun is utterly improper for 
 exhibiting opaque bodies, being polarized and decom- 
 posed by them, so as to give rise to all sorts of optical 
 deceptions : neither can it be modified or dulled by 
 refraction through rubbed glass, or any sort of semi- 
 pellucid substances, and thus rendered fit for use : by 
 reflection from white unpolished surfaces it may, but it is- 
 then no stronger than the light of the atmosphere. 
 
 6th. Method of observing Opaque Objects by Day-light, 
 plain or condensed. Employ the mounting (No. 2) recom- 
 mended for transparent objects by artificial light. 
 
 If your object is contained in a slider, insert it into the 
 holder in the usual manner: turn your back diagonally 
 towards the window, and place the engiscope on a table, 
 in a convenient position for observation in that direction, 
 so that the light of the window may fall full upon your 
 object, without being intercepted by your head : the 
 nearer you are to the window the better, provided its 
 direct light does not get into your eyes. You then pro- 
 ceed to the adjustment of the focus, &c. as before de- 
 scribed. If your power is considerable, or the day dull, 
 you will require a condensed light : to obtain it, attach 
 the condenser, 'f 9 to the neck of the body, by means of 
 its clamping screw and split socket, 'g ; unscrew the 
 double convex lens from the other, and remember to use 
 only the plano-convex, with its plane side presented 
 towards the light, and its convex side towards the object : 
 slide it up or down upon the neck of the body ; and adjust, 
 
OBSERVING WITH AND MANAGING ENGISCOPE. 201 
 
 also, by the help of the joint, v, until you get a good 
 spectrum of light upon your slider. (It will be well, in 
 the first instance, to form an image of the window upon 
 your object, and then to push the illumination a little 
 closer to it, so as to obliterate the image.) You will in 
 this way get the maximum of brightness : The crown of 
 the convex side of the bull's eye, or condenser, when 
 properly adjusted, will be about an inch, or an inch and 
 a quarter, from the object. 
 
 If you think it more commodious, you may attach the 
 condenser to the stage, instead of the body (only, in this 
 case, if you move the optical part, you will lose your 
 light). To effect this, remove the setting of the lens 
 altogether from the split socket, by unscrewing the 
 joint, v, and fix it afresh, by means of its screw, into the 
 square bar, fig. 14, which insert into the square hole in 
 the stage. The adjustment is effected now by pushing 
 the illuminator up and down in the stage, combined with 
 the motion of its joint, v, as before. I think, however, it 
 will be found advisable only to attach the condenser to 
 the stage when artificial light is employed ; for, in this latter 
 case, the weight of an additional lens is applied to the 
 condenser, which might overload the body, if depen- 
 dent from it. 
 
 If you wish to view opaque terrestrial living objects, 
 put them into the suitable live- boxes, with the pierced 
 covers, to admit air. These said boxes, with one or the 
 other of their lids, may be used to hold nearly any 
 object. 
 
202 MICROSCOPIC ILLUSTRATIONS. 
 
 7th. Method of observing Opaque Objects by Artificial 
 Light, either plain, condensed, or reverberated, by Silver 
 Cups. Use the mounting (No. 2), as before: close the 
 shutters of your apartment, if you mean to observe in this 
 way in the day-time : procure a telescope candlestick, or 
 one which has an arm moving up and down upon a bar, 
 so that it can be adjusted to any height as a candle burns 
 down (the sliding-tube of the pillar will, in some mea- 
 sure, serve this purpose, if you have not a proper candle- 
 stick), and get a wax taper, or, what is better, a piece of 
 old rushlight, to afford your illumination. Don't laugh ! 
 but do as I bid you : and, moreover, a pair of scissars, to 
 trim it. A piece of rushlight (please your worship) well 
 trimmed, gives a remarkably steady light, without flaring 
 or flickering, and is just as intense as that afforded by oil, 
 or tallow, or gas itself, burnt in any other way. The 
 intensity of the light is the thing wanted, not the quantity. 
 Perhaps a wax rushlight might be an improvement. Now 
 this rushlight, adjusted to the proper height, is to be 
 placed before your stage, and as near to your object and 
 object-glass as may be, without burning them : but you 
 must take especial care that its direct light does not get 
 into the body of the engiscope, or it will utterly destroy 
 all distinct vision : on this account it must be always 
 placed a little on one side of the objective : and if the 
 focus of the said object-glass is longer than half an 
 inch, it will be highly advisable to use a conical 
 shade (fig. 19), drawn over it, reaching almost to the 
 object itself; which will, moreover, always be some 
 
OBSERVING WITH AND MANAGING ENGISCOPE. 203 
 
 security against burning 1 it, or injuring the object- 
 glass itself. The object, if below a certain size, will 
 of course be seen upon a ground of some sort, and this 
 must be a black one : if on a slider, it must be gummed 
 upon black paper ; or, if mounted upon a cork cylinder, 
 having a pin thrust through it, the surface of the cylinder 
 must be blacked with lampblack and glue, before the 
 object is applied to it : but if it is of a nature to be held 
 by the nippers, Mr. Lister's black ground (fig. 22) must 
 be employed to bring it out in true relief; for it is far 
 blacker than any other, and stifles all heterogeneous 
 light much more perfectly. This is attached to the 
 stage in the same way as the live-box or slider-holder ; 
 and is, in fact, the same piece of apparatus which con- 
 stitutes the false stage for the simple microscopes, and 
 for dissecting 5 with the top and bottom, 'n, 'p, applied 
 hi their proper places, to close it up. In the present 
 case, its dark abyss is to form the ground against which 
 the object is examined. At the other end of the needle 
 is a piece of cork, contained in a bit of brass tube. This 
 is to receive the pin by which insects are retained in 
 their drawers in entomological cabinets, which can thus 
 be taken out and examined without injury ; more espe- 
 cially if the pins have been thrust through them diago- 
 nally, which is a great convenience when applied to the 
 microscopes, as it allows their backs and bellies to be 
 freely presented to the action of the object-glass. The 
 nippers, seen of their natural size at fig. 23, are drilled 
 through at s, so as to lay hold of the head of a pin, and 
 
204 MICROSCOPIC ILLUSTRATIONS. 
 
 retain it firmly, so that it can be twisted about in any 
 direction, without risk of detaching it. This will be 
 found a most useful contrivance for holding the cork 
 cylinders, and all objects whatsoever mounted on pins. 
 The swivel which carries the nippers, &c. is thrust into 
 the arm, and in that manner used, as shewn in the plate. 
 
 The mode of observation by plain artificial light 
 corresponds with that by plain daylight. It is one of 
 the simplest and best ways of viewing opaque objects ; 
 and the contrast of the strong divergent lights and 
 shadows cast by the rushlight, brings out, and enables 
 us to comprehend, the foreshortenings and perspective 
 of the complicated details frequent in irregular opaque 
 subjects, and thus to verify their true nature, construc- 
 tion, and relative position, much better than, I think, 
 any other sort of illumination. 
 
 Never forget that, when observing with the candle 
 before the stage, you must hold your breath, or have a 
 handkerchief tied over your mouth : a single puff" is all 
 that is necessary to cause the flame of the light to play 
 over the object and destroy it (and thus in fits of absence 
 have I destroyed many valuable ones). What renders a 
 rushlight so useful is, that it will not, of itself, flare over 
 your object : it can, moreover, on account of its small- 
 ness, be got closer to an object with impunity than any 
 other artificial light ; and thus, in fact, gives the most 
 intense sort of plain illumination ; for the intensity of 
 light decreases according to the square of its distance 
 from the object. 
 
OBSERVING WITH AND MANAGING ENGISCOPK. 205 
 
 When observing faint, sombre, and dull opaque ob- 
 jects, especially if high powers are used, a vast increment 
 of apparent brightness is obtained, if all light is excluded 
 from the eye, save that of the visual pencil. Thus, it 
 will be found a great assistance if a large blackened 
 pasteboard shade, nearly a foot in diameter, is made to 
 fasten upon the eye-piece, and thus completely to ex- 
 clude the direct light of the candle, &c. ; or still better, 
 if a black hood, like those used by astronomers, is made 
 to cover up the face completely, but with an aperture for 
 the body of the instrument. Such an arrangement also 
 saves us the trouble of shutting one eye ; and the en- 
 largement produced in the aperture of the iris, by keep- 
 ing the eyes in the dark, enables the faintest rays to 
 aifect the retina. 
 
 Condensed Artificial Light is procured by the action 
 of the double illuminator, f f; whenever it is used by 
 lamp or candle-light, the double convex must be screwed 
 on, still preserving the convex side of the piano, or bull's 
 eye, next the object. The split socket must be clamped 
 tight in such a position on the neck of the body, that it 
 shall present the face of the illuminator in a lateral 
 position relative to it, so that the rays of the candle, &c. 
 shall fall conveniently upon it ; its distance from which 
 should be about an inch and a half, and its distance from 
 the object should also be about an inch, which will give 
 the maximum of illumination, with a large spectrum, so 
 as to illume a considerable space. I think, whenever 
 
206 MICROSCOPIC ILLUSTRATIONS. 
 
 this condenser is used, the flame of an Argand lamp 
 should be employed, shaded, however, by an external 
 copper tube, with an aperture in its inferior part just 
 large enough to expose the flame, placed over the usual 
 glass one. This will give increased effect and splendour 
 to the object, by preserving the apartment in gloom, and 
 thus allowing the iris to expand itself. 
 
 I do not think a more intense light is got with an 
 Argand lamp than with a rushlight, but certainly a 
 far greater quantity of it. 
 
 Silver Cups, or Specula, afford a very brilliant and 
 intense light, almost without shadow, because it plays 
 vertically upon the summit of an object, like the sun of 
 tropical climates. This is the only species of illumination 
 which will bring out many opaque objects properly (a 
 fly's foot, or human hair, for example) ; but for others, 
 requiring shade for their verification, it is altogether 
 improper. (The markings on the scales of butterflies, 
 &c. are a good illustration of this position.) 
 
 Each object-glass must have a cup attached to it ca- 
 pable of adjustment, by being moved up and down upon 
 the tube in which the object-glass is set, so as to cause the 
 focus of the latter to coincide with that of the cup, (the 
 maximum of brightness cannot otherwise be easily 
 attained). The object must always be held by the nippers 
 or mounted on a cork cylinder, when it is to be illuminated 
 by cups : when the nippers only are employed, the disk, 
 'r, must be placed between them and the illuminating 
 
OBSERVING WITH AND MANAGING ENGISCOPE. 207 
 
 lens, &c. so as to prevent any false light from getting 
 into the object-glass, and also to furnish a black ground. 
 
 Remove the bottom of the black box, 'p, and fix the 
 condenser, n, in such a position, by means of its tight- 
 ening screw, that its convexity shall be inserted into the 
 bottom of the said black box, which will be found its 
 best position either for day-light or candle-light. The 
 engiscope must then be placed on the tripod-stand 
 mounting. No. 1, and directed against the sky, (the 
 best way, perhaps, of using the cups, with day-light], 
 or it may be used in an horizontal or vertical position, 
 and the light reflected by the oval mirror; but when 
 artificial light is preferred, use the mounting No. 2, 
 and place an Argand lamp about an inch and a half 
 from the condenser, n, taking care that its flame is con- 
 centric with it, and a most intense illumination will be 
 procured, even with powers equal to the l-60th of an 
 inch focus. Cups, when properly managed, give a 
 brighter light than can be procured by any other 
 means. They may be used without the assistance of 
 the condenser, but, of course, in this way give a much 
 more feeble light. 
 
 Please to remember that the condenser under the 
 stage, and that represented attached to the body, are 
 expressly and exclusively devoted to opaque objects. 
 
 8th. Mounting for Dissections, fyc. Dissections are ge- 
 nerally performed on opaque subjects, though it may be 
 sometimes necessary to dissect transparent ones also. 
 
208 MICROSCOPIC ILLUSTRATIONS. 
 
 The erecting eye-piece must, of course, always be em- 
 ployed, by sliding it up and down in the body ; with 
 various objectives the power may be made gradually to 
 advance from a mere nothing up to the l-20th of an inch 
 (which I conceive to be the utmost which can be used), 
 and still allow sufficient space between the object and 
 the object-glass for the anatomist to manage his imple- 
 ments, and to procure the necessary illumination. 
 
 Mount the engiscope in the vertical position, and see 
 that the bar is locked fast in its front leg, to give the 
 utmost degree of stability ; place it on the tripod stool, 
 before recommended, with the front of the stage before 
 you, and the pillar next to the window (a diagonal po- 
 sition for yourself and the instrument relative to it, will 
 be most convenient, to allow the light to fall freely on 
 your subject.) Use the single condenser, n, to furnish 
 your light, if necessary; it may be moved round the 
 neck of the body, to suit your convenience. You may 
 also employ the lengthening piece, 'i, if you please, which 
 will allow you to fix the stage on the pivot and sliding 
 socket, i y and give a lower position for it. If you use the 
 latter, see that it is clamped very fast, that it may not 
 give way with the weight of your hands. 
 
 If you do not find it advisable to employ the false stage, 
 fig. 22, which procures a separate rest for the little 
 fingers, while the thumb and index finger operate, drop 
 the piece, fig. 27, into the aperture of the stage, and fix 
 it by turning it one quarter round. Then place the 
 piece of cork or wood, &c. on which you dissect in the 
 
OBSERVING WITH AND MANAGING ENGISCOPE. 209 
 
 smaller aperture of this, and proceed. I should recom- 
 mend you to get the leaves of two Pembroke or other 
 tables placed on each side of you, to rest your elbows 
 upon at pleasure, and likewise to procure several pieces 
 of board, about one and a half inch thick, to place under 
 the instrument, to depress or elevate it without the help 
 of the slide in the pillar (which, when called into effect, 
 always unlocks the bar.) 
 
 Your operations can always be carried on under the 
 surface of liquids, by the help of the direct or diagonal 
 boots. 
 
 When you wish to get at the side of your subject with- 
 out disturbing its surface, mount in the position, No. 2, 
 and turn your stage round on the pivot, i, until it comes 
 to be in the plane of the bar : every lateral view can then 
 be obtained at pleasure, by turning the object round, 
 and you can operate in this position as well as in the ver- 
 tical one. You may also, by help of the ball and socket, 
 tilt the body at any required angle, and still adjust the 
 stage to the horizontal position, as before, taking care to 
 pinch it fast by means of the screw, /, before you begin 
 to work again. 
 
 There are no tools for dissection equal to very fine 
 scissars,they always cut without dragging or tearing ; 
 there is, however, the greatest difficulty in procuring 
 them. The Sheffield workmen are the only ones capable 
 of making them sufficiently delicate. Miniature scissars 
 have been made only half an inch in length quite perfect; 
 now, a pair constructed with blades like these, but in 
 
 p 
 
210 MICROSCOPIC ILLUSTRATIONS. 
 
 other respects like those for some operations on the eyes, 
 (i. e. without bows), having one arm inserted in a long 
 handle, and the other left short, playing against a feeble 
 spring to keep it open, is the sort of tool required. It is 
 held like a pin, and cuts by the pressure of the index 
 finger on the short arm. You may, of course, dissect 
 with simple microscopes of low powers as well as with the 
 engiscope, but the present stand is not so well adapted 
 for them as a separate one would be. 
 
 9th. Mounting the Diamond and Sapphire Microscopes 
 for opaque objects. It is one of the vices of simple mi- 
 croscopes that they can only shew opaque objects with 
 cups at least if their powers exceed one quarter of an 
 inch. The lower foci shew opaque objects admirably 
 with day- light, the stand being in the vertical position. 
 When used for plain day-light in a horizontal one, all 
 that is necessary is to assume such an attitude relative to 
 the window that the head shall not intercept its light. 
 For example, if you sit with the bar horizontal and pa- 
 rallel with the window, the light will naturally fall be- 
 tween the stage and the magnifier upon the object. When 
 cups are employed, they act precisely like those attached 
 to the engiscope, and magnifiers thus mounted are 
 managed in the same way. 
 
 10th. The Amician Catadioptric Engiscope is attached 
 to the present stand by means of the arm, fig. 20, and is 
 managed precisely on the same principle as the refracting 
 
OBSERVING WITH AND MANAGING BNGISCOPK. 211 
 
 aplanatics. The body turns round upon a swivel, by 
 means of the pinching screw, to suit the convenience of 
 the observer ; and the only real peculiarity in observing 
 with the reflecting engiscope is, that the optical part is 
 always at right angles to that of the refracting one in any 
 given mode of mounting or observation just as the eye- 
 tube of the Newtonian telescope is relatively to that of a 
 refractor, cateris paribus ; but this instrument, as well as 
 the simple and compound microscopes, are probably best 
 on their appropriate stands, of which full and particular 
 descriptions will be given hereafter*. Jt is only for the 
 sake of those who should like to have the three instru- 
 ments jumbled together, that they are adapted to the 
 same stand in the present case. 
 
 Courteous reader, I have endeavoured to supply the 
 place of a viva voce lecture on the instrument, and to in- 
 fuse into thee such knowledge as I possess, touching 
 the management of the aplanatic engiscope, &c. The 
 directions given will, I hope, be found more explicit, 
 clear, and intelligible, as well as more full, particu- 
 lar, and diffuse, than any others hitherto given in print. 
 Valeant quantum valere possunt. By their assistance thou 
 shalt be enabled to enter into a course of researches very 
 nearly as profitable to thyself arid fellow creatures as if 
 thou wert engaged in the sublime and important occu- 
 pation of determining whether the small star of Bootis, 
 is of a greenish blue, or bluish green ; or whether some 
 nebula is very gradually or very suddenly much brighter 
 in the middle. 
 
 * Vide Micrographia, chap. i. 
 
APPENDIX. 
 
 No. 1. 
 
 On the Optical Phenomena of certain Crystals. 
 BY H. F. TALBOT, ESQ. F.R.S.* 
 
 SOME time ago I had the honour to communicate to the 
 Royal Society an account of my invention of the pola- 
 rizing microscope. This instrument possesses so great 
 a power of developing the internal structure of trans- 
 parent bodies, even in their minutest visible particles, 
 that I feel confident the employment of it will lead to 
 many new and interesting results. At present I mean 
 to confine myself to the description of a phenomenon 
 which shews strikingly the beautiful order and regula- 
 rity with which nature disposes the fabric of some of her 
 minutest visible works. 
 
 The object I speak of is a kind of minute crystalliza- 
 tion which may be obtained in peculiar circumstances, 
 and I doubt not in many different ways ; but the manner 
 in which it has presented itself to my observation is as 
 follows : 
 
 A crystal of borax is placed in a drop of phosphoric 
 
 * From Phil. Trans. Part I. for 137. 
 
214 APPENDIX. 
 
 acid, somewhat diluted, upon a plate of glass, and then 
 moderately heated until the crystal dissolves in the acid. 
 It is then set aside to crystallize. It is well to prepare 
 a number of these plates at once, varying the relative 
 proportion of the acid and salt, in order that the desired 
 kind of crystallization may be found in one or other of 
 them; for there is a considerable variety in the crystal- 
 line forms obtained by this method, some of which indeed 
 are very singular- But when that kind of crystallization 
 takes place which it is more particularly my intention to 
 speak of, the field of view of the microscope is seen 
 covered with minute circular spots, each of which is like 
 a tuft of silk radiating from a centre, and is composed 
 of a close assemblage of delicate acicular crystals forming 
 a star. But besides these, are seen interspersed among 
 them a number of circular transparent bodies, which are 
 evidently modifications of the former, being, in fact, tufts 
 or stars of acicular crystals, in such close assemblage as to 
 be in optical contact with each other, and to produce the 
 appearance of a single individual. Now let us suppose 
 a group of these circles to be under examination with the 
 polarizing microscope, and when the polarizers are 
 crossed we observe the following phenomenon : The 
 field of view being dark, the little circles become lumi- 
 nous, and we see upon each of them a well defined and 
 dark cross, dividing the crystal into four equal parts. 
 All these crosses are placed similarly, and are parallel to 
 each other, and their direction remains unaltered when 
 the crystals are turned round in their own plane by 
 
ON THE OPTICAL PHENOMENA OF CRYSTALS. 215 
 
 revolving the plate of glass upon which they stand. This 
 beautiful appearance can be seen with a moderate mag- 
 nifying power. I measured the diameter of some of the 
 larger crystals, which I found to be from I -300th to 
 1 -600th of an inch. But there are many much smaller, 
 and indeed they may be seen decreasing in size, until 
 nothing remains visible of their structure but the four 
 luminous quadrants, appearing like four minute dots of 
 coloured light placed close together. 
 
 I proceeded to examine the circles with a high magni- 
 fying power, and under favourable circumstances of illu- 
 mination, and I observed in them a very admirable 
 structure. 
 
 Each circle has upon it one or more coloured rings 
 arranged concentrically, but the number as well as the 
 colour of these rings is different in different individuals. 
 
 The innermost ring is deeply coloured or black, and 
 incloses a central space of white light, which is traversed 
 by the arms of the cross intersecting in the centre. This 
 part of the cross, which stands within the innermost ring, 
 is beautifully well defined, and perfectly black. The 
 general appearance resembles the figure 98, in Brewster's 
 Optics, which is a representation of the rings seen in 
 uniaxal crystals. It especially resembles it in the circum- 
 stance above mentioned, viz. the more defined outline 
 of the part of the cross which is within the innermost 
 ring. 
 
 We have hitherto supposed the polarizers to be crossed; 
 but if we place them in a parallel position we shall see 
 
216 APPENDIX. 
 
 a phenomenon complementary to the above. The circle 
 now presents four patches of coloured light, one in each 
 quadrant ; and we generally see near the centre four 
 black or obscure spots, which correspond to the arms of 
 the cross in the other position. 
 
 Such is an outline of the microscopic appearances 
 presented by these little crystals, which are probably the 
 minutest bodies in which so complicated an optical struc- 
 ture has hitherto been witnessed. I find that the smaller 
 circles are, the more perfect is their form, and the 
 brighter their colours. 
 
 These crystals, as I have already observed, probably 
 consist of spicula diverging from a point, but which are 
 in the closest possible contact, and in a state of complete 
 mechanical cohesion. It seems to follow as a conse- 
 quence from such a structure, that their density must 
 increase from their circumference towards their centre. 
 Now it is worthy of remark, that Sir David Brewster has 
 discovered very similar phenomena by polarized light in 
 the crystalline lenses of certain fishes, which are known 
 by direct experiment to increase in density towards the 
 centre. Indeed the figure which he has given of the 
 lens of the codfish in the Philosophical Transactions for 
 1816 (Plate XII. fig. 1,) is so like the appearance of one 
 of the crystals which I have described, that it might be 
 supposed to have been intended for a representation of it. 
 
 Having pointed out this resemblance, I may also men- 
 tion another class of facts to which I think those I have 
 described possess a considerable analogy. I mean the 
 
ON THE OPTICAL PHENOMENA OF CRYSTALS. 
 
 optical figures which Brewster has discovered in spheres 
 of glass whose density was rendered variable by heating 
 them. 
 
 He says* that " if we take a cold sphere of glass and 
 immerse it in a trough of hot oil, placed in a polarizing 
 apparatus, we shall observe a black cross with four sectors 
 of polarized light. If the sphere is turned round, it will 
 exhibit in every position the very same figure. If we 
 now suppose the trough to be filled with such spheres, 
 they will exhibit the same phenomena in whatever direc- 
 tion the polarized light is transmitted through them, and 
 even if they were in a state of motion. A fluid com- 
 posed of such spherical particles would exhibit the same 
 polarizing structure in every possible direction, and even 
 if it were in a rapid state of gyration. If the particles 
 possessed the structure that produces circular polariza- 
 tion, the fluid would develop the phenomena exhibited 
 by oil of turpentine, &c." 
 
 And againf , " The structure of the particles of a cir- 
 cularly polarizing fluid must be exactly the same along 
 every one of its diameters ; that is, the structure must 
 be symmetrical round the centre of the particle, or 
 analogous to that which takes place in common polariza- 
 tion when a sphere of glass has its density regularly 
 increasing or regularly diminishing towards its centre." 
 
 I have quoted these remarkable passages at length, 
 because it appears to me that what is there advanced 
 
 * Library of Useful Knowledge, art. " Polarisation of Light," p. 51. 
 t Ibid. p. 45. 
 
218 APPENDIX. 
 
 merely as a hypothesis, acquires a considerable degree of 
 probability from the facts which I have stated, since I 
 have succeeded in rendering actually visible circular 
 particles of excessive minuteness, in each of which 
 the microscope detects the very structure imagined by 
 Brewster, viz. the black cross and four sectors of light. 
 So that it appears not improbable that the circular 
 polarizing properties of fluids may be owing to the 
 presence of multitudes of particles similar to these, which 
 they hold in solution. 
 
219 
 
 No. 2. 
 
 Further Observations on the Optical Phenomena of Crystals. 
 
 BY H. F. TALBOT, ESQ. F.R.S.* 
 
 SECT. 1. In my former paper on this subject I have 
 described the remarkable circular mode of crystallization 
 which sometimes occurs when borax crystallizes from a 
 solution in phosphoric acid. 
 
 I have stated that when examined by the polarizing 
 microscope, a black cross and four sectors of light are 
 seen upon each crystal ; and upon that kind which is 
 most easily and frequently obtained, there are seen in 
 addition one or more rings of vivid colour. Some 
 deviations, however, from this usual form occur occa- 
 sionally ; one of which, being extremely beautiful as a 
 microscopic object, deserves a separate mention. This 
 variety of crystalline circles differs from the one first 
 described in the following particulars. 
 
 1. The circles are much larger, attaining the diameter 
 of one-tenth of an inch ; whereas those first observed did 
 not exceed one-two-hundredth of an inch in diameter. 
 
 2. They are flat, whereas the former ones were convex, 
 and frequently, I believe, of a spherical form. 
 
 * From Phil. Trans. Part J. for 1837. 
 
220 APPENDIX. 
 
 3. In consequence of which probably, they are seen to 
 exhibit no coloured rings, but only a cross. 
 
 4. The cross is brightly coloured, red, green, blue, &c. 
 upon a white ground (the polarizers being supposed to 
 be parallel to each other). This has a beautiful appear- 
 ance, especially when several circles seen at once have 
 crosses of different colours. 
 
 5. When the polarizers are placed at right angles, the 
 phenomenon complementary to the above is seen. For in- 
 stance, the circle which presented a red cross upon a white 
 ground now presents a black cross upon a green ground, 
 
 6. In an intermediate position of the polarizers, the 
 circle just mentioned presents a red cross alternating 
 with a green one, thus dividing the circle into eight 
 sectors of coloured light. 
 
 Other circles present other colours, but they all follow 
 the same analogy, and the crosses upon all the crystals 
 are in a parallel position. 
 
 7. These crystals last longer than the former ones. 
 I have found some of these retain their structure for two 
 months ; the former kind seldom last in perfection more 
 than a day. 
 
 8. It sometimes happens that their circumference is 
 imperfect, and presents a notched or jagged outline. 
 These have a very beautiful appearance, and have been 
 almost universally compared by those who have looked 
 at them to highly-coloured flowers with four petals ; the 
 cross upon them being so dark as to have the appearance 
 of being a division between the petals. 
 
ON THE OPTICAL PHENOMENA OF CRYSTALS. 221 
 
 All the circles, when viewed by common light, appeal- 
 transparent, white, and very uniform. If they are com- 
 posed of acicular crystals diverging from a point, these 
 latter must be exceedingly slender and numerous, and in 
 perfect optical contact, since a high magnifying power 
 does not render them separately visible. 
 
 SECT. 2. With respect to the chemical nature of 
 these crystals, it appears to me evident that they consist 
 of boracic acid. They are obtained by dissolving borax 
 in phosphoric acid; and it may be inferred that this 
 latter substance unites with the alkali, and isolates the 
 boracic acid. In order to see if this supposition were 
 correct, I dissolved boracic acid in alcohol, and I found 
 that a drop of this solution evaporated on a plate of glass 
 frequently yielded an abundant crop of the crystalline 
 spherules. But these are generally exceedingly small, 
 requiring a high power to display in them the cross and 
 four sectors of light, and they speedily grow opaque ; for 
 which reason they are not so well suited for observation 
 as those prepared by the former method. They establish 
 the fact, however, that this mode of crystallizing is a 
 property of the boracic acid. It is highly improbable 
 that it should be peculiar to that substance, but I have 
 not yet met with it in any other. 
 
 SECT. 3. Explanation of some of the Optical appearances. 
 1. When any doubly refracting crystal is examined 
 with the polarizing microscope, (the polarizers being 
 
222 . APPENDIX. 
 
 transverse to each other, and the field of view conse- 
 quently dark,) if it be turned round in one plane, it is 
 seen to grow four times luminous and four times dark in 
 the course of one revolution. This I have found to be 
 universally the case with all the substances which I have 
 tried, and it also is in accordance with theory. 
 
 2. In the case of an acicular crystal one of the optical 
 axes always coincided with the axis of figure, or length 
 of the crystal ; so that if a crystal of this sort appears 
 unilluminated, all the others that are either parallel to it 
 or perpendicular to it are likewise dark, 
 
 3. It results from the above, that a circle composed of 
 acicular crystals diverging from a point must present the 
 appearance of a black cross, and that the crosses on all 
 the circles will be parallel. 
 
 4. With respect to the rings of colour, they are a con- 
 sequence of the variable thickness of the crystalline circle 
 at different distances from its centre. Their being visible, 
 and indeed very conspicuous, upon a body of such small 
 diameter, arises from the very energetic action of boracic 
 acid upon polarized light. 
 
 SECT. 4. I have remarked that the circular crystalli- 
 zation of boracic acid is frequently entirely superseded 
 by other modes of crystalline formation ; which circum- 
 stance appears to be chiefly owing to the presence or 
 absence of combined water. Some of these variations 
 deserve to be particularly specified. 
 
 1. Instead of circles there often occurs a formation 
 
ON THE OPTICAL PHENOMENA OF CRYSTALS. 223 
 
 of crystals resembling two opposite sectors of a circle 
 combined. This form may be traced in different crys- 
 tals, from its commencement when the angle of the 
 sector is small, through all degrees of increase, until at 
 length the opposite sectors unite and form a complete 
 circle. 
 
 2. The crystals are frequently of a very irregular elon- 
 gated shape, which does not approximate either to a 
 prismatic or a cylindrical form. This stem, as it may be 
 termed, subdivides itself at both extremities into an 
 immense multitude of diverging fibres, giving it the ap- 
 pearance of a bundle <Jf elastic filaments firmly held to- 
 gether in the central part, but with its extremities left at 
 liberty to diverge*. 
 
 3. Another variety resembles in the same way irregu- 
 lar steins or branches, which, however, instead of being 
 subdivided, are abruptly truncated at both extremities 
 perpendicularly to the general line of their direction. 
 
 4. Sometimes, on the contrary, the ramification is 
 much more developed, and then resembles two plumes 
 united by a common stem. 
 
 5. Crystals of regular geometric form. These appear 
 to require the presence of combined water. 
 
 Whichever of these formations occurs, it is for the 
 
 * This appearance is iiot very uncommon in the crystallization of other 
 substances, though I believe it has not yet been described. The divergence 
 of the filaments suggests the idea of electrical repulsion as being at least its 
 primary cause. 
 
224 APPENDIX. 
 
 most part seen in all the crystals at once, to the exclusion 
 of any of the other forms. 
 
 SECT. 5. These crystals generally undergo a sponta- 
 neous change in the course of one or two days after they 
 have been formed. Those (No. 4) resembling plumes 
 usually break up and resolve themselves into small 
 rhombs and other geometric forms. The elongated 
 crystals (Nos. 2, 3) undergo a remarkable change. They 
 become traversed with innumerable fissures transverse 
 to their length, and thus break up into thin plates, which 
 either cohere loosely or separate entirely. 
 
 SECT. 6. All these forms are very pleasing objects for the 
 polarizing microscope. This arises from the very high de- 
 polarizing power of boracic acid, which enables its thinnest 
 plates to exhibit colours of great variety and brilliancy, 
 and causes even its dust or smallest particles to appear 
 luminous. The more energetically any substance acts 
 upon polarized light, the closer and more crowded are 
 the band and lines of colour which appear upon its 
 crystals. These isochromatic lines, of which there are 
 
 frequently many alternations, denote lines of equal thick- 
 ness in the crystal. In the case of boracic acid, when 
 anhydrous or nearly so, these lines are more crowded 
 than in any other crystal that I have yet examined, inso- 
 much that to exhibit them distinctly is as fine a test of the 
 performance of a microscope as to resolve the more diffi- 
 
OPTICAL PHENOMENA OF CRYSTALS. 225 
 
 cult lines on the scales of a butterfly's wing, or any other 
 of the known test-objects. And in many cases the 
 microscope only indicates the existence of a still more 
 delicate structure, which, at least in its present state, it 
 has not power distinctly to exhibit. 
 
 SECT. 7 On Analytic Crystals. [ now come to de- 
 scribe a property of crystals which I met with while em- 
 ployed in pursuing the above investigation. This is, the 
 power which certain crystals have of analyzing polarized 
 light in a manner analogous to the tourmaline ; for 
 which reason I shall propose for them the name of 
 Analytic Crystals. 
 
 If I am not mistaken, this property has been hitherto 
 confined to the tourmaline and a few other natural 
 minerals : and it has not been known that their effects 
 could be imitated, much less surpassed, by crystals arti- 
 ficially made. I trust, therefore, that it will be of some 
 interest to describe a method of procuring such crystals. 
 
 In the following experiments it will be understood 
 that the analyzing plate of the microscope (or the pola- 
 rizer next the eye) is removed. 
 
 1. A good example of this kind of crystal is obtained 
 by dissolving the sulphate of chromium and potash in tar- 
 taric acid by the aid of heat. A drop of this solution 
 placed on a plate of glass soon yields by evaporation 
 filmy crystals, which very frequently have the charac- 
 teristic property of tourmaline : that is to say, that if 
 
 Q 
 
226 APPENDIX. 
 
 polarized light is transmitted through them, in one posi- 
 tion they suffer it to pass freely, but if they are turned 
 round 90 degrees, they arrest and ahsorb it entirely. 
 
 When the experiment has been successfully conducted, 
 the crystals will not in this position allow the smallest 
 portion of light to pass. 
 
 If now we consider the extreme thinness of these crys- 
 talline films, it will appear how energetic must be, their 
 action upon light ; since, although white and transparent, 
 they are able to produce an absorption equalling that of 
 the best tourmalines, notwithstanding that the effect of 
 the latter is aided by their natural dark colour. 
 
 But if these crystals are analogous to the tourmaline, 
 they must have the power which that substance has of 
 analyzing the light that has been transmitted through 
 other crystals. 
 
 Accordingly, if we place in the path of the polarized 
 ray a plate of sulphate of lime of a proper thickness, the 
 crystal which before absorbed the light and appeared 
 black, becomes splendidly coloured with that colour 
 which the sulphate of lime produces, and which a tourma- 
 line would shew if it were employed as an analyzing plate. 
 
 On reversing the polarization of the ray (or turning 
 round the crystal), the complementary tint appears. The 
 same results occur if the crystal is employed in the first 
 place to polarize the light, and tourmaline or calcareous 
 spar is used to analyze it, so that the analogy or rather 
 identity of effect with the tourmaline is complete. 
 
OPTICAL PHENOMENA OF CRYSTALS. 22/ 
 
 1 will now mention some other crystals which possess 
 the analyzing property, but not in such a high degree. 
 
 2. Boracic acid. If dissolved in boiling water, it yields 
 in cooling irregular crystals which have considerable 
 analytic power. A crystal which in one position is so 
 translucent as to be hardly distinguishable from the 
 water in which it floats, is in the transverse position 
 very, strongly defined. It does not become dark all over, 
 but only in its outline. 
 
 If now we employ it to analyze the tints of sulphate of 
 lime, its outline becomes beautifully coloured. Nothing 
 can exceed the delicacy of colouring which a number of 
 these crystals exhibit when viewed* together ; those which 
 He in one direction appearing, for instance, green ; those 
 in a transverse direction, red. The appearance is very 
 unlike any other optical phenomenon that I know of, in 
 consequence of two colours being seen in strong contrast, 
 and without any intermediate tints ; and also from the out- 
 line only of the objects being coloured, while their interior 
 remains without colour. It is only when the crystals 
 have a fibrous or striated structure that the tint extends 
 over all their surface. 
 
 The boracic acid has the same analytic property, and 
 precisely the same appearance, when it crystallizes from 
 a solution of borax in phosphoric acid. The plumose 
 crystals of it (No. 4, supra) are very delicately coloured 
 with the two opposite tints. 
 
 I obtained a very beautiful result by placing a drop of 
 phosphoric acid upon a group of circular crystals. This 
 
228 APPENDIX. 
 
 caused a fresh deposition of boracic acid upon them as 
 nuclei, which assumed the form of very delicate cilia, 
 spreading in all directions as from a centre. These 
 fringed circles shewed the analytic property in an admi- 
 rable manner, exhibiting four quadrants coloured alter- 
 nately with complementary colours of great vivacity. 
 
 3. Another instance which is worthy of mention is the 
 oxalate of potash and chromium, a salt whose optical pro- 
 perties have been investigated by Sir David Brewster*. 
 If some gum arabic is added to a solution of this salt, and 
 a drop of it put between two plates of glass, it abandons 
 its usual mode of crystallization for another, which re- 
 sembles a microscopic vegetation composed of minute 
 prisms growing one out of another, and variously ar- 
 ranged in sprigs and branchlets ; while in other places 
 it assumes an undulating capillary form, much resem- 
 bling in miniature the tufts or locks of a species of 
 Conferva which is seen growing in pools of water or in 
 the sea. Now these objects are possessed of a high 
 analytic power, insomuch that, when a plate of sulphate 
 of lime is placed beneath them, they assume a colour of 
 great intensity and splendour, which is changed for the 
 complementary tint when the polarization of the incident 
 ray is reversed. 
 
 4. Nitre. If nitre and gum arabic are dissolved to- 
 gether in hot water, a drop of the solution put on a glass 
 plate yields very good analytic crystals. These have a 
 
 * Philosophical Transactions for 1835, p. 91. 
 
OPTICAL PHENOMENA OF CRYSTALS. 229 
 
 branched or plumose appearance, and assume beautiful 
 colours in polarized light when a plate of sulphate of 
 lime is placed beneath them. The microscope shews the 
 colour to reside principally in the outline, but to the 
 naked eye the whole film appears coloured. As these 
 films may be obtained of large size, the phenomenon cah 
 be well seen by the unassisted eye. 
 
 A very interesting experiment, and one which throws 
 much light upon the cause of these appearances, is to 
 transmit a beam of polarized light very obliquely through 
 a small prism of nitre immersed in gum, and viewed 
 with the microscope. Its outline then generally exhibits 
 two colours instead of one ; for, while the edge of the 
 prism which is on that side from whence the ray of light 
 comes, is, for instance, of a red colour, the opposite edge 
 will appear green. Reverse the polarization of the light, 
 and these colours are exchanged one for the other. This 
 observation enables us to explain the origin of the pheno- 
 menon in a satisfactory manner, and to shew why it only 
 occurs in crystals possessing strong double refraction, 
 like nitre, in which the refractive indices of the two rays 
 are materially different. 
 
 When a ray of common light is incident upon such a 
 crystal, and therefore divides itself into two rays oppo- 
 sitely polarized, both rays are transmitted through the 
 central parts of the crystal, which are bounded by parallel 
 planes, or by planes approaching to parallelism. But 
 when the bounding planes of the crystal are much in- 
 
230 APPENDIX. 
 
 clined to each other, and therefore refract the light in 
 the manner of a prism, the refractive indices of the rays 
 may differ so much, that while one of them passes freely 
 through such a prism, the other cannot pass at all, but 
 suffers total internal reflexion, and is thereby dispersed ; 
 just as if the prism had a larger refracting angle with 
 respect to that ray than to the other. Therefore if two 
 oppositely polarized rays are presented successively to 
 such a crystal, as in our experiment, one of them will be 
 transmitted, and the other not. That this is the true 
 explanation appears from this, that when the oblique 
 planes are well formed and clearly denned by the micro- 
 scope, the colour also is accurately limited by the same 
 boundary : so that while this part analyzes the tints of 
 a plate of sulphate of lime, the rest of the crystal is 
 inactive. 
 
 It may be inferred by analogy, that the same cause 
 produces the analyzing power of striated or fibrous sur- 
 faces, and of those in which the striae are too minute to 
 be discernible (as in No. 1, supra, page 225) : for it is not 
 the property of all crystals with striated surfaces to have 
 the analytic power, but only of such as are doubly 
 refractive in a high degree, 
 
 I have said that the capillary crystals (No. 3) possess 
 the analytic property, although their diameter is often 
 evanescent even with a microscope. An important infer- 
 ence may be drawn from this, viz. that a ray of light imme- 
 diately on entering one of these crystals, subdivides itself 
 
OPTICAL PHENOMENA OF CRYSTALS. 231 
 
 into two rays of different refractive indices, or at least 
 that the thickness of crystal which is requisite to pro- 
 duce this effect is insensible to observation. 
 
 When nitre is made to crystallize in gum, it often 
 shoots into prismatic crystals, which are very interesting 
 objects, the more so, that they are of a permanent nature, 
 and not liable to spontaneous change. When examined 
 by polarized light, these prisms, in one position of their 
 axis, frequently disappear completely. This arises from 
 the refractive power of the gum being equal to that of 
 one of the two rays in the crystal. Reverse the polari- 
 zation of the ray, and the crystal appears, as it were, to 
 start into existence, acquiring great strength and black- 
 ness of outline, and, not unfrequently, entire opacity. 
 Again, when the sulphate of lime is interposed, this 
 opacity disappears, and the crystal becomes brightly 
 coloured. Since it is probable that many better methods 
 may be found of obtaining this kind of crystal than have 
 hitherto presented themselves, I have hopes that it will 
 be possible to obtain large and permanent artificial 
 crystals, which may possess the advantages of the tour- 
 maline without the inconvenience resulting from its dark 
 colour. 
 
232 
 
 No. 3. 
 EXORDIUM*. 
 
 BY C. R. GORING, M.D. 
 
 THE discovery of a set of objects for ascertaining the 
 defining and penetrating powers of microscopes, has 
 founded a new era in the history of those instruments. 
 The thick aplanatic object-glass for diverging rays, and 
 the Amician reflecting microscope, have in consequence 
 been perfected. The substitution of diamond and sap- 
 phire lenses, for those made of glass, in the single micro- 
 scope, with the ingenious apparatus for illumination 
 contrived by Dr. Wollastori, may also in some measure 
 be attributed to the same source. It may surely be 
 affirmed that no improvements at all commensurate to 
 these in magnitude and importance are likely to be made 
 hereafter. Microscopes are now placed completely on 
 a level with telescopes, and, like them, must remain 
 stationary in their construction. 
 
 In the present work I have endeavoured to establish 
 the mechanical part of the construction of microscopes, 
 
 * From First Edition, with corrections. By C R. G. 
 
EXORDIUM. 233 
 
 that is to say, their mountings, carriages, and apparatus, 
 &c. on true and philosophical principles, and I flatter my- 
 self not totally without success, most of the better class of 
 instruments, which have been made of late years, being 
 more or less constructed according to the plans I have 
 suggested. This, however, is a subject on which no man 
 must pretend to dogmatize or dictate, for such is the spirit 
 of contradiction, and of renitency against conviction, in 
 mankind, that they will make it a point of honour to go 
 the wrong way if you urge them too forcibly on in the 
 right one, just to shew that they will have their own way, 
 and are determined to think and act for themselves. 
 
 Though it may savour somewhat of egotism, I cannot 
 refrain from stating some of the difficulties which I 
 experienced in making the drawings of the living objects, 
 though I know very well that people of first-rate talent 
 explode difficulties, and will hardly allow of their exist- 
 ence, except with bunglers and half-taught amateurs. 
 I wish I had been one of those favoured individuals with 
 whom the most arduous achievements of all sorts 
 
 " are no more difficile, 
 
 Thau for a blackbird to whistle." 
 
 HUDIBRAS. 
 
 If any portrait painter had to execute a likeness of 
 some person afflicted with chorea, who could not be 
 prevailed upon to be quiet for more than half a mi- 
 nute together, who was perpetually jigging about the 
 apartment, and exhibiting his tail instead of his head, 
 
234 APPENDIX; 
 
 &c. &c., I think he would be compelled to admit 
 that he had undertaken a task which would at least 
 require time and patience for its completion ; but if, 
 in addition, the said person was to be removed to a 
 distance, so that the artist could only see him with a 
 powerful telescope, and had to follow his motions as 
 well as he could, and be content to catch a glimpse of him 
 crossing his field of view now and then, according to the 
 humour and good pleasure of the said individual, I 
 think the worthy limner would begin to think that after 
 all there was some difficulty to contend with. 
 
 Now this case I conceive to be an exact parallel to 
 that of drawing living objects with a microscope. 
 
 Their incorrigible restlessness so baulks and baffles the 
 artist, that he is frequently compelled to lay down his 
 pencil to regain his lost temper, and fresh courage to 
 proceed : in many cases his best resource is to study the 
 object till he has got all its features by heart then to set 
 them down on paper study again, and gradually cor- 
 rect them : by the time he has made half a dozen rough 
 sketches he will get pretty near the truth : he may then 
 commence a regular drawing. I can safely say that I 
 have drawn many of my objects five or six times over 
 before I could arrive at my portraiture. I have heard a 
 great deal about shutting one's eyes after having made an 
 observation, and drawing from the impression left on the 
 retina. This, I suspect, is better in theory than in prac- 
 tice, and it is evident that, unless the object is stationary, 
 
EXORDIUM. 235 
 
 the last impression must be so confounded with the 
 preceding ones that no distinct image can be left. 
 
 But it will probably be asked, why I did not kill my 
 object before I drew it, instead of giving myself all this 
 unnecessary trouble ? I answer, that I never could make 
 a drawing to my mind from a dead aquatic larva or 
 insect. Such is the extreme delicacy of their organs, 
 and so rapid their decomposition in water, that long 
 before a drawing can be executed, the main and capital 
 parts of the internal, and even external structure, in 
 most subjects, will become confused and unintelligible. 
 If they are removed out of the water, a still greater 
 change is effected by their drying : their colours vanish, 
 and their whole appearance is totally changed ; to say 
 nothing of the fact, that a dead aquatic insect so loses 
 the peculiar distinctive character derived from its 
 favourite attitude and position, that though the com- 
 ponent parts might, in some instances, be correctly given, 
 the tout ensemble would be scarcely recognized. Some 
 of the proportions length of the body and legs,&c. may, 
 ho\vever, be measured from the dead subject ; and this 
 help I have always had recourse to when I could spare a 
 specimen for destruction. All attempts to preserve these 
 creatures in spirits, turpentine, &c. are useless, from the 
 excessive corrugation these liquids produce. I also made 
 various attempts to confine them in a narrow compass, 
 so as to prevent much latitude of motion ; but was com- 
 pelled to desist from the project, finding that they got 
 
236 APPENDIX. 
 
 into constrained and unnatural positions, and injured 
 themselves by struggling. 
 
 At the period the drawings were made, it is proper for 
 me to observe, that the happy method of preserving 
 aquatic insects in Canada balsam or varnish was not 
 discovered; still, however, a live subject is far superior 
 to a dead one, under any circumstances whatever ; which 
 proposition I consider so self-evident that I shall not 
 set about proving it. 
 
 1 suspect that other individuals have met with the same 
 obstacles as myself in making drawings of living micro- 
 scopic objects ; for I think it may be affirmed, without any 
 illiberality, that, with very few exceptions, those of my 
 predecessors are proportionally more rude and incorrect 
 than those of any other subjects of natural history what- 
 ever; their various lineaments and features being fre- 
 quently false, and exaggerated to a degree amounting to 
 caricature ; nevertheless, they have been handed down 
 from one bookmaker to another, ever since the days of 
 Swammerdam, as if they needed no improvement what- 
 ever. I have made my drawings at that period of the 
 growth of the larva or crysalis in which I thought it 
 made the best and most interesting object, and that they vary 
 greatly in their appearance, according to the degree 
 of maturity to which they have arrived. There are also 
 many varieties, very closely resembling each other, which 
 it is not very easy to particularize ; but I am quite con- 
 fident, that when the genuine object is procured, in the 
 
EX6RDTUM. 23/ 
 
 state in which I drew it, the correctness of my execution 
 will be recognised. 
 
 The specimens which I have selected are those which 
 experience has shewn to excite the strongest emotions of 
 pleasure and satisfaction in the great mass of observers 
 of all ranks; whom I have always found to be most 
 delighted by comparatively large living objects, seen with 
 medium powers. In fact, they seem to afford the same 
 sort of gratification with a menagerie of living wild 
 beasts on the large scale ; and most certainly many of 
 them wonderfully emulate the ferocity, voracity, cunning, 
 and cruelty of the mammalia. They prey on each other, 
 and fight with a degree of determined obstinacy not 
 inferior to that of any beings whatever. They have like- 
 wise a thousand diverting pranks and humours, quite 
 peculiar to themselves. In addition to these amiable and 
 amusing qualities, they possess such a high degree of 
 transparency, that their unique and beautiful internal 
 machinery is as clearly perceptible as if they were made 
 of glass ; so that, without any dissection, we can unravel 
 all the mysterious workings of their nature ; such as the 
 circulation of the blood, the pulsations of the heart, the 
 peristaltic motion of the intestines, arid the play of every 
 muscular fibre. This property of transparency is not 
 possessed by any other living beings with which I am 
 acquainted, except the animalcula infusoria. I may 
 observe, that no perfect insects present so many facilities 
 for adaptation to the microscope, or can be so easily 
 preserved and managed ; so that the larvae, &c. appear 
 
238 APPENDIX. 
 
 to me peculiarly and exclusively devoted to the conside- 
 ration of microscopists. 
 
 Their natural history will be given by Mr. P., with 
 every particular likely to interest the reader concerning 
 their peculiar habits and instincts : such as their mode of 
 locomotion, and of taking their prey ; the food which 
 they most affect; their relative dispositions towards each 
 other, and other beings with whom they associate ; with 
 full directions how, when, and where to procure them ; 
 and how they may be preserved in health and vigour for 
 observation, &c. &c. 
 
 I do not feel myself called upon to state more con- 
 cerning the nature and object of this work ; and 1 humbly 
 hope that truth and justice to myself and partner, and 
 our joint publication, does not require me to have said 
 less. 
 
 I shall conclude this introduction by a vindication of 
 microscopic science and its votaries, from the aspersions 
 which have been cast upon them by the inconsiderate ; 
 many of whom have been pleased to assert, that micro- 
 scopes have, of late, received a degree of patronage from 
 the most illustrious and distinguished savans, to which 
 they are not legitimately entitled. Were they applicable 
 to no other purposes than the dissection of blackguard 
 vermin, the observation of stinking ditch-water, or the 
 amorous passions of ants and worms, I should, perhaps, 
 for argument's sake, admit that they were but the tools 
 of a puny, pitiful pedant, whose passions and amusements 
 were of a trifling, if not of a degrading complexion : but 
 
EXORDIUM. 239 
 
 I would ask whether, in the hands of men like Bauer, 
 they are not applied to the development of the most 
 curious, important, and interesting details of anatomy 
 and physiology, which, without their assistance, could 
 never have been known ? and whether the finest and 
 most delicate parts of the structure of animals, in their 
 extreme penetralia, are not rendered equally intelligible 
 with the coarsest and most evident parts of their fabric, 
 by means of these instruments ? 
 
 None are apt to treat microscopists with more contempt 
 than some astronomers, and even mere star-gazers. 
 
 I shall always possess the most profound veneration 
 for astronomy, as the most sublime of all the sciences ; 
 but star-gazing is a distinct department, though fre- 
 quently confounded with it ; and is, in my opinion, little 
 better than downright microsophizing. To the telescope 
 certainly belongs the inspection of the great and sublime 
 works of the creation ; to the microscope belongs the 
 petites and beautiful ones : if the former shews us the 
 world above, the latter exhibits the world beneath us. 
 
 I must here positively chant the vast superiority of a 
 microscope, as a hobby-horse, over a telescope, at least 
 when the latter is used merely as a star-gazing instru* 
 ment. Let us suppose you have, at a great expense, 
 procured a very fine telescope. How many hours in 
 the year will you be able to use it ? Not above one 
 hundred, says Sir W. Herschel, even if you lose not 
 one in other avocations. How many colds and quinzies 
 are you to get by exposure to the night air, when you 
 
240 APPEN'DIX. 
 
 might be much better employed in your warm bed ? 
 How many disappointments are you to endure from 
 finding the wind suddenly shift into the wrong quarter, 
 after you had counted on a good night's sport ? How 
 many scoldings and curtain lectures from your wife ? 
 After all, when you have seen all the double stars, clus- 
 ters, nebulae, within reach of your instrument., together 
 with the planets, mountains in the moon, and spots on 
 the sun, &c. together with any curious terrestrial objects 
 about your habitation, and exhibited the same to your 
 friends, what remains but to dispatch your charmer up 
 to the garret, as of no further use ? You may, certainly, 
 on high festivals, transport it down again, in order to 
 convince people what a prodigiously scientific personage 
 you are, even to be able to possess such an instrument, 
 and to know the right end of it from the wrong one. 
 
 A microscope, on the other hand, is available at all 
 times and seasons. There is no end to the number of 
 objects it may investigate. In the solitude of the most 
 dismal prison, you would only need a lamp, a microscope, 
 and some vegetable infusions, to furnish you with an 
 inexhaustible fund of amusement, totally independent of 
 other external objects. When employed in researches 
 on animal and vegetable physiology, where are your in- 
 vestigations to terminate ? Your life will not suffice 
 to study sufficiently the wonders of the minutiae of 
 natural history. 
 
 The supreme Author of Nature has been pleased to 
 bestow so exquisite a degree of finishing upon 
 
EXORDIUM. 241 
 
 many of his works, that they can be only appreciated 
 by man with the assistance of the microscope. Surely 
 he who is but a work of God may be allowed to admire the 
 works of his Creator, without incurring derision or ridi- 
 cule, even though they are minutiae. Trifles are said to 
 take only with frivolous minds ; but minutiae are not ne- 
 cessarily trifles, as it will be easy to prove. It is not 
 only, in my own opinion, unscientific, but even swinish 
 and ridiculous, to contemn any thing merely on account 
 of its minuteness. To say nothing of the hackneyed ar- 
 gument, that greatness and littleness exist only by com- 
 parison, I will ask, if the Automaton Chess-player had 
 been made on a scale of l-20th of an inch to a foot, or 
 even much less, it would, in consequence, become despi- 
 cable as a work of art ? 
 
 Suppose some individual, greatly distinguished by his 
 talents in ship building, in making astronomical instru- 
 ments, or steam engines, &c., was also to evince a passion 
 for making minute automata and watch- work, such as 
 tarantula spiders, minute singing-birds, musical seals, or 
 even such curiosities as a coach drawn by fleas, &c. 
 would it shew good breeding, or good taste, to despise or 
 ridicule his minute labours, while we admired his 
 grander and more imposing works ? 
 
 Now it does appear to me, that the Supreme Being 
 does, in some sense, resemble such an individual ; for 
 his power loves to display itself in every way in which it can 
 be displayed, whether upon the minute or grand scale, in the 
 creation of animalcules, as well as of fixed stars and I 
 
 R 
 
242 APPENDIX. 
 
 cannot help thinking myself, that those who spurn and 
 scoff at the minute works of God, he they what they may, 
 while they affect an admiration of his great wonders, are 
 guilty of a species of impiety, and must be either liars, or 
 hypocrites, or fools. 
 
 Men are perpetually wondering what can be the use of 
 bugs, and fleas, and wasps, and such kind of vermin, and 
 speak of them as absolute blots in the escutcheon of the 
 Almighty. The use of these little insects is surely to 
 teach man a perpetual lesson of humility. He is extremely 
 apt to fancy himself the only being of real importance in 
 this planet, and that every thing in it has been made for 
 his exclusive use and accommodation : whereas a very 
 little consideration must teach him that the said fleas, and 
 bugs, and wasps, &c. are intended to enjoy themselves in 
 their own way just as he does ; that is to say, without 
 greatly considering the convenience, comforts, or happi- 
 ness of other beings, I consider it as certain that bugs 
 were intended to prey upon man, as that man and the 
 other pi'Bedaceous mammalia were intended to destroy 
 the weaker animals. If a bug ([ beg pardon a cimex 
 lectularius) could reason, it would probably suppose that 
 man was of no other use in the creation than to prepare 
 its habitation and supply its food ; and would think that 
 no better evidence need be adduced to prove the immense 
 importance of a bug than that such bountiful provision 
 had been made for it, &c. 
 
 If mere utility is to be made the standard of excellence, 
 what a large part of that of most of the sciences consists 
 
EXORDIUM. 243 
 
 in their effect as counterpoises to superstition and barbarism ! 
 and in this point microscopic science surely has its voice 
 among the rest. All men are apt to despise their neigh- 
 bours' pursuits, and to dignify their own as the only ones 
 of real importance and value : but while the use of the 
 microscope is sanctioned by the examples of POND, of 
 AMICI, of WOLJLASTON, of HERSCHEL, and of BREWSTER, 
 who disdain not to relax from their severer studies in 
 such pursuits, he must be very hardy and fastidious in- 
 deed who dares to deride and condemn them. 
 
 Great disgrace has been brought on microscopic 
 science by the manner in which it has been perverted to 
 the support of preconceived opinions and hypothetical 
 views, as well as to a spirit of wonder-making. I hope 
 that a new and golden age of observation will now 
 
 commence. 
 
 C. R.G. 
 
 MARCH 1837. 
 
244 
 
 No. 4. 
 
 Observations on Swammerdam's method of Dissecting and 
 Preparing Objects for the Microscope*. 
 
 Referred to at page 121. 
 
 IN the preparation of objects, no man was more success- 
 ful or more indefatigable than Swamrnerdam : in minutely 
 anatomizing, in patient investigation, and in curiously 
 exhibiting the minute wonders of creation, he stands 
 unrivalled, far exceeding all those that preceded, as 
 well as those that succeeded him. Deeply impressed 
 and warmly animated by the amazing scenes that he 
 continually discovered, his zeal in pursuit of truth was 
 not to be abated by disappointment or alarmed by diffi- 
 culty, and he was never contented till he had attained a 
 rational and clear idea of the organization of the object 
 whose structure he wished to explore. 
 
 We have only to regret that we are ignorant of the 
 methods he employed. To discover these, the great 
 Boerhaave examined with a scrupulous attention all the 
 letters and manuscripts of Swammerdam, and has com- 
 municated the result of his researches, which, though but 
 
 * Adams's Essays on the Microscope, p. 134, from the Life of Svvammer- 
 dain, by Boerhaave, ed. 1758. 
 
OBSERVATIONS ON DISSECTING, ETC. 215 
 
 small, may enable us to form some idea of this great 
 labourer in the field of science. 
 
 His chief art seems to have been in constructing very 
 fine scissars, and giving them an extreme sharpness; 
 these he made use of to cut very minute objects, because 
 they dissected them equally ; whereas knives and lancets, 
 let them be ever so fine and sharp, are apt to disorder 
 delicate substances, as in going through them they gene- 
 rally draw after and displace some of the filaments. His 
 knives, lancets, and styles, were so very fine that he 
 could not see to sharpen them without the assistance of 
 a magnifying glass ; but with them he could dissect the 
 intestines of bees with the same accuracy and distinctness 
 that the most celebrated anatomist does those of large 
 animals. He was particularly dexterous in the manage- 
 ment of small glass tubes, which were no thicker than a 
 bristle, and drawn to a very fine point at one end, but 
 thicker at the other. These he made use of to show and 
 blow up the smallest vessels discoverable by the micro- 
 scope to trace, distinguish, and separate their courses 
 and communications, or to inject them with very subtile 
 liquors. 
 
 He used to suffocate the insects in spirits of wine, in 
 water, or spirit of turpentine, and likewise preserved 
 them some time in these liquids ; by which means he 
 kept the parts from decomposition, and consequently 
 collapsing and mixing together ; and added to them such 
 strength and firmness as rendered the dissections more 
 easy and agreeable. When he had divided transversely 
 
246 APPENDIX. 
 
 with his fine scissars the little creature he intended to 
 examine, and carefully noted every thing that appeared 
 without further dissection, he then proceeded to extract the 
 viscera in a very cautious and leisurely manner, with other 
 instruments of great fineness; first taking care to wash 
 away and separate, with very fine pencils, the fat with 
 which insects are very plentifully supplied, and which 
 always prejudices the internal parts before it can be 
 extracted. This operation is best performed while the 
 insect is in the pupa or chrys.alis state. 
 
 Sometimes he put into the water the delicate viscera 
 of the insects he had suffocated ; and then shaking 
 them gently, he procured himself an opportunity of 
 examining them, especially the air-vessels or trachaea, 
 which by this means he could separate, from all the 
 other parts, whole and entire, to the great admiration of 
 all those who beheld them : as these vessels are not to be 
 distinctly seen in any other manner, or, indeed, seen at 
 all without damaging them, he often made use of water, 
 injected by a syringe, to cleanse thoroughly the internal 
 parts of his insects 5 then blew them up with air, and 
 dried them, by which means he rendered them durable, 
 and fit for examination at a proper opportunity. Some- 
 times he has examined with the greatest success, and 
 made the most important discoveries in insects that he 
 had preserved in balsam*, and kept for years in that 
 
 * It is probable that the beautiful plan now adopted of mounting and 
 preserving insects in Canada balsam is the same as that here alluded to ; 
 notwithstanding it may be considered as a new discovery, because the 
 
OBSERVATIONS ON DISSECTING, ETC. 24J 
 
 condition. Again, he has frequently made punctures in 
 other insects with a fine needle, and after squeezing out 
 all their moisture through the holes made in this manner, 
 he filled them with air, by means of very slender glass 
 tubes j then dried them in the shade ; and, last of all, 
 anointed them with oil of spike, in which a little rosin 
 had been dissolved, by which means they retained their 
 proper forms for a long time. He had a singular secret 
 whereby he could so preserve the nerves of insects that 
 they used to continue as limber and perspicuous as ever 
 they had been. 
 
 He used to make a small puncture or incision towards 
 the tail ; and after having gently and with great patience 
 squeezed out all their humours, and great part of their 
 viscera, he then injected them with wax, so as to give 
 and continue to them all the appearance of healthy, 
 vigorous living creatures. He discovered that the fat of 
 all insects was perfectly soluble in oil of turpentine : 
 thus he was enabled to shew the viscera plainly, only 
 after this operation he used to cleanse and wash them 
 well and often in water. He frequently spent whole 
 days in thus cleansing a single caterpillar of its fat, in 
 order to discover the true construction of this insect's 
 heart. 
 
 His singular sagacity in stripping off the skin of cater- 
 pillars that were on the point of spinning their cones, 
 
 account given by Swammerdam is not sufficiently explicit to point out the 
 mode of applying it. 
 
248 APPENDIX. 
 
 deserves particular notice. This he effected by letting 
 them drop by their threads into scalding water, and 
 suddenly withdrawing them ; for by this means the 
 epidermis peeled off very easily ; and when this was 
 done he put them into distilled vinegar and spirit of 
 wine mixed together in equal proportions, which, by 
 giving a proper firmness to the parts, afforded an oppor- 
 tunity of separating them, with very little trouble, from 
 the exuviae, or skins, without any danger to the parts ; 
 so that by this contrivance the pupa could be shewn to 
 be wrapped up in the caterpillar, and the butterfly in the 
 pupa. Those who look into the works of Swammerdam 
 will be abundantly gratified, whether they consider his 
 immense labour and unremitting ardour in these pursuits, 
 or his wonderful devotion and piety. On one hand, his 
 genius urged him to examine the miracles of the great 
 Creator in his natural productions ; while on the other, 
 the love of that same All-perfect Being, rooted in his 
 mind, struggled hard to persuade him that God alone, 
 and not His creatures, was worthy of his researches, 
 love, and attention. 
 
 THE END. 
 
 WILSON AND SON, PRINTERS, 57, SKINNER STREET, LONDON. 
 
Preparing for the Press, 
 1. 
 
 DESCRIPTIONS OF VARIOUS APPARATUS 
 
 FOR ILLUSTRATING THE 
 
 INTERFERENCE OF LIGHT. 
 
 2. 
 
 IM AIL 
 
 ILLUSTRATING THE 
 
 WONDERS OF THE MINUTE CREATION; 
 
 of tfje 
 
 BY ANDREW PRITCHARD. 
 
 " By the beauty of the creatures, proportionably, the mightiness of the Maker of them 
 is seen." Wisdom of Solomon. 
 
 THE design of this Work is an attempt to elucidate those parts of the creation, 
 which from their extreme minuteness are known to us only by the aid of the Micro- 
 scope. So manifold and various are these productions, all equally wonderful and 
 beautiful in their kind, and alike indicative of the wisdom and power of " the Maker 
 of them" so full of interest is the study of them, that there is no branch of Natural 
 Philosophy to which our attention may be directed with a greater promise of instruc- 
 tion and delight. 
 
 Whilst the press absolutely teems with Works explanatory of organic and inorganic 
 bodies, might we not expect, that the Illustrations of these Works would chiefly result 
 from the personal observations of the Authors ? Such, however, is not the case. 
 Strange to say, they are chiefly copies. A series of original Drawings, therefore, illus- 
 trating a Work of this description, taken from the finest specimens of Microscopic 
 objects ; such as the minute structures of the animal creation the respiratory apparatus 
 and various locomotive, manducatory, visual, natatory, and other organs of Insects 
 portions of bone, skin, scales, &c., &c. the structures of woods, plants, recent and 
 fossil, ferns, mosses, algae ; also, the manner in which the circulation is carried on within 
 them the curious mode of aggregation in divers geological productions ; and a mul- 
 tiplicity of other highly interesting matters, it is thought, will not be unacceptable to 
 the public. 
 
 From the number of years the Author has been occupied Collecting his materials, 
 and the thousands of beautiful preparations which have passed through his hands, no 
 ordinary opportunity has been afforded him; and it is with this belief, that he now 
 comes forward with a Work on this branch of Natural Philosophy. 
 
 London : Whittaker and Co., Ave Maria Lane. 
 
ADDENDA TO LIST 
 
 CONSTRUCTED BY 
 
 ANDREW PRITCHARD, 
 
 !L[l m A M OJ) F A T 00 K d ^ 
 162, FLEET STREET, LONDON. 
 
 1. PRITCHARD'S Apparatus for illustrating the Interference of Light, with 
 
 Telescope and 100 Screens ............ 15 15 
 
 2. - ditto, with 50 Screens ...... ^ . . . . 10 10 
 
 3. - ditto Pocket Apparatus .......... 086 
 
 4. - improved Eye-piece Polariscope, with Reflector . . 220 
 
 This instrument exhibits the colours more intense, and the rings better defined, 
 than the Slider Polariscope, invented some years ago by Mr. P. and now generally 
 used. It can be applied to any compound Microscope. 
 
 5. - improved Polarizing Apparatus for the Microscope .. 1 11 6 
 
 By Mr. P.'s new arrangement of the Prisms, the field of view is greatly extended, 
 and any eye-piece can be used. 
 
 6. - ditto ditto large Prisms and a double image one . . 440 
 
 7. The Apparatus 4 and 5 fitted to Microscopes .. .. .. .. 2126 
 
 8. Stereoscopes ................ 0150 
 
 This curious instrument, invented by ProfessorWheatestone, illustrates the pheno- 
 menon of single vision with two eyes. 
 
 9. Heliostats, with spring Clock, for fixing the sun's image in any position 880 
 10. Pritchard's improved Gas Microscopes . . . . . . <18 to --- 
 
 1 1 . - ditto ditto with polarizing and interference Apparatus --- 
 
 12. - - new portable Gas Generators and Holders . . . . 880 
 
 This apparatus generates with one charge only sufficient gas for several exhibitions, 
 and retains it any length of time ready for use. 
 
 13. Prism Telescopes for shewing the lines in the Spectrum . . . . 770 
 
 14. Pritchard's Vertical Microscopes, single, doublet and triplet 5 5s. to 10 10 
 
 The doublets and triplets are all constructed with thin piano, convex lenses, and 
 not common globules. 
 
 15. - Achromatic Microscopes [see Microscopic Illustrations] 
 
 16. Improved Magic and Phantasmagoria Lanthorns. . \ 12s. 6d. and 3 13 6 
 
 Sliders of Astronomical, Natural History, Costume, and Comic subjects. 
 
 17. Selenite Ornaments and Wedges for polarized Light .. .. .. 076 
 
 18. Pairs of Lenses for exhibiting Newton's coloured Rings . . . . ' 10 
 
 19. Nichol's improved dumpy single-image Prisms, each .. 7s. 6d. to 14 
 
 20. Pritchard's new Fine-Adjustment Microscope Bodies .. .. .. 1 11 6 
 
 This construction is simple, more precise in its action, and of more general utility 
 than any contrivance yet invented for the purpose. 
 
 21. Microscopic Objects, Is. each and upwards, or in cases at 6s., 15s., 21s. and 560 
 
 These objects consist of thin sections of Recent and Fossil Woods, Coal, Jet, Char- 
 coal, Oolites, Flint,Teeth, Bone, recent and fossil. Insects and Dissections preserved in 
 Balsam. Zoophites, Ferns, Algse, Fuci, Mosses, Shells, Scales, Ditto in Flint ; Fossil 
 Infusoria; Crystals, Madrepores, Sponges, Tests, &c. These are prepared in a very 
 superior manner, and many of them cannot be had through any other source. 
 
 22. Specimen Cases, containing Examples of the above, each 8s. 6d. and 15 
 
 ACHROMATIC AND REFLECTING TELESCOPES. 
 
 OPERA GLASSES, Single, Double, and Solid. 
 SPECTACLES IN EVERY VARIETY OF MOUNTING, WITH CRYSTAL AND CLASS LENSES. 
 
 BAROMETERS AND THERMOMETERS. DOUBLE IMAGE PRISMS. 
 
 FRESNEL'S RHOMBS. BARTON'S IRIS BUTTONS. TOURMALINES. POLISHED CALCAREOUS SPAR. 
 INTERFERENCE MIRRORS AND PRISMS. 
 
 antr ii-Hcrctfcogu ^pparatutf of all fctntttf. 
 
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C .R.GORIl'G M.D. 
 
 
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