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Lea diagrammes suivshts illustrant la m^thoda. ita lure. 1 J 1 2 3 4 5 6 I'. A rRKAllSK 7^ ON LIGHT, VISION & COLOURS; (jOMrm«iN(j A THEORY ON ENTIRE NEW PRINCIPLES, DEDUCKD, BY OIU'.AT CAUh. AND STUDY, FROM COMMON NATURE, EXPLANATORY OF MUCH I'HENOMKNA NOT BKFORK EXPLAINED OR U.N DEH STOOD. BY THOMAS BRETT, ESQ OOUNVY OF PRBL, C. W. Wh. '4 TOllONTO : FEINTED AT BLAOKBURN's CITY STEAM PRESS. 1858. M««aMMiikiM fore CoI< PREFACE. It is with extreme diffidence that I venture to lay be- fore the public a book of philosophy on Light, Vision, and Colours. Entered according to Act of the Provincial Legislature, in the year 1858, by Thomas Brett, in the Office of the Registrar of the Province. world, without one dissenting voice ; and that the opinions T have here expressed are in diametrical opposition to those I have before mentioned, it is only reasonable to an- ticipate an amount of opposition from those whose inter- ests and prejudices are affected by improvements and strongly protected by wealth and talent. Under such circumstances, I find a greater demand upon my courage than I ever before experienced ; but when, on the contrary, foi Cc '„. I .Th. iU wor nen and laur wor] I h thosi ticip ests Btror circu than PREFACE. It i, with extreme .Uffidence that I venture to lay be- fon, the pubhe a book o.^ philo«.phy on Light, Virion, ,„d ■ Under all eircumstance,, a gencrou, and enlightod pub- 1.C W.11 speedily diacern the „erit. or defeet, of a work ..o.atted for their eon.idoration, and an author may HI..7. e.-p.ct a candid opinion will be given thereon, but ;'"" ' ""'*' *"=" "■« «»bj-t before me h„, oeeupied tl-oc,.».derat.on of the greatest talented philosopher, the wor d ever produced, and that the opinions of those emi- nent per«,ns have been sanctioned and recorded, and again and aga.n conarmed by all the learned and intelligentTom- .u«mt,es an every country and kingdom throughout the world, wthout one dissenting voice; and that the opinion, I have here oppressed are in diametrical opposition to those I have before mentioned, it is only reasonable to an- fcpate an amount of „pp„,itio„ fro,„ those whose inter- ests and prejudices are affected by improvements and strongly protected by wealth and talent. Under such c.rcumstances, I find a greater demand upon my courage than I ever before experienced ; but when, on the contrarj- It. PRKKArE. I Ionic at thft iraportancft of the (lubject that htM engaged my attention, and fcol a oonviotion of the purity of the motives which hivo imluGjd im todavolop;) tho truth, with, ftlsu, a firm e.jnviotion that certain phenom-na in light and vision oould not be aoouuted for under cithm- r)f .tho pre- Bont existing thcorion; and that scienci! in that depart- mont must remain .stationary until a complete revolution in the theory of li;^.ht and vision could bo effected, X come to 'the resolution that nothing should deter me from ondoavonring to add my mite to the improvement of scioncp, by endeavouring to correct tho numerous misun- dnrj^tandingi that abound in the minds of millions that are itt extreme anxiety upon i subject that has never been satisfactorily explored or explained. When 1 take iuto considoration that vrhcro there h such an amount of learning' and wealth, there must also bo an amount of indupen(l)nce, and that in most ins^anoe.-i it requires le.s3 labour to establish truth than it does to maintain errors; and also, that I believe there i« in exi.stcnco a ola.ss of ^2mm% both wealthy and loarnod, who.=je whole aim is dir.jted to philo^jphrc pursuits, and whose ambition is to arrive at correct notions, and whoso honour and in- te-rity are onl> equalled by their dis-usfc at mjan jvctions. I therefore feel a consolation that the merits of this work arc under better protection than wore the works of Galileo and Copernicus in their days. ■ ^ • ;'iir,PAvery by the Author arc tho following • l8t. That .the eyes of animals produce genuine ligit a2 ' fi. Pwr.>.\rD!. and tlint tyulight jMirtaktw of ail thu prop.rfiiw or qualifi cation.s :i|»|H'rfaiiiinK to fhu oth-r li^htoCtim luiivorjio, rmoh nn rt'flcction and rcfriuaioii 2nd. Thii» Vvj^ht .loos not bririL' thn imngoa of objoctii to our eye* to k* a. it., but lluil the uye with vlnion oxti;n«I«d. takes perception of objoctn in thiar roHpootivo loculitiw, 3rd. That he han nolved tho (juention amongst optical writors, why w»! do not leo double in as much on wo have two eyes. 4th. That ho has solved the qucntion amongnt optical writers, why wo Kt^n thin^ft in thtir natural .-^tate. 5th. That jr taking perception of objcotj. 12th. That ptTooptionH of aistant objects art obtained by a union of viBJon, which I term umversai eye-sight. I3th. That ifu'cnu' -^ lighl is capable of passing through tho different lenncs 0*";^ iclcacopo, it in not capable of faking an imago with it through thonj, f.nd that if an image could bo tnkcn through thou- it would be so mutilated *>y the process, that it would not ha known or identified 14th. That Koeiuer, the Danish astronomor, bailed to di«Jcover tho tnictjauBo of tho lateness of the appearanwi of Jupiter's satellite, after it emerged from behind the planet. 15th. T'.iat Dr. Bradley failed to discover the true cause of the aberation of the stars. U)th. That ho gave a correct cxpliuiation of what has been called tlio phenomena or illusion ol* the lenticular sterco.scope, lately brought to a hi-h state of perfection by 8ir David Brewster. 17th. That he explained the true cau^c why wc can see a coin lying at tlic bottom of a bowl, by means of pouring water into the bowl, although wc could not see that same coin without the use of the said water. Vlll. PAEFACE. 18th. That he satisfactorily demonBtiates and explains, by the use of a convex lens, that the eun, the moon, and the principal stars, do not send their image with any light of iiicirH Ui the eye. r t Sti-a I>«in The The i L!gl B«t]< E^Al CONTENTS. SECTION I. Pa** Strange Appe«r«ncefl.— Incompetency of Light to cowTey ImftgHi of Ob- jects to the Eje.— Ocuiar DemonMtrations upon Vision.— Mystery. The Ctiuse why we do not see Double, and why we aee things In their Natoral States 13 SECTION II. DflDJODstration on Beflecfcd Light by a Looklnp-glaBS and r-andle.— Obeer- ▼atlons on tha sanio may be denionstruled by the San, Moon, or Stars.— A man in front of a Ivooking-glass.— Experiment upon Vision by the use of rolt?B.— ObaervatiuDB thereon, 20 SECTION III. The cause of Light— It ia produced by many suhetances — Genuine and Ck)mpoiind Light.-— Ita subatiincei not the quickest traveller, U it car- riee Image* with it.— Error and miannderstandlng about Light, 2S SECTION IV. The Fieetnetsof Electricity and Light 99 SECTION V, Light dc<;s not rebound like a Ball— Reflection and Refraction explained.— Rough frurfrtces do not Jicflect Ligh.— Light has three qualific»tion», all eeeeutial to Vielon, 85 SECTION VI. B* (lectors and producers of Light. — Experiment on a Oaa Barrol. — Ex- pt'rlment on a Lamp in an Orchard, 88 SECTION VII. \, ^ E'Afr— a BuflBciency for all purpose*, 40 X . CONTENTS. sf:ction VIII. PA»r. f h« ^lar Sjstem.— The Sun and Moon causa Tides, and in th» same man- ner they form Day and NigLt. — The Snn and PlanetH illuminate !!pace between them 42 SECTION IX. The Eye. — The opinion of Optical Writcrn. — Th» Author's opinion (iifftrent. Description of the Eye.— Obgervatlons.— The Cornea.— The Vitreous Humor and Convex Lens are powerful Reflectors, ftl SECTION X. The Vision contains all the Media between the eye and the Object.— An Image is something.— How Po'^ceptions are had.— Vision is influenced by Gravitation.— Tho Cornea ia the seat of liring Light.— The Crystal- lino Ijeus.— Vitreous Humour.— Agneous Humour. — The Media in the Night a good Conductor. —The Media in Day-time a bad Conductor. ••-Keason, 5fi SECTION XI. The Lenses iu th» interior of a Telescope. — Passage of Light through thorn. The reader is requested to form hia owu opinion thereou.- The Agent that passes through a Lens can have but one Substance.— ObserTationa on aTeleacope, M SECTION XII. The Abenatiou of th»» Stars.— Uoemer and Dr. Bradley at fault,. 66 SECTION XIII. Tho lenticular Stereoscope. — A description of the lenticular Stereoscope as brought to a high state of perfection, by Sir David Brewster, and pub- li.shfd iu London, 69 SECTION XIV. 31ore observations on Reneition and IJofraction, 79 SECTION XV. An extract from Publication. — Comment thereon.— The MastBVDiece 82 CONTENTS. XI. i»r. 42 51 SECTION xvr. Paoe. CoJouw in tl.o Atm,«p)H.,v.-n,e Prism uud Sir Isaac N,.wt«n.~(>i,inio„ thereon.~n.o Atmosphoro contalnfl a variety (^f Fluids—The Fluids we earthy productions from opaque and transparent substances- Colours aro sereii In number, exclusive of whifo—Eye-light and tho light of the Uulverso destroy all Colours—Names of Colours ern.nc- ously arranged— Colours can only be seen in certain degrees of Light respectively— The Prism as a magnifying power— Observations .ipon Window— The light of the Eye obliterates lino Shadows— Blue ether can bo looked through to a great distance, and White may be discerned by certain means— Shadows projected by Wiiidow llfirs «(fi 55 «0 66 69 79 SECTION xvri. Production of two Shadows— The Umbra and Penur^bra.-I£ow porceiveil. Observations on the .Shadows and Prism.-Tho llainbow— The Solar Spectrum, 99 82 I I i I A T II i^ A T I S E 0.4 LIGHT, VISION AND COLOURS. i SECTION I. fitrai.;?o Appearances. Incompetency of Light to convoy Inmgei of Objects to the Kye. Occular Demonstrationa upon Vision. Mystery. The Cauao why we do not see Double, and why we see things in their Natural State. It has always appeared strange to me that images of objects should be brought by light and placed upon the retina of the eye — particularly so — when we consider that the eye has a better view of objects when stationed in a dark place and viewing any object in the light, than it would have if stationed in a light place and viewing objects in the dark. Suppose a man standing at the bottom of a well, he would see a thing more distinctly at the top of the well than he would see the same thing if he was stationed at the top, and the thing he was looking at were placed at the bottom. It appeared equally strange to me that the image of a star or planet shovild bo brought, and partly conducted; B 14 A TREATISE ON LIGHT, through a telescope by lij:;ht, and placed between the eyc- lenH and the licld-lena there to be viewed. But, above all other considerations, the strangest appearance was that light .should bring to my eyes the image of any shadow whatever. If I look at the moon, T behold the different Hhadows projected by the hills and mountain.^ upon the moon*H disc ; so long as those shadovv.s are screened from the sun'.i rays wo see them, but, no sooner do they become exposed to the sun than they are obliterated by the sun's brilliancy and rendered invisible by the very same body that we are to consider the fuithful bearer of all images to the eyes. It ylso appears very strange to me, that modern pliiloso- phy should attribute to light a regular progessive motion, sufficiently correct and undeviating as to be used as a standard for several purposes of the greatest nicety, and to lay it down as a rule that its motion is not instantaneous, as the ancients considered it, but that it progresses regu- larly about 192,000 miles per second. Appearances so at variance with my understanding, and apparently so with nature, created in me a desire to de- velope the truth, and unravel the apparent mystery con- cerning them, and, to accomplish the desired object, I considered it most prudent to consult common nature and begin at the bottom, making sure that, if I could but follow nature through all her intricacies and windings, I should not be led astray. Thus, taking nature for my guide, I resolved to throw off all prejudices and former iuipressioris that might influence my consideration, and thereby enable myself to look common nature fullv in the fat-e. I resolved to com- 1 t VISION AND COLOURS. 16 1 I menco thinking in earnest, and after thinking, considering and comparing, I at length r.rrived at certain conclusions, and, by degrees, I discovered certain criteria which, though humble in their appearance, afforded me substantial assistance, because founded upon nature. If we examine the capabilities of light we find it in- competent to the task of transporting or conducting the images of objects to our eyes, for both light and image must, of necessity, pass thousands of miles transversely through all the light that is moving out of the path, or orbit, of a planet or satellite that it might be despatched from or proceeding to. I cannot see how an image or a shadow, or the image of a shadow, or any other faint substance, could be conveyed to such distances through so much light without being ob- literated, nor through so many atmospheric fluids without interference ; and though light itself mighu find its way through tranaverse currents of light, its own thinness of substance, and its imponderability, would greatly assist its passage or admission, yet, it would undoubtedly have no chance of carrying images with it. Where, I would ask, is the consistency of considering that light brings the images of objects to our eyes, as millions of us gain perception of objects at one and the same time from all parts of the universe. How could light answer so many demands from numberless eyes and how receive our requisitions for dispatch of business ? Light would require a previous communication from the eye, to enable it to answer so punctually all the uncertain and irregular demands with such amazing dispatch, millions of millions of images to be dispatched momentflrilv ! 16 A TRKATISK ON LIOIIT, besides all this, lijj;ht would rocjuire life ; no inanimate fiubsfancc could pcrfunn tin oiTico, whoHO duties rcijuire so much disorction iind punctuality. The more I tiii»»k ofnuch things, the more unreasonable does it appear that light should travel undeviatingly 2,880,000 ujiloH, in fifteen seconds, leaving the imago of Jupiter'H satellite; such an unerring performance to be required, of such an unsteady lluid, is too unreasonable to be entertained. Suppose I am looking across a field : I see a tree ; if I shut my left eye I see the tree with my right eye, i*^ I shut my right eye I sec the tree with my left eye ; but, when I open both my eyes, I only see one tree ; now, as the right eye viewed the tree it must be evident that vision unites the right eye with the tree, and that vision must have been on the right side of the nose, so, as the left eye viewed the tree, it must be evident that vision united the left eye with the tree, and that vision must have been on the left side of the nose. If it was, as is generally believed, that light brought the images of objects to our eyes, it would undoubtedly have brought me an image for each of my eyes, and I should have received as many impressions with my two eyes, con- jointly, as I received with my two eyes individually; and, in case of viewing a plurality of trees, I should have received two perceptions for each tree j but, be there many or be there few, 1 only receive one perception for each tree or just the same )\umber of perceptions with both ray eyes as another man receives who has but one eye. It seemed so mysterious to me that a person who had but one eye should gain as n:any perceptions of a certain number of trees as I could gain with both my eyes; the i VISION, AND COLOUR.s. 17 I! the plscnoiuenou seeinccl to ovcrpowor uio, and I found my curiosity nrouscd, and having recourHO to a book of Phi- losophy upon the subjoot, tlio author's account only added mystery to mystery, it was there said, *' As the image which is formed by a convex lens, is inverted as ropecis ob- jects, so muht the inia«;o which forms at the bottom of the eye. It has therefore been a «juestion amongst optical writers why wo gee objects in the natural position, and also, why we do not see double, inasmuch as we have two eyes. Various explanations of tlieso facts have been offered chiefly foujidcd upon optical principles, no"c however ap- pear to have given general satisfaction." 1 therefore nuide it a particular portion of my ewfiuiry, and, by a series of experiments and a deal of consideration, I plainly discovered that light did not bring images of ob- jects to the eye, but that the eye, with vision extended, tak perception of objects in their respective locality, and this is the sole cause why we only get one perception for each object with both eyes, or in other words why wo do not sec double, inasmuch as we have two eyes. It must be evident, that if the two eyes send forth a stream of vision each, and those two streams after passing the nose, unite and form but one stream, the amount of both united will only be one stream and can take but one perception in the locality of the objects, but far different would it be if an object had to send its image to the eye, iu that case it must either send two whole images in the the same manner that it sent them individually, or, two halves of an image, which p occss it never performs. It is therefore very clear why we do not sec double. And the same may be said about images being formed at the b2 18 A TRKATISE ON LIOIIT, bottom of the cyo, ntul why wo Heo thingH lu their natural position. The cause of this i« simihir to the former. There \h no image fonnod at the bottom of the eye, and the eye by ex- tending its viMion to the objeet, »cvn things in their natural position, and also in their riglit phiee. When I look with only one eye at an object .singly, that one eye occupio?* nearly all the npaco between it and the object, and when I look with both my eyes, they together only occupy the same space as the one did; therefore, it is clear to be understood that if perceptions are taken by a mass of vision, leaving t.io eye never to return, only one perception can be had of one object at one time, anv more with two eyes than one ; but, if light brought the images of objects to the eyes when both were in readiness to receive them, i^ certainly would use both eyes alike and give each nn image. I will not trouble the reader with all the expedients I have had recourse to before bringing myself to the afore- mentioned I onclusions, suffice it to .say, that I have fully satisfied myself, and I will endeavour to satisfy the reader with more facility and less labour. To avoid misunderstandings, it is neces.sary here to mention that it is not contended that the light of the uni- verse does not reflect or refract, but it is strongly denied that light brings the imagesi of objects to our eyes by any progressive motion. It is also denied that light has any regular progressive motion; and it is also asserted that there is no need for such a regular progressive motion ; and it is also asserted that the eye is a producer of light, and by it, united with the light of the univer.se, takes pcrcep- tio frw thi ob shi VISION AND COLOUBft. 19 tioDf of objects in their roKpoctivc iocalition, and that tho light of tho cyci poMOSSOM tho qualitiuN of reflection and ro* fraction us all other lij;;ht iloos, and to tho f^eneral liu^ht of tho univcri^o may he attributed tho illutaiuation of all objoctH, and rendering them viMiblu with tho cxcoptiouM of 8ha'4, which li^ht renders iuviBibic. :' t 20 A Tlir.ATlMK ON MailT, SECTION 11. DflinHtiMtrAtliin t>n lti>i:(. ObarrTfttiunt on tUi* Mi«nii> iiinjr )m« i* 'iii<>tiMtruei-/ the 'tin, M(N>n ur HtHr*. A mitn in front of m l^uoklnKK'-*"*' Kk(i«rim< nt ii{k>ii Vitiun by tlui iim of 1\>U i, ObMr- VAliun* titorvon. It huH always been considered that when a person looks at n mirror he nee.s the imago of Home object that liglit han brought to the ghtss and reflo'^'t* haek to tho cyo, but after various considerations I lind that liglit neither brings the iunigo of objects to the glass, nor reflects them back to tho eye, it merely illuminates the object, and the light of the eye, performs that office of reflection at the glass, and falling back upon the object where it takes its perception. Suppose I take a looking-glass and lay it flat upor. a t? blc, and place a candle on the opposite side of tho table, at some distance from tho glass, and somewhat elevated above it, and then place myself on the other side of tho table in such a position that I can clearly see by reflection the flame of the candle by looking at the surface of the glass, and then I procure some person to assist me. First I shut my left eye and direct my assistant to mnke a mark upon the glass where the flame of the candle is seen by my right cyo] ntist. I hold my head steady and shut my right cyo, and divt cf uiy issistant to make a mark upon the glass whore the flame '^f the candle is seen with niy left eye. Next, with a steady head and both my eyes open, I toll luy assistant to make a njark whore the flame of the candle ^ seen with both my 0}es, having done thus, I perceive t Vlfinv AVI) rui/»URl. 2i ^ tho threo ntrkl upou tito "InM* will corro MpuuU with my Hfo ey:n and the conti 0 tho flame Hoea on rho glans removed from placp to phice, whiltst all the time tho candle ronminod Ftationnry, and all tho motions of the flame eorre8p'.*icled to tho motions of tho eyclii'H ; in sueh case Hurely it muf** be that tlio light of tho ovo proceeds to the glass and is reflc(t«;d Loiu thence to the candle where perception is had. Similar oxporiuients raoy be made on iLo*siin, in the day time, by ♦he use of a looking-glass, and on tho moon or t'ao stars, by a pool of vuter in tho night. They will all currob.»rato the fact that light docs not, by either direct approach or by vcHection, pcrfoni'. any otfice of bringing imago.«» of objocts to the eye, but thj't cye-liglit i.h entitled to the credit of taking perceptions of objecls in their res- pective locality. Try another experiment. Let a man place himself in front of a looking-glass and ob.scrvj what he sees whilst his eyes aro directed to what arc ci\\\»d their images. The vision of the eye, in this case, passing to the glass, must dir(!Ctly meet the vision that is returning from tho glass, and, if any image or substance, whatever, was returiiing from tho glass to tho eye, it would cause i':terference with the vision pa.ssing from tho eyo to tho glass, and no perceptible substance could be there without being per- ceived ; but there is no such inte.fereuco, all is calm 22 A TREATISE ON LIGHT, and clear"' perception, and the, so-called, images appear to bo as far in the rear as the man is in front of the glass, although the glass itself is, perhaps, no more than the eighth or tenth part of an inch thick. The only conclusions we can possibly draw from this, are, that the apparent distance in the rear was the real distance in front of the glass, and that every particle of matter between the man's eyes and the glass, whether atmospheric uir or fluid contained therein, formed part and parcel of his vision, that was completely doubled about at the glass and continued back to the eyes, and that the so- called images are the eyes in reality. If I take a thin rod or pole and set it up about two feet distant before my eyes and place another pole at a consid- erable distance, say a furlong, in front of the first, and then arrange myself behind the first pole, so that the centre between both my eyes is in line with both poles, then, with my left eye shut, the vision from my right eye will pass the right side of the first poll and spread on both sides of the distant pole to a great extent, and then, with my right eye shut, the vision from my left eye will pass the left side of the first poll and spread on both sides of the distant poll to a great extent. Thus the vision from both my eyes will cross over and intermix with each other and pass through each other, and unite with each othor, yet they do not incommode each other, nor in the smallest degree interfere with each other, there is no obstruction, nor is tht, vision of either eye int. "fered with by any image intended to be conveyed by any mean? to either the one or the other of my eyes, but the vision used by one eye is also used by boih my eyes. t VISION AND COLOURS. 23 SECTION III. The ca„«e of Li^Lt. It Is pro.Jucci by many substances. Gnnnino and Componn.l Light Its SubHtance« not the quickest travollor, if it cnrri.H In.Hgcs with it Error and miBunderstauding about Light. Light is the brilliancy or mere shine of bright substtmces, whether transparent, opaque or in a fluid state, as a diamond or other bright gems, polished steel or iron, or other smooth sub.stances, or water and other liquids, or the dif- ferent atmospheres of the sun and planets with their components, such as fluids and vapours that float in them, so also the ejes of animals, and, in short, everything that shmes and will produce light in an otherwise darl? place, may be considered as a producer of light. 4 ,| . '^'^ ^^^^'^ ^^ produced by so many substances, and has its abode amidst so many vapours and fluids, it is very natural for us to err whilst considering if to partake of so many dif- ferent colours; tlicre can be no wonder that we have what IS called white light, red light, blue light, and many other coloured lights ; but, although light and, colours may per- chance bo produced by one and the same parent, and at one and the same time, they are not one and the same th'ng, for light is imponderable and invisible, whilst colours are both ponderable and visible, anu ihe propei lies ot light and colours are as much opposed to each other as the.r weight and visibility are difi-erent, for light illuminates and shines, whilst colours cloud and darken the appearance of ^any object. It should therefore be borne in mind that ^ i„ me kinds of vegetables are debarred from compound light, containing all its adulterating colours, they will loose or exchange their colours; such as celery, endive, lettuce, and cabbages, if either covered with mould or tied up close will grow white, and the green will forsake them. I think we ought to impute this exchange of colours to the exclusion of those colours that associate with light, and not to light itself. Genuine light has no colours either to withhold or bestow. We also err when we suppose that the gay colours of 26 A TRKATISH ON LIGHT, the rose ami other flowers reeeivo their colours from light ; as light nlways has its associates convenient, they, by their capabilities of coloiir-givi!istancos that are heavier than the atmosphere, are pressed towards the earth by the weight of the atmosphere, and all those substances that are lighter than the atmosphere are, by its weight, pressed upwards from the earth. Now, if light did not possess the power of evading the pressure of the atmosphere iu some degree, or by some means, it undoubtedly would be poised up to the top of the atmosphere, or farther from the earth tlian any fluid that possesses more substance than itself; but the flncuess of its substance, giving it a chance to remain below, and taking shelter within the particles of other fluids nearer to the earth, helps it to avoid the pressure of the atmosphere, and the universal attraction, which is everywhere by its nature, may always retain light as a companion. I therefore think, we may fairly consider that we believe the cause of their fleetuess is owing to their subtilty, that renders them free from the pressure of the atmosphere and the annoyance by fricti* n. These two fluids, possessing so much freedom from the , annoyance of friction and the pressure of the atmosphere, ^ and also possessing the power of passing through other fluids, a^" almost free agents, and they must be at times more in readiness to move when an impulse is given than t^'^ grosser fluids are; and there can be no doubt, but, 32 A TUKATIHC ON LIGHT, that u« electricity movo.s more quickly through <,'ooiurfuoo is cullod tho line of n'flot'tlou. No .tccuunt has ever been ^ivcu about tho light of tho oyo, in fact it has never boon considered as possessing tho same qualities and properties as other light does; but if wo consider that tho light of tho eye possesses all tho (juali- tios and properties that other light docs, (which is certainly tlio case ), when wo arc looking at a candle by the help of a looking-glass, the lino of incidence for the light of the candle is the lino of rofleefcion for the light of tlic eye, and tho lino of incidence for the light of the eye is the line of reflection for the light of tho candle, consequently both the lines aro lines of reflection light. Had the writers upon light possessed the necessary knowledge, that the eye posscised all those qualities and properties, and as the light of the eye and the light of tho candle, in passing to and from a looking-glass, must pass directly through each other in consequence of tlie fineness and softness of their pi..perties, so light is the least capa- ble of all substances to possess a sufficient o-'iantity of :M A I Ki \ i I ^^ "N i.inriT, elanticity io urontr* n rrh«mii«l win n fulling upon ii refli'ct- ing iiurfucc. Hoforc a poMoo can wf these lomonu, byriijth tors are ' degree ) it ever j;ht of a he light ure that •efloctor, the ap- ng light ) the re- ctcd ray u^h. efleetiiig lange its of light one line eturning ctly back / I V h Whtn t ray of li^ht is made to full u|Mtii a rffl«*ctor oh- liquely, tliut portion of the ray that falls nn the reflector is chanjfed in xhape, and the refractor's Il^^'ht nhiuing ihroujih thin lijiht in itsehanged nhape, takes it* dli»;>.lIon, accordinjr to the naid shape, and a line of refle-tion i« pro- duced according to the line of incidonco. It folK»WM, that the more obliquely the light in allowed to fall upon a sur- face, the nu>re ji« Ua shape changed upon that fturfaco, and the retlet'tor again shining through thai ehanp d si ,pc, Bcndrt forth itn reflected ray accordingly, which will bo e<|ual to the incidental ray; ho, in all cascH, the tllrection of the inc'dcntal ray forms the Hliapo of light upni tho surface, and by the shape of light upon tho surface the reflector forius tho reileetod ray, and in all caMCrt the ar.gliir* of incidence and reflection are according to the hliapo of the lii;ht lying upuii the surface at th«». angles. As the angle of the incidental ray U increaHod or de- orciirtcd, 80 the light at the angle is chang<^d in whape, and so, also, is the rttleeted ray increased or decreased; the reflecting .surface j»erformH its duty by shining thn,ugh any bright substance that falls nn it, and tao powers of sliinii g arc according to sihape. Ai it is with reflection of light, so it is with rol'raction IS in passing through the diflerent media, light changes its shape at every surface it has to pa hi. n.atcriNls of h.'iit, and liL'Itt coiituinnd in hin atnioHphoro. IIo fliil n..t object to the sun being conniilercJ a gJubo of tire, m the iin<'i(>nts coriMidcr.-d it; neither have any of tlie PhiluBophers that have loUuwtd in his train attempted to do 80, but time nnd comniou scdhc have uimost eradi- cated the absurd notion about the sun beiujr a globe of fire, without the assistance of highly cultivated Philosophy. I shall hereafter endeavour to explain and Hupply the deficiency I have «poken of as wanting to show how the planets are moved round in their orbits by th.^ j^ avit-.tion of the fiun, j.nd how the planets reciprocate and move the «un, in return, on hin axis, and how the sun and planets, by a system of reciprocity, cause each other to form day and niffht for themselves with their respective materials.- It is not eoiiHJsf.eut >vith the an.Munt of wisdonj bestowed upon the creation of the s.Iar 'system, that a globe of fire •hould be placed in its centre, HF.d that globe composing more than (..ne half of the systen., for the purpose of warmino. nud lighting all the planets of that systen.; par- ticularly so, as they are situated at sucli a variety of dis- tances, without any regaid to tbdr size. Were the .sun u .«!ubc of fire of his dinjcnsions, and tha planets were de- pending on hiu, lor light and heat, we might rcasonablv expect that the sn)all planet Mercury, in his nearness to the sun, wou'd either be njelted to a fluid or calcined to a .Mnder: while the large planets, Herschol and NepJune, at their immense distance, would be froze into a solid ball of ice. As our nature leads ua to admire the beauties of the creation, so it also leads us to censure any obsurditiea. that may be discovered that are calculated to deform it. ▼tHioN AND ror.oiras. 48 If wo cnaso to consider the sun a« h p^lobo of firt% our cw- rioHity in excited to know whiit he i.s couipMud of, for cer- tain ho \A n gh)bo of Moiuo sort, auil uiitil wc can, by any meaiiH prove to (ho contrary, wo ought to cormider him a globe of materiul.H, like any other gh)bc of the solar sys- turn, and j.urijtkinir of .siniilar uaturi', jiropertics and qualitioH. If carefully exnniincd, tlio «un ro^yuible.s the other pluiicts of the system in their motion'^, aud iiccording to the uuiverHjil law of gravitation, his councctiou with the other parts of the «olnr ayateui prove him to pohsichs some of our nature, and, as he contains more than one tialf of all the 8- .stem, it mu.st be an idea appertaining to the ex- tremes of absurdity, to suppose the sun to be in anything, or in any wise, diftereut from the remainder of the sys- tem, composing the minor half; that \m motions are like the planets, — as he move», they move, — as he turns round, they turn round ; he has spots on his surface, as all the planets have, and he shines, as all the other planets shine, aud he proves himself to govern the planets, by the same system that the planets govern him, and they together govern by a system of reciprocity, under the general or universal law of gravitation. As the sun and the planets govern by a system of reci- procity, under the universal law of gravitation, bc they prove themselves to posses^ natural principles, alike to each other; and it appears to me, that the sun in his bril- liancy, does not exceed the brilliancy of the earth, more than his bulk exceeds the bulk of the earth, and if it were possible that an eye could be placed without the limits of the earth's atmosphere and all her fluids, it 4(; A TUKAXiSfcl UN LIGHT, Would not bf un)io dnzzlctl .than bj looking; at any other planet. Wiuit itiakcs thu Mun bri>;htor tliuti nii) othur planet is, thut ho, by his groat jjowoih of uttrai'tion, nlwiiyH buM tho ^routuHt portion of tho fluidH of our utni(»Mi>borc on tho ttidu next hiniHcIf, and wo huvo to look tluouu'h them when wc look at tho sun. The oxplutiation \ promised to ^ive, to nbow how, by frravilation, tho planets are continued niovinu' in their oi bit.s, is us follows : — It ill ki that smooth ball, [)la('ed upon a smooth horizontal surface or j)lano, re((uiros but littlo force to drive it along and tho smoothor tho bull and plane the loss force is reciuired to keep it in motion ; but as an horizontal piano gradually leaves tho burfaco of tho earth, in tho same manner that any straight polo would leave tho surface of a round tub or barrel if laid aeros.s the round part of it, so u ball rolling across an horizontal plane must, necessarily, as it leaves the round shape of tho earth, become more and more impeded in its passage by gravitation, because it is going further and further from the centre of the earth ; whereas, if a ball were so placed as to roll in a curve line round the earth, it would not be so impeded in its passage by gravitation, as it would always keep at tho same distance from the earth. Now, suppose it were possible that we could put a flat glass belt, in the shape of a hoop, round the body of tho earth, and thia belt was made perfectly smooth and exactly round, then if a glass ball was made perfectly smooth and exactly round, and placed upon the belt, we might in such :i case, consider that tho powers of gravitation would be pqual on each side of the ball, and by its lying upon a circle so i ii VISION ANU riil.uURS. 47 j»mo(tth and ho tMjui-distnut from tho contro of tho earth, Riid tho H'noothiit'M of tho Mirftiee, wouhl ft!mo«t annihilate tho friction hetwecn them. In such a Htiito nntl position the most triflir)^' force wuuld be re<{niro(l to put tho ball in motion, and still 1(*sh, if any at uU, W(uiUi bo required to keep it ^'oin;.'. This would bo oomin^ m near aH possiblo to a perpetual motion, and would as near m poj^sible re- Homblo a planet in its orbit, only tho planet would have no belt to roHt upon, and consequently would bo entirely froo from friotion. Tims, then, a very smull anumnt of force is Hufficiont to keep a planet in motion, and wo have to point out, nocordinir to our opinion, -whoro that small amount of force is to come from. Tho sun beinir ronsid(M-ed a :,dobo of materials like th » earth or iiny other planet, must possess an immen.se atmos- phere, loaded with fluids, a.s all the planets are htaded, I'roduc'-.g heat, light and electricity, but this largo atmos- phoro may be spread over his surface. There can be no more fear of aniuuils being scorched beneath it th.m there is of tho animal.^ of tho planets. They can be as well sholtored under a largo atmosphere as n small one. Whilst we acknowledge a system of gravitation between the sun and all tlie pbr.ots, it is only reasonable to admit a system of gravitation between the suu and tho different atmospheres of the planets, or between the sun's atmos- phere and tlio planets' atniosphero" it is the same. Tho quick-moving flaids will be fir.^.t attracted and soonest affected by it, ai'd it is understood that tlio powers of at- traction aro at one iime positive and at anotlier t'me nega- tive or ropulstive, as the bodies it is acting pon aro more or less loaded with electricity. So, on tho morni'^ir side 4h A THRATiift ON hUlHV . ui' the oarth, ai HuiiniM)» ilta fluiilH in tint KfcmoHphere will be leitN )ouUt;«J, iiimJ coiiK(!<|iicotly in more rcadinrns to bo uttrucU'd poftitivcly, whereby the earth will b<; {tiiDrd round in \u:t tiiurnal motiun ; and, on th« «venin{^ «idc of lht3 cMirth, ut Munsot tbu fluidH urn b/udud witti electricity, Mui tbnreroie uo^^utively nttrafted or n^pulMfd; fo by tbfi ihornln^ uttruotion tho HuidH iu tlie iitniuNpluTO are pulled towurdtt tbu .sun, and by the ovunib^ uttructiou tho fluid* in the titiiionpherc are rcpuined und i'urccd from tho t»un| thurt th<; earth is round in bor orbit und tb(; light- jriviny fluidH aro attracted and kept conntnntly on tho sid© uext the sun, hence wc hnve day und ni^iit with our own inutcriulM of lii^ht und b«Mt by tbo iiclp of the pow«»rB of gravitation or attr.ictiun of tiu; Mun. And wUilHt w** aro ptininji oCtb daynand nigafs by boinj: rolled round tlie nun in one your, by thy nuiuo power of ottraotion tho li^ht-plvinJ; fluids in the sun's atuuitiphere Hre all l\i\fi time influenood and pulU-d round bin body by the {^ruvitutioa of tbo eartb, whereby a great unoiunt of liplit and heat must be ujovinjr round the huh an the earth moves round. This process of influence is HuITicient to uiovu the brilliancies in tho suu's atinOHphere to a j^rcat extent, and tliereforo a j;reat body of lii^ht and heat are kept on that ^idc of the sun next the earth, the accumu- lation of v.-hi;.h, iui;^'ht produce a summer HcaBon on the «un'H disk once a year, and always leaving behind it a great npace for winter c for other planets, each to form a Bcason likewiHO, and thus tho phmets situated us they are in the ecliptic, some more oblique to tho sun than others, would each of them form one summer round and upon the gun's body, consequently, a variety of seasons would be k'^ V JNIOV AVTi fiiToffflg. 49 41 p«rfornM.d by tho iiitn.n.ni pi,,,,, ts accnrfiin- lu their poni- fcion tu thf mill »nriod« of rov..luti«,n , ami m th.^,- Muiaons would not (K!|»onU upm any oF tlic plunetr* tor nupplicw r.f lij^ht Hiid hotU. but only U|n)ti thiMr n.s«iMtaiu;i. in tho oxchan^o of pravitation, mo «ach planet mi^jht cuuho h mimn, aco<,rdin>r to the m.v and tho p^Tiod of itH annual revolution, t.xt«Midin,< f is ni tho Hun'H rquator towurd.s uHch of his p^.lus, accord inu- t. .ho position of each planet to the nun, whilnt eudi planet would enjoy « day and u ni-.'ht for oach diurnal m^riod upon ir> .sxi.s. It dec;, upponr that a syHtcni of reciprocity htiwci r. the sun and the planets in natural, whil.st wc take a natural «urve> ..; it, and that it is as essential an it is natural to the convonionco of life upon the suuh di.^o, and that he should pos^seas a variety of 8cii«onH, sf, that th(. fluids con- tained .n itM atmosphere should bo divided, an a .neann of prcfc viiig a moderation of heat, instead of bavin.' the whole drawn toKcther i,.to one (lintrict, wlureby tlu. animal y vej^otableereatior. might be .scorched in one sectio,. and froze in another. If we remove the ab.urd notion, that the .un rurninhea the planetH with light and heat, and consider that our in- tercourse with the .un in by a recipn;city of gravitation only, what then becomes of the system of radiation of light from the sun ? It is true we obser^'e the shadows pro- jected by straight lines, but none of those lines can extend any faj-ther towards the sun than the opa-iue ])ody tliat projects then.. It seems contrary to our system of nature t.mt so tine and subtle a body as light is, should, by a;, v moans, be transmitted from tho surface of the atmosphf c E 50 A TREATISE ON LIGHT. to the i^urfaco of the earth. It is contrary to the rules of gravitation that a light 8ubstancc whould find itfl way to the bottom or nearest station to the earth. Besides, if light was sent from any body in straight lines to project shadowH, what i.s there to prohibit it from spreading behind the body of a planet, and daniiiglng or obliterating the shadow ; such a fluid as light vould re({uire something to keep it in a straight line, an 1 I know of only one power that can bo considered competent to the task, and that power ia gravitation, which .seems peculijirly adapted to control light substances: and us wc consider the sun as the main source of gravitation, and as we constantly see all the lines by the sides of shadows in direct lino towards the sun, and all the genuine light of thn solar system continu- ally i-ctained in front of the sun, and also in front of each planet ; and, apparently in obedience to both, the sun and planet that arc always appearing between them, and always &( jmpanying the latter in its orbit. When we consider all these circumstances, we can have no hesitation in con- cluding, that instead of emitting rays of light and sending them for our use, he exerts a power directly contrary, and drags all the light to the front of each planet by his powers of gravitation, and leaves the rear of the planets and all other opajeets we cannot expect to find shadows long enough to reach eitlier tlic lens or its focus, and, consequently, we liave to look for other causes than images made of shadows for perceptions ; Init first it may be necesnary to take a g'anco at the construction of the eye. There is, around the back of the eye, a \ery tough coat, called the sclerotic, that gives insertion to certain muscles that roll the eve-ball and c;ive direction to any object. The interior of the sclerotic is lined with a coat almost en- tirely made up of blood vessels and little arteries, which, by their internetting, cross one another in almost every possible direction. This lining is called the choroid coat. The surface of the choroid coat, inside, is thickly covered with a slimy figment of black colour, and tliis again is overlaid by a serous shed called Jacob's membrane ; inside of all this is what is called the retina. These coats form the rear of the eve, and in the interior there is a quantity of vitreous or glassy humor, and in front of this glassy humour is suspended what is called a double convex lens ; in front of this lens is the iris, and a certain opening in it is the pupil ; and in front of the iris is a chamber filled with a watery fluid, and this wateij fluid fills up the space between the iris and tlie cornea before mentioned. Into the rear of the sclerotic and choroid coats, from the brain, comes the optic nerve for each eye, and spreads itself out upon the interior surface of the retina. Here, it is worthy of particular notice, that the optic nerve, coming from the brain, spreading all over the retina, and extending until ' V VISION, AND COLOURS. 53 » tf it unites with the beautiful transparent ahell called the cornea, wliich forms the front of the eye-ball. There can bo i\h doubt but the cornea itself— being of a nervous substance — forms part and parcel of the optic nerve, wisely contrived for the express purpose of forming a seat for the life of the animal, the light of the eye, and the other light of the universe, to unite for the conveni- ence of taking ncrceptions of distant objects. We know tluit in the cornea the most acute sensation of feeling is manifested by the simple touch of a feather, and the perceptions so taken by feeling must undoubtedly be transmitted to the brain by means of the optic nerve, of which the cornea forms all the front section, and is entirely transparent. As the cornea possesses the power of taking perceptions by feeling, and passing them instantaneously throughout the nervous system, as well as to the brain itself, as all other members of the system do so, therefore, it must be reasonable to consider, that the cornea, in taking perceptions by seeing, will immediately transmit those per- ceptions that are taken in the same place by the same route ; and it would be very unreasonable to suppose that when a perception is once within any portion of the ner- vous system — as within the transparent cornea, where there is a massive substance composed of nerves all leading to the Jarain— that it will again leave that mass of nerves to pass through a watery fluid where there are no nerves at all, then to enter the iris and pass through it and the nerves within it, then again to pass through the convex lens, and on leaving the lens to pass through the vitreous or glassy humor ; and after all it would have to get into the optic nerve at the back of the eye on the retina, without the 54 A TREATISK ON LIOHT, benefit oi* h transparent [)i)rtal for its entrance, as it does not appear that the ojitic nerve is any where provided with the necessary accommodation to receive perceptions by sight witli a transparent entrance, only at the cornea, which forms the front section of the optie nerve, and is united with it by transparent fibres. The reader can judt'e for himself which paHsa.<:;e is the most practical and the most reasonable of the two now mentioned for perceptions. The vitreous or glassy humor, in the interior of the eye, seems wisely constructed for the pur- pose of supplying the lower lens with genuine light, and the lens itself seems as wisely adapted lor the very neces- sary and useful purpose of a powerful reflector, it being constantly supplied with living light; and the glassy hu- mor and the lens, in junction, send their livinj; litjht throuo-h the iris into the transparent seat of vision, where life and the light of the eye are seated, in junction with the other light of the universe. As it can be satisfactorily demonstrated by various means that the light of the eye is capable of obliterating a faint shadow, and as nothing but light is capable of accom- plishiTig such an act, so it may be considered that the eye is constantly emitting light when uncovered by the eye-lid, and it would be extremely unreasonable, and in fact unna- tural, to consider that the eye which oblitorates shadows could, at one and the same time, emit light from the eye and receive a shadow or image into the eye. 'J'he idea is . contradictory oi' itself. V VISION, AND COLOIJRH. 65 SECTION X. I The \Mon containn lUl tho Media between the Kye and the Oltject.— An Im.ige i.-4 »om«»thliiK.— llow I'erc^ptionu are hiid.— V JNioii is inniiancn.l by (JiHvltiition. — TheCornftft Is the seat of living Light.— Tho Cr.VHtallin.; I^n» —Vitreous Humour.— AKneouH Huniuur.— The Alodia in the Night u good Conductor.— The Media in Da.v-tinio a bad Conductor.— RfnHon. The viRioii of tho eye ccntains the eye-light, wliich in the natural production of the eye, together with all the media between the eye and the object, whether it be glass, water, atmosphere or fluids within it. The line of vision for the eye is the size and shape of the object at the thick end which is next to the object, and of the size and shape of the eye at the thin end Avhich is next to the eye. The small end of Jie line of vision may be used individually, by its respective observer.^, but the large end next to the object must be used in coLipany with all eyes that are viewing the same side of an object at the same time, and therefore the range of vision is general and generally used, and the very same particles or fluids that form vision for one eye — particularly when looking at largo and distant ob "is— may be used for vision, or lines of vision, for hiinuicds and thousands of eyes ;hat arc em- ployed looking at one and the same object at one and the same lime, therefore the whole universe is one commoii joint-stock of vision for the eyes of all persons desiring to make use of it, and may be considered tlie greatest joint - stock the world possesses, and the most commonly used ; aud therefore it is no wonder that our curiosity is excited 5G \ TREATISE O.V LIGHT, to (liscovor liow, md by what m'jan-«, vision can iiifurm us that tlicru iire ctiitain iiunieiisc bodies circuhitiii;r in space, in rejiular orbits, and iicrfbiniinj^ certain rcNohuions in certain pcrloda of time. All thi.s inlurniatiun i.s gained by vision, or by the help of vision, and without vision none of it could be Jind ; yet. vision, amidst all its generosity has not satisfactorily revealed to us how, or by what means, itself has been working, whilst such an amount of informa- tion has been gained. 1 think, if our vision had been exerted as rnucli to dis- cover and disclose its own natural way of working, as it has been exerted to discover the motions and properties of other .substances; and assisted as much in the one case as in the other by the ingenious contrivances that have been bestowed upon matters and properties not more deserving assiislanctj. a greater progress would have been made in the right way; but as it i.s, but little is known, and mankind have been content to consider that the light of the univer.se brings imnges of objects to our eyes at the rate of 192,000 miles per second of time, such of my readers as have considered the inconsistencies men- tioned in the former part of this book, and can, at the same time, find courage to discard all former prejudices, will feel no dllliculty in coming to a right understanding concerning the natural workings of vision. The getting into a right understanding is simple when once got out of a wroiig one, but the greatest difficulty is to get rid of pre- judice; and the absurd notion that light brings us an image of every thing we see, must be got rid of. An image must be something, be it ever so smaJl, and if it be small enough to be carried by light, it would be i r i VISION, AND ("OLOITRS. 57 i obliterated by light, and if it be large, light could nut bear the burden of it; besides, any number of images, whether large or small, would ehoke up the small avenue leading through the eye to the brain. The idea is ridiculous; and still more ridiculous is the notion, that the perception of a fixed star can be conveyed to the eye in the short space of the twinkling of an eye. The nearest fixed star considered to be about sixty-two billions four hundred and eighty-one thousand five hundred millions of miles, and to suppose that a perception of a star at that distance can bo brought over that space in that time, by light or by any other means of mere motioi., is mere madness. The vision of the eye comprises all the transparent media before the eye, and must be subject to, and influenced by, the universal law of gravitation. Every large body of light must influence a small body; and as the greatest amount of fluid is, by gravitation, kept on the front of every planet, and light is generally kept in the general stream of gravitation, which is towards the sun, so the light, outside of a planet's shadow, influences and attracts all the light from the rear of a planet, and thereby leaving it in darkness. As the eye is purposely constructed for taking percep- tions, so it is furnished by nature with means adequate to the purpose. The cornea, being a scat of living light, being always loaded, and situated in front of the eye, and receiving life throughout the difiercnt fibres from the optic nerve, leaving the agneous humor immediately behind it, and the crystalline lens immediately behind the agneous humor, and the vitreous humor inmiediately behind the crystalline lens. 58 A TUKATISR ON LIOIIT. The cry.stallino lon^ pcrtbrms the office of a l:i?np, and is 8Upph"(Hl with light matured and conducted to it by the vitreouH fluid. Tlic arjuoous humor in front of the lens, or lamp, maybe useful in straining and regulating the issue of the light from the lenn to the cornea, and the whole of the appanituH Po Kituated in rear of the cornea, mnturo and send their pr<)duotioii>4 of light directly through the load of life and light seated in the cornea, from wl\cnee it emcro^-s and divorgingly spreads upon all objects, parsing through all transpanMit fluids in the atmosphere; but finding a limit at every oparjue substance, it is thereby united with the light of tlie universe, and all the light of the universe is vision, for all eyes generally and each eye individually. All the transparent media may be considered eye-sight, and all the ojwque substances, that limit the range of vision, may be considered perceptions. I have observed that the light of the eye and all other light is under the influence of gravitation. Taking all things into consideration they all conspire to prove such to be the fact ; and the media that light has to pass through must bo considered as a conductor of light, and, like gal- vanic electricity, light has its good conductors and bad con- ductors. The media, on a fine night, when the atmosphere is free IVom vapours and less loaded with bright fluids or brillian- cies may be considered a good conductor. The light of the eye is attracted by the light outside the shadow of the earth very rapidly, as far as the extent of the atmosphere, which, probably, is as far a,s a conductor is required, for the light must then and there be all alike and independent of atmospheric influence ; for, in the space beyond the at- ( VIMIDN ANr» roLorRH, 59 moBpherc, all must be mm vast amount of vision, that extends to and cncloHew mII objects. The media in tlie diiy-tinio in always a bad conduetor for eye-light, for if the utniowphcre iu free from vapours it is alwayH loadcni«a tu ilin iiiti>rlitr of B Tml^vofm.— Viumago >tiii hit owu o|iinloi) tlu-ruuii.— Thti At^nt thM pAfiMM throiigli K I.Huf cnn havo Mit on** HiihNtntn-* — OhMrrvKtloim on « Tulpwiopfl. The IcnwM that form tho interior of a telescope may fairly bo considered U) form part und parcel of the vision of the eye that is lookiiij^ through them. The Hide of the convex lens next the eye extends tlie vision. Home parti- oles of vision pa.ss directly throujjjh the whole of the lenses at the centre, which is called their axis. Some particles of vision diverge and pass through at some distance from the centre, and some diverge and pass still wider, until the whole of the hemisphere of the Icnsc next the eye is formed into vision, extendctl and magnified. Wc will now consider the state and situation of the vision which wc find in possession of the whole of the hemisphere next to the eye of tho first lens, and observe, that as the vision for tlie eye at the first focus advanced by diverging and taking possession of tlu; first hemi.sphere of the lens, so it will, by leaving pos.session of the first hemi- spcrc, converge and advance to the focus of the second lens, and th'ough ♦ru.isparcnt lenses and through itseir; to no other substance than a shine can we ascribe such a ([ualification, and from BO other substance can we ascribe such a qualification, an J from no other substance than the eye (which is expressly eoustruet.od by nature for the purpose) o ii wo expoet such a shine, There is something wonderful in a telescope and wou- fJerful discoveries have been made by it. and wondjrful IV ^ J VISION AND rOLOTRS, 05 goiiius liuM been cniployod to I'lilar-e and rxtoml tliow dis- coverk\s, but the most wonderful of all to mc if, that amidst all tliu IcaniiuLr and iinjiroved talent so cin}iloyed. none liavc noticed tliis sini[)le i'aet, whilst light has so often engaged the attention of so many eminent philosojjhevs. It would appear as tliough this part of the subject of jtiiiloso- pliy had been slighted, and yet I have too great an opinion of the many who have allowed it to escape their iu.)tice to suppose that tlu^y woald allow their curiosity to be limited in any one instance ; but so it lias buen, and that whilst the person was inventing what is called the spider legs and placing thiun in the focus (»f the eye lens lie never thought that the vision in looking by them would have to roam and extend to the object to take a pe'-ceptiou, but considered that every distant object sent its image to keep company with the spider legs of every telescope niade use of to view it; but so it has been, and from that and other events and circums'tances, it appears to me that every invention must be preceded by some striking event, thougi' perhaps very trifling in appearance, to actuate the mind before curiosity can be aroused ; and that genius cannot work without a criterion, and although c jerions are constantly staring us in the face, some trifling event in requisite to make us re- cognize them. It must therefore follow, that philosophy is a compound of the consequents of events and circuiii stances. f2 ^ J fi6 A TRSATISK OW MOHT SKCTiON XII. 'II." .M..-,ati..n of tlM StarJt.-Ronnio. .irvl Dr UrHdloy at f,.ult. h i.s ^^^nenUly considfro.l that tlic abornition of tho Starr* iH a corroboration of rhc prisscnt tlicory of ro.^nhr progi-cssivc motion in Ijoht. Rocnior, the Dani.sh astrono- mer, found a consi^ijucnt in the lateness of Jupitor',H satellite in enier-ino- from heliind tlie planet, where it had been eelipsed. and when he thou-ht he had discovered a rc-ular and steady progressive motion in litrht, he was so far satisfied with it as to consider it a sufficient cause for the lateness of the appearance of the .satellite. Some years after, Dr. Hradley found a conse.iuent in tlu^ abcrrition of the stars, and .uivin- thin-s a hasty consideration, he came to the cfmclusifn that the proo-rcs.sive motion was the cau.^e. The difference of the velocities of licrht discovered by these two per.sons was three tliousand miles per second of time, lloemcr '.'! discovery gives 1 92,0(10 miles per second, and J)r. Bradley's discovery gives 195,000 miles per 8ccond. Had they each of them took more time to have considered the subject, it is probable tliev might have cor- rected their understanding, and finally have arrived at dif- ferent conclusions, which might have been nearer correct, and consequently more satisfactory. It is evident that tliey both consictered that light brought images of objects to the eyes to be seen. This belief was handed down to them from the ancients, and without hesitation thej adopted it and built upon it without sufficiently criticizing; vrsiON, AN'D roLonw. 67 and coiiij»iirin,<>; it. with natiiiv; h.-nrn tho rrmr of hnildini; upon a rotten fonndation. It i8 also evident that they did not know that the eye- sight was LMMiuine light, and that the li-)it of tlie eye and the light of the universe possessed the power of obliterating shadows and shades ol' all deseriptions ; neither did they consider the nature of the duties they were inijiosing upon light, compelling it to carry dark fluids that the brilliancy of light always ruins wlien coming in contact. Neither did either of them consider the midtiplieity of velocities of light that must surround both Jupiter and the earth, by being continually forced out of the paths of those planets, and tluit the motion of ligiit in the conveyance of those images would be transverse to the path of light with its passenger, when past^ing from Jupiter to the earth, r>r from the earth to Jupiter. If Dr. Bradley had rejected the prevailing idea about light carrying images of objects, he might probably have discovered a means of bringing himself correctly to the conclusion tliat the aberration was 20} seconds for all the stars. Surely it is inconsistent to suppose that liglit could bring images from all the stars, whose difference of «listance from us is so great, in the same space of time. Everybody knows that it requires a longer time to per- form a long journey than it does to perform a short one ; but everybody does not know, but a great many persons do know, that there are as many degrees and minutes in a small circle as there are in a large circle, and that the farthest distant fixed star describes the same number of circles round the earth, eitlier daily or annually, as the nearest fixed star does ; and each of these circles, great or ()8 A TREAT ISK ON |,r(}frT small, cohtMiii :{(iO^ jind for dl tiic ht, ir.s i,s 20} Nt'i-onds f "•> 'i>')rc; :,n,l a- the aberration >»' each, and no imro, I think the cuuso of tl.e aborrutio,, of tb. stars is occasioned hy a httlc tinie lost, whilst the eye is lookin^^ outwards for a perception, instead of bein.u- occasioned by lio-ht havin^- to travel 2,880,0(10 miles, to overtake the earth' in her orbit in the short space of fifteen seconds of time. T thereforj icel fully conli.lent that when the true cause of the abern tion of the stars shall be determined, it will have no con- nection with any regular pn.j^ressive motion in liuht. f VIMON, AND 0(>I,oirR8. 69 SKCTFON XIII. / •niP lODfictilrtr H>i.1 rr..^^!or. ,,,,(1 p.,Nish»-l In L<>n(l>>ii. I have copied an jiocoiuit of this iiKstrumerit, togctlior with an explanation of it, given by the publisher, whose notions of vi.sion and perceptions of objects, taken by the eye, arc strictly in unison with the popular notions of the present day ; and as my notions upon similar subjects arc strictly at v.-uianeo with both the author and the public, T have considered it essential, to a riglit understanding, that the whole should be exhibited in such a manner, that the curious may first read the instrument itself in its own na- tural language, then read the author's account of it, and then read my account of it, after which, they can think, deliberate, and decide, between me iind the author, as to which account coincides most with .the natural account given by the instrument itself •' It is fourtcoi\ years since Mr. Wheatstone first gave publicity to his views, on Mhat he termed fomc previously unobserved phenomena of binoculnr vision, that is sight with two eyes. An instrument he had invented, was at the same time described, by means of which, two separate diagrams representing different perspective views of the same object could be made, when viewed in a particular way, so to unite as to form only one figure having the ap- pearance of a perfect solid. Hence the name of the in- 70 •V TRKATISK ON lUillT, 5:'::ir:i;;:,r"" -< - -- -^^^ .0... ,,„, -i;":i:^:::;r:;;:;:-,:;:f'^ "-■■ t)tcn III (•..ii8c.,uoncc of the princiT,lo of ih • discoveries of xXeipco T Ubot I) "'^''/^. '^ '^''^ «^>- ^l^e no sooner diroffor^ f^ +i i ' ""^'""^^n was Slum- 's VIk;0N. ANtI COLOIRB. 7] "", :'"''"";>■ ••^«'«««'-'» '■>■ «hich external „l,j,....s „„ «en ■• In U. earliest f„„„«, ,],„ „,e,-e..»c„,«. .,„„.is,ej ufa box o„to.„„„ two plain n,irrors, about fi,,,.- inehe., ^1,' ^-d «t r„l.t anKle.s .0 eaol, other. („„„, ,., Tl^b ' oppoM te as oorrcHponJin, nnrror, the .eflected iun.-c Hit e}es oi tijo observer TIi...... *i ana, .Rented an apparently «oiM ™,,..anee. to nil U FKuircs, fji.it It scomod incred ble the l-iw*- •• (,'on,bi„in,. theory with praetiee, .Sir Davi.l Brewster Z:r .^";:''"«"^/'"^ ";'1--J >'.>on Mr. Whea.: , e- ■'1'IMi.tu^ , .he nioxt popular f„rn, „f the in.trnn.tnt i. the o«lK, t iMt ,, forn,od or con,«tructecl by „,«.,., of a len, (figure 2.) ; we will refer to it .nore partieularly by a jZ' de then taken up ,„ earnest by an eminent philosophic-,! ;"•; ■■;""""' -'kor i„ ,Vi,, wl.enee it found ., way I l^^-'ountry. at the exhibition in ,851, an., sine'thi 't .a been made ,n London in ahuost every variety „f ..—, and ..old a. p™e.s fr^^^^ as.sL!ed\:'di::r t:-:' f*^""'^' "'" ^""•^"^"''^^ uyaia„iam.s, the diftculties are not entirely re- ■:<' eJ. Kveu the eon.ple.e instru.uent with its appropriate ;:ra:Vi;t''-''-'^"'-'^p^-Miuieefrer: . -- ■ ..n.. .{ >h.y e^pre^s approval or .urpri.«. it seeu.s 72 \ TllEATISK UN LKJIIT. by way of eompliinfnt or t'usjtoiii, iimn than tumi n just appreeuiiioii of tlm eHocts prnduecil." " Lit us try to (ixpljiiii what iiiiiiht not iiiiproj-t-rlv be lurnuMi the opticiil illiHioii of the storesoofK;, that is the inelhoil l>y whicli two cli-.siinilar pictiucH driwii on flat Hurfacc'rt, ami phiceil several inches appart, may be made when viewed with both eyes pcrleetly to coalesce, and to present the appearance ol' only one fi<;ure. not however, any longer a flat surl'ace but raiitsed or depressed in its pro- per proportions, so a.- nut to be distiiiuui^lsable IVom a solid." "11 v.e take a pi(;co of card board, about nine inchc;? t>4uare, and fold it like a book, then net it on one end, with the folded edge towards the observer, the s I, r-,i than wlien both eyes are open." '• Try nnother experiment— to a fixed point, about two fe..'t from the inside of a window of a rooni, attach a thread with a wiiight at the bottom to make it answer the purpose of a plumb-lino, look steadily at the thread with both eyes and get it exactly in line witli one of the upright bars of the sash ; without moving the head, direct the eyes towards the sjish bar. and two threads will be seen, one in its former position and another at the right side of it ; repeat thv es- .. VISION, AND COLOURS, 73 periment, and v/lmi the thread is again in line with the iMh-bar, shut tho left eye, and the thread will appear to change it.^ position several inches to the right fide of tho bar; again, adjust it with the rightfeye only, then j?hut that eye and look at it only with the left, the thread will retain its position ; do the same thing with the left eye, and thin Fhut the right, and tho thread will chnrge its position as before * The distance to which the thu j:d ap- pears to move, dopends on the di^^tanco t.t which the ob- server stands from the window." " Look at an object out of doors, as the nngle or window frame of a building, and try the experimcnis just described, and the plumb-line will appear fifty or a hundred feet to cither side of the central line, according to circumj-tunccs." " These examples may be considered very .simple and un- meaning, but they serve to illustrate the principles of bino- cular vision quite as satisfactorily as if we had recourse to expensive instruments or apparatus; our two eyes, under ordinary circumstances, perform their duties ^o readily, so perfectly, and to ourselves so unconsciously, that we scarcely ever think of them as being in reality a' pair of tlie most beautiful optical instruments ; nor does it occur to us, that each external and visible eye transmits to the Internal and invisible eye a separate picture or image of the objects within its range; two distinct pictures are however admit- ted into the darkened chamber behind the eyes, although * It is probable thai the habit of shutting the left e^e when plumbing objects, might be the cause of this apparent «lifference in the power of the ej-es. The writer is not sure that it exists in all persona alike. UlV 74 A TflKATIHK ON LIUBT. ono picture only is Mx^n. TUu inner aye is ttuppiitiU with UMichiuvry fur making suitable tuijuHtineotH, no that two UiM-8iiiiilur partM or iiuugen of the Maiau objectit arc iiiBtintly combined and converted into one, Notiiing is mure certaiu in followin^i; out the experiment with the cardboard and the phuub-lino, than tliat in looJiint; at an object with \hA\i cycH, there arc parts on the left liand of tlie observer, which it is physically inifiosHiblo nhould be seen with the right eye, and fue rersa. The nearer the object Ih to the observer, the more readily is this law determined ; hence, thcrel'oro, it is plain, that the images or pictur'is received upon the retina, are not exactly alike, and yet they are so instantly and jHirfectly united, that we never detect the difference, and never should know that more than one picture had entered the eyes." " So it is with the ears ; the softest whisper or the loud- est report, whether enterinfj; both ears with e(|Ual force, or one ear with greater force than the other, is so perfectly equalized with the inner ear, that we are conscious of only one vibratory impression, although the external cars are two-fold in their mechanism. If we try to apply these principles to the stereoscope, there will be no longer any difficulty in understanding it." '' Obtain two pictures or diagrams of any particular object — in such forms asrtould present at a certain distance, when viewed by each eye separately, or, in other words, let the picture vary in perspective to a given angle — such pic- tures, if taken for Mr, Wheatstone's reflecting stereoscope, when placed in the instrument and viewed with both eyes, will so unite, tl at a few lines traced upon paper seems to stand out in full relief; two separate pictures of a solid VWIO^f, A!fD nOLOUIlfl, T5 being apparontty changed into tho object intended to be peprcnentod. Nor \b thin all, tho real pioturefl cannot b« aeen in iho plaooH they actually ocriipy, whilHttho combined imago is »ecn at a part of tho inMtriiinnnt where there is no picture of any kind." '* In the lonticulttr (Hir David Brewjoct«; but, in my or ' ' it far as li^'ht and vision aroc^nocrncK'., he hr»left»auch mih for improvoiuout, wIiIIhI giving? a dosoription of ih** ' m- mont, and m;ikin>; obsorvationw thcrw)n. Tho Htorco»oopo is a very plousin;.'; and amunin?; instni- nuiut, calculated to arouKO our curio»ity, increaBO our know- lod.m\ P-.d oorroct our understanding, with respect to the truo thoory of light and vision, whioli cm woll be found out by lir.st discovcriur the workinT of the cy** upon the ma- ohino or by tho working of the machine upon tlio eyes. I shall procoou to make some remarks upon tho author's observation:^, and then leave tho reader at liberty to ooneider and judge for hirsolf lie sayd, that each external or vi5«iblo eye transmits to tho iuterual or invisible eye, a separate picture or image of tho object within ittj range. To allow this, wo must certainly admit, that every entire animul hius at least three eyes, that is, two external and one internal. Ho says, that two pictures are adiuittod into the darkenod chnmbev behind the eyes, fathnu'^h only one picture is seen. IIo has not told us what has become of the other picture. He says, that the inner eyo is supplied with miwjbinery for making suitable adjustmcnte, so that two dis-similai parts or images of the same object are instantly combined and converted into one. •/ than ▼moil A!ft> e^L09MI, n w He Km nnt given uh * denoripttnn of thiM mochinery for convortinj? two objfotH into one, and nii it uurpuMcth my nadcrttunding, I muMt lei it \mm, and loave the rouder U) frame on opinion for himM^U*. Ho nays, licnco thcrctor«, it ia pluin, ilmt tlie iningca or picturca received upon the retina are not exactly nliiie, and yet they are no in«tnntly and fx^rlectly united, that wo ncTcr detcet any dilVurcno**, and never fhotild Icnow that more than one picture had entered tlie eyen. Ho has not told us how ho eame po«<«.»8acd by a know- ledge, that they are m instantly and perfectly united, that wo never detect any difference, nor how he cume to know thnt even one picture hud entered the eye. If wo receive all this as a satisfactory explanation of the stereoscope with the eyes, or the workinj^H of the eyes with the stereoscope, we must consider that the pliLnoinena ih not explained, for we have a greater tnpk to unravel the internal machinery than we had to understand the external illusion; certainly, we hj*vo, by an intricate and circuitouH journey after truth, arrived at some place still farther distant from it, for intricate as is the working of the Btercoscope, it bo- oomcfl totally eclipsed by the internal machinery that unitet two dis-similar objects ko instantly and ko perfectly. He says, nor is this all, the real pictures cannv I be eeen in the places they actually occupy, v-hil't the combined image is seen at a part of the instrument, where there Ls no picture of any kind. If the author would try to correct himself, and diaard the notions about images and pictures, he would soon per- ceive that the two objects taken respectively by each eye, are both actually seen at the places they respectively occupy, o2 78 A TREATISE ON LIQUT, and arc only apparent!; seen where there is no picture of any kind. First, consider that perceptions are taken externally, and blot out of memory all about images and pictures and in- terned machinery, then direct both your eyes to where the mirror meets in a wedgc-likc shape, your vision there falls upon the cd^c of a wedge and is divided in two halves, the right half slides along the right side of the wedge-shaped mirror and j^rocceds by refiection of the eye to the right lens; the left half slider, along the left side of the wedge- shaped mirror and proceeds by reflection of the eye to the left )cus ; the two halves of vision infringing upon their re- spective lenses double back upon their respective objects, where they arc purposely placed to bo seen, and where in reality they are to be seen, and where in reality they ought to be seen, and like all other objects, arc seen in their respec- tive stations, and the combined images will not be seen at a part of the instrument where there is no picture of any kind, but only apparently seen there, that is, at a place where the vision from both eyes separate and make their first bend or reflection. There it is, that things that are seen by reflection always appear to be seen ; but in point of fact, the vision doubles back upon the object and takes its perception, and I consider this to be the law of taking perceptions, be there ever so many bends or reflections in the vision or light of the eye ; at the first bend nearest to the eye, the object always appears to be seen, but in reality it is always seen at its respective station, be it ever so distant. '/ VI8I0N, AND COLOURS. 79 of SECTION XIV. ' More obsorvations on Reflection ani Ilofiii-ticvii. In all oases oi" reflection and refraction, ^' .th tlic light of the eye and the other light of the Universe perform their several offices, by similar means of shining in straight lines as far as the different ledia, through which they pass, will afford them a passage, or until the said media admits a greater or less portion of their light, in such manner as to increase or decrease the amount of light passing through them, and by such means, to produce a change of shape on light, which change of shape, as I have before said, pro- duces a change of direction; and that in vision, every crook or bend in it, may be considered a reflection or refraction of the eye-light, and though there may be a plurality of bends in the vision, the eye-light proceeds from bend to bend, until it falls upon the objects, and whether the ubjcct be opaque or transparent, or fluid, it matters not ; at the first bend, the object is always ap- parently seen, because there is no return of the eye-light in vision, and the same holds good in all cases, whether with the kaleidescojx), the stereoscope, or any other instrument ; it is the reflection of the eye that follows the object, by any number of bends in vision, from bend to bend, or from re- flection to reflection ; when this position of the working of the eye is understood, a correct notion may be formed about light, sufficient. to dispel all apparent phenomena and illu- sions. 80 A TREATISE ON LIOHT, A great misundorstaiiding cxieta In the consideration respecting the diflferont refractions produced in ligh*;. The solution of placing a coin in an otherwise empty bowl, and placing the bowl in such a position to the eye, that the coin cannot be seen at the bottom of the bowl, the edge of the vessel intervening ; in this state, if the eye is kept steady in its station, whilst water is poured into the bowl, the coin instantly comes into view. Now, the consideration in this case is, how is this coin brought into view ? does the light of the universe raise the coin to the top, or docs the vision of the eye, as it unites with the water, swell in extent in the same manner that it docs when it enters any other kind of prism, whatever, whether composed of water or crystal. In all cases, when we look at a lens or crystaJine prism, we water prism, the vision instantly swells or increases in ac- cordance with the nature or shape of the media it so enters. Now, this water in the bowl, that the light of the eye or the vision of the eye has entered and united with, is a medium and composing part of the vision of the eye that has now swelled down to the coin, where it tikes its per- ception without obstructing or disturbing the coin that is lying close down to the bottom of the bowl. I have tried this solution in eveiy possible way I oould think of, with straight viewing and oblique viewing, with strong lights and with partial lights, with opposition lighta and with conjunction lights, and from all I can discover, the light of the -miverse has no more to do with • lan to illuminate, whilst the light of the eye is swelled at the entrance of the wat<,r, as it does swell wh("i it enters any prism whatever. VISION, AND COLOURa. 81 One observation, however, may be made, that is, whilst the coin ia lying in the middle of the bottom of the bowl, it is apparently seen at the middle of the surface of the tho water, at top, and if we can only allow the light of the universe to ftill upon tho water, at one side or point, and at the same time tJlow the eye to pass round at a distance, 80 as to complete a revolution, tho coin is apparently seen from all points of the edge of the bowl, whether looking towards or from the light let in upon it, which would cer- tainly not be the case if lighi had anything to do with for- warding images to the eye. There is no phenomena in solution of this description, a right understanding is all that is req'iircd. and to get at a right understanding, wo must travel by the right road to it. I may be observed, that ttie other light of the universe is subject to enlargement, as well as tho light of the eye, but it must bo ridiculous to suppose that light raises the image of the coin from the bottom of the bowl to the top of the water. Twenty persons may be arranged around the bowl, and each, without difficulty, gains a perception ; but how would twenty images be, sent there oy light, or how could tvrenty eyes be supplied with ar image, a piece out of one solitary coin ; it would seem that the farther wo travel from the natural path the more we are bewildered, and that it conse- quently, follows, that ♦' ; more wo correct our notions by comparing them with n ture, the more clearly will wc be enabled to see tLi"ni,^h what is now considered mystery, illusion, i-nd phenomenon. 82 A TRKATIHE ON LIGHT, SECTION XV. An extract from Pobllcation.— Commont thereon. — The Maattirpioce. The foUowinp; ia an extract from a publication, purport- ing to be a production by Thomas Dick, LL. P., called ** Celential Scenery, or the wonders of the Planetary Sys- tem displayed," and i^ published at Philadelphia, in the year 1853:— At page 227 of the said book is the following, and upon it I have made a t-omment : — " From what we have stated above, in relation to the phases and motions of the moon, it is evident that the moon is a dark body like the earth, ard derives all its light from the sun ; for its enlightened side is always turned towards that luminary. It likewise derives a faint light from the reflection of the sun's rays from the eartli in the same way that we derive a mild light from the moon. And {IS the earth has an uneven surface composed of mountains and vales, so the moon is found to be diversified with simi- lar inequalities. It is owing to these inequalities, or t\c roughness of the moon's surface, that the light of the sun is reflected from it in every direction ; for if tlie surface of the moon were perfectly sn^-joth, like a polished globe or speculum, her orb would be invisible to us, except perhaps at certain times w hen the image of the sun reflected from it would appear like a lucid point. This may be illus- trated by the following experiment: — Place a silver globe, perfectly polished, about two inches in diameter, in the sun; the rays which fall upon it being reflected variously, VISION. AND COLOURH. 89 4 according to their several incidences ui)on the convex sur- face, will come to our eye only from one point of the globe, which will therefore appeiir a small bright spot, but the rest of the surface will appear dark. Let this globe then be boiled in liquor used for whitening silver, and placed in the sun, it will appear in its full dimensions, all over lu- minous ; for the eftect of that liquor is to take off* the smoothness of the polish and make the surface rough, and then every point of it will reflect the rays of light in every direction." Mr. Dick .speaks of the image o*' the sun. being reflected from the moon, would appear like a lucid point. It should be understood that there is no image of the sun, whatever, reflected fi-om the moon to the earth, nor from the earth to the moon. I have not a silver globe two inches in diame- ter to experiment upon, but I have that which will answer my purpose full as well : I have a pair of convex spectacles, which are all sufficient for the purpose that I am about to explain. I will now endeavor to demonstrate what T have, in different parts of this book, endeavoured to explain, that is, that eye-sight is eye-light, and eye-light reflects and refracts as tdl other light does, and eye-sight, by reflection, lakes iKjrceptions of objects in their respective localities,' and the other light of the universe, commonly called sun- light, only illuminates objects and renders tliem risible. Let a person take any convex glass—an eye-glass, the top of an opera glass, or a glass from a pair of spectacles, either will do very well ; let him hold the convex glass to the sun as he would hold any other mirror to enable him to see the sun by reflection, whilst shutting one eye, let him look upon the surface of the glass with the other eye 9i A TREAT 1 8E ON LlfJAT, and he will perceive two bright lucid spots upon the glaw. Those two npots, may, by a movement of the glaas, be made to move round each other at any distance to the extent of the glass; they may be made to approach each other, to eclipse each other on the centre of the glass, and again to extend from each other to the extremities of the glass. When these two spots appear to eclipse each other, it is then only that they have any connection with each other, and it is then only that there can be a misunder- standing about which is and which is not the incidental or the reflected ray between the light of the eye and the light of the universe. It is only when those two spots are in that one p<:iBition upon a convex lens that the line of re- flection for one might be taken for the line of incidence for the other. Were a person making use of a plane mirror the ease would be very different, as every incidental ray from the eye might be taken for a reflected ray from the light of the universe, and every incidental ray from the light of the universe might be taken for a reflected ray from the light of the eye. We are tiicrefore indebted to tlie convex lens for preventing us from falling into errors that we would be liable to Ml into whilst making u.se of a plane mirror. If we examine these two spots, one of them appears as though it floated upon the surface of the glass, and, from its position and circumstances, it must be formed by the lio'ht of the* universe, in a diiectiou from and between the gun and the spot itself. The other spot does not appear upon the surface of the glass, but has the appearance of being ft long diatanQe through th^ glass j and from its posi- tion and cirfiumBtanoea itmuat be- formed by the light of VISION, AND COLOURS. d I -, the eye in a direction from and between where the gpot itaeU* has the appcaruiice of being seen on the glass and the sun. By a careful insptctioti these two spots cannot be mistaken for each other, for move the glass how you will, and put thorn in what poiition you will, each of them support distinct appearances, and even when they are, by a movement of the glass, brought in such a position as to partially eclip.«e each other, that is, when the spot that floats upon the surface of the glass seems to cover one half of the sp(»t which appears to bo at a great distance through the glass — which is at a time when each of them ft»rm un eqlial angle with the sun and the eye — then, in that case, as in all others, there is a visible distinction ; the one is always floating upon the surface of the glass, whiljt the other li apparently seen at a great distance through the glass, and tlip ibrmer always appears much nearer the eye than the latter, whilst the latter always appears more clear and distinct than the former. The position and appear- ances of these two spots denotes them to be produced by two incidental rays of light ; liie one that has the appearance of floating upon the surface of the glass js produced by the light of the universe, that shines some little space beyond the upper limb of the glass, and tl)ereby foiling obliquely upon it, and subsequently reflecting ' in^J the atmosphere. The other, that iias the appestrance of being seen a long way through tho ^lass, is prodiiced by theliglitof the eye that shines softie little space beyond ^ka tipper limb of the glass, coutrariwise to the fonjaer, and ■ftiereby follin^^ oblicjuely upon the glass, and. sul)sequently^ "tieflecting back, it takes perception of th6 body of ^e suii. •Here we have the light tf the unlvelife '6f sun-light shiiaing 80 A TBIATIBK UN LIQOT. upon one spot of tho lens and reflecting into the atmosphere, whilBt wo have the light of the eye shining upon anotlier Bpot of the lent* where the light of the universe does not meet it. These Hpot« demonstrate, Hufficiently and Batisfaetorily, what I could not by other mean8 Hatisfaetorily explain. In ficctions 1 and 2, I endeavored to explain the 8ame flubjcct by observations upon a plane mirror, and my ideas were then, as they are now, that the light of the eye took pcrcrption of objects, and the light of tho univerBo illumiu- atod and rendered them visible ; but with a plane mirror the task wart a difficult one, for the incidental ray of the eye- light waa always meeting the reflected ray of the light of the universe, and the incidental ray of the light of the universe was always meeting the reflected ray of the light of tho eye, and the one was always takciu for the other ; and I could not possibly procure a criterion that was sufli- cicnt to illustrate my own ideius until, perchance, I wan looking uv a book, which acknowledged Mr. Thomas Dick to be its author, and on reading about the moon I dis- covered the words, — " When the image of tlie sun, reflected from it, would appear like a bright lueid point." An idea instantly struck me, that a convex lens would be the iden- tical implement for the consummation of my wishes ; and immediately T made application, first to my spectacles, then to the lens of my opera glass, and both ol' them gave me immediate satisfaction, for us the light of the eye and the light of the universe have each a line of incidence, inde- pendent of each other, so their lines of reflection must bo independent of each other too ; and as there is no difficulty in determining the claim of each to its respective inciden- ! I 1 VWIOPf, AND OOLOfnWI. 87 f I I tal spot upon tho glaan, m) I muAt conflidor thn object fuUj accomplished. Should the observer find any difficulty in distinguishing the difFercnec between the spot produced by the light of the eye and the spot produced by the light of the universe, he can conveniently satisfy himself by making use of a candle, instead of the light of the universe, to demonstrate upon the lens, and the spot that is produced by the light of the eye will exhibit to his view, not only the flame of the can- dle, but also the candle itself; thus he will have occular demonstration that the light of the eye takes perceptions of objects in their respective locality. It is evident that these two spots are produced by differ- ent lights, for each of them are inverted towards each other. When examined by a lens and a candle, the one that ap- pears to float on the glass has all the appearanct* of a sha- dow projected by the genuine light of a candle, showing through the common light or flame of the same candle, and leaving an imperfect shadow of the candle upon the glass; whilst the one that has the appearance of being seen at a great distance through the glass has all the appearance of the candle in as full perfection as though the eye were looking directly at the candle ; and the fact that they both are always inverted towards each other denotes that the lights that produced them must come upon the gla^s fr(»ni different directions, for, whichever way the one pointfj, the other always points to the contrary ; that is, if the point of one is towards the north, the point of the other is always towards the south, and if the point of one is towards the east, the point of the other is always towards the west, and 88 A TRRATWr. O.f MOHT. «uoh is thn ojuw In ovory poiition that thov may be made U> rxjcupy. As for tho lucid point upon the silvor globo, Mr. Dick hafl (Idccivod Iiiiiii^oir. It can oojiily bo a8c;crtaiiu!(l that th« «pot ho Hpciiks of i.s a production of tho li^ht of tho uni- verse, yet it is not the spot wlicrc tho sun \n »oon, and con- nequently tho pioturo of tho sun is not by it roflectod to tlio oiirth from tho moon. Surely Mr. Dick must have been in orror when ho con- sidorod that the surface of a perfectly polinhcd silror globe docs not reflect so much light from one part us from another, because he merely sees a lucid spot in one place. Tho cause why tho bright polished globe could not be scon in •un-light was because there was too much light, and conse- quently no colour; for too much light destroys c(»lours, and renders objects invisible, hence the necessity of boiling tho silvor globe to give it a colour to render it visible. The bright spot or point that Mr. Dick spoke of was a colour, and consequently not so bright as the invisible por- tion of the silver globe that was exposed to sun-light. This was an error occasioned by not knowing tliat light and colours are distinct substances, and that genuine light is always invisible because it is colourless. The polished silver globe, before it was boiled, was brighter, and a greater reflector oPlight than it was afterwards, but it could not be seen because it was void of colour. » • TWION, AND C i ColuurB in the Atmo#pJipr«.— Th« I'rism •ii.l Kir I»(t«"- N«wton.— opinion thorton. — Tho AluirMiph«ro rontninn h tarlfty of FlultU— Th« Kluldn um ^rlby pro- (lucUuni from opaqiw and transfMireiif iutmUncM.— Colour* tr* —na In iiuiub«r,»x<'liiHlT« of wlilt...— Kye-llkiht and tliw light of tb« tnivaraa d»>«trojr tkU Colour*— NaniM of ('(.lonrn «!rronooii»ily •rrannrd.— Colours cnn only Im» iwen in ctrtttin iletriMi of Light rf-HptwMrnly.— The PrUman a inaRnlfyli-jf power .--Ol»rv»lion» n|.on m Wl«dow.--The light of the Ky© ohlitt-ratM \a* Shadown — IJIuf cthir ran Iti looked lliroiinh to a (i;r»mt dlMfiuui-, ami White may b« dlsciTnod by crfiiin nn'»n«.-«hndow« iiroj^cti'd hy Window Ban. By the use of the Prism seveo diflereiit colours arc revealed to our view, viz. : red, orange, yellow, green, blue, indigo and violet. Those Hcvcn colours hnve been terr^cd tlic solar spec- trum, because they have been considered a production of solar light. Sir Isaac Newton and many other eminent philosophers have considered that tho whole of those seven colours arc a production from white light, by virtue of the Prism, and that white light contains the wholo seven. That the Prism possesses the power of separating white light into those seven colours, and when t^'ov employed a second Prism, with power equal to the first, they supposed that they had found a means of returning all those colours into white light again. If Sir Isaac Newton considered that the white light he spoke of to be, and to contain, all the atmosphere, and all fluids therein contained, his ideas, so far, were close upon the subject, but the idea that the prism possessed the power of separating all those colours to be seen, and that a second ii2 mmm IMAGE EVALUATION TEST TARGET (MT-3) V A o ^'tfii :/. ^ :/. 1.0 "i^ I I.I 1.25 us 1^ |2.5 2.2 12.0 U III 1.6 Photographic Sciences Corporation qG \ m \ \ ^^ \ O^ f- v'''" %^ ' f1> 23 WEST MAIN STREET W2BSTER,N.Y. 14580 (716) 872-4503 ^ Lo be considered (opaque and transparent. As the earth supplies the atmosphere with exhalations consisting of colours and fluids, so the atmosphere supplies the earth with colours and properties to the aninjal, vegeta- ble, and mineral creation. We have many instances how vegetables, when deprived of air, loose their colour, and when again a free circulation of air is allowed them they regain their colour, and, it is also understood, that those plants that in their growth are ex- cluded from the air, do not give carbon like those that receive the fresh air, and we know that no animal can live long or enjoy good health without the enjoyment of fresh air, and also, we observe how petrefactioii is carried on by the three elements—earth, air and water— all these things may be considered as proving that the whole system of nature is conducted and matured by a system of reciprocity, between the animal, vegetable, and mineral creation, on one part, and the atmosphere and all its components, on the other part. My belief is, and I have strong reasons for my belief, that the opaque substances of the earth supply the atmos- phere with colours, by exhalation, and that all those colours 92 A TREATISE ON LlflOT, must be amazingly fine or .subtile ; and further, that all the earthy transparent and bright substances, by their shining, supply the atmosphere with light. If the exhalations of the opaque substances are amazingly fine, as they are so thinly scattered in the atmosphere, what must be the fineness of the mer shine of those bright sub- stances that undoubtedly produce light in its genuine state. As all the colours in the atmosphere are a production from opaque ponderable substances, they must have a ten- dency to create darkness in the atmosphere, and as light is the mere shine of bright shining substances, it is imponder- able, and in quality or substance, it surpasses in fineness anything we can think of ; nevertheless, when all the pro- ductions, both opaque and transparent, are intermixed in the atmosphere, they compose, what I denominate, common light, and this common light, together with the atmosphere itself, bears a white colour. The colours seen in the atmosphere, are seven, including green, which is a compound formed of yellow and blue ; these are the colours generally seen by the help of the prism. As the white or common light which comprises the whole of the colours I have mentioned, and can only be seen by the naked eye in some particular position, and does not make its appearance by the use of the prism, so I shall omit mention of it for the present, whilst discussing the use of the prism. Eye-light, or otherwise eye-sight, which is the produc- tion or mere shine of the eye, ai'-d, an account of its fineness, it is possessed of the power of passing some dis- tance through the dark ether, composing common light ; this distance may be about as far as our vision will range VIHION, ANl> COLOUIW. 03 on a clear day, that is until it reaches what appcarn to be the canopy of blue 8ky ; but if assisted by a strong light, at a distance, as the light between the eye and the suu, then those two lights know no bounds, but by their shining obliterate all colours and substances between thenii that is, they by their fineness nhinc so easily and freely through the whole, that neither colours nor fluids can be perceived, this is the case upon all occasions, when the eye-light ineeti any strong light, in the absence of any intervening opaque substance. All persons that have previously written upon the colours, have given them in the following order, viz. : red, orange, yellow, green, blue, indigo and violet ; had they possessed the knowledge, that all those colours arc produced by two shadows, which is evidently the case, viz., a light shadow, that produces the yellow, orange, and red, and a dark shadow, that produces the blue, indigo, and violet, they certainly would have begun with one extreme of the colours and ended with the other ; instead of which, they have be- gun with the extreme of what is produced by the light shadow, and ended with the extreme of what is produced by the dark shadow, thus they have the extreme dark grade of each shadow at the ends respectively, and the light grade of each shadow at the centre respectively ; thus, inst(!ud of having red, orange, yellow, green, blue, indigo, and violet, by leaving out the green as a compound, tht^y should have had yellow, orange, red, blue, indigo, and violet, for aa the yellow is produced in the strongest 'ight, those two colours should have formed the extremities, and the remainder the intermediate grades, for it is evident and strictly essential to be understood, that no two of those six colours can be 94 A TRRATIHK ON LmHT, seen m one griuie of a shadow, that is, the yellow cannot be seen m tho sanrifl grade of shadow m the orange, the orange cannot be seen in tho game grade as the red,' the red cannot be seen in the flame grade as the blue, the blue cannot be seen in tho same grade a« the indigo, the indigo cannot be seen in the same grade as the violet, neither can the violet bo seen with the same light as whit<>. As none of these colours ca.i be seen, only in a certain degree of light, in the shape of a shadow respectively, on account of the brilliancy of the eye, and the strong light of the universe, so ncHhcr of them can be seen without having its particles magnified by tho use of the prism, and by having the eye that is viewing them lowered in its' bril- liancy, by the use of the same instrument. The prism generally in use is made of glass, a description of it is unnecessary. It has a high magnifying power transverse to its shape, viz. —when it is held horizontally, all the objects perpen' dicularly to it are greatly magnified in size, whilst those objects that are parallel to the prism are not at all magniiied. I can, at pleasure, increase or decrease the apparent size of aJl objects, by increasing or decreasing the size of that side the prisrr next the object, this is done simply by turnin- the prism up or down, whilst looking at any object ; fo^ instance, if I am looking out of a room, through a window and holding the prism horizontally, if I turn the prism up' ward, the side of the prism next the window is enlarged, and consequently my vision also, and the panes of g)a5sare greatly increased in length, but not in breadth ; if I turn the prism down, the side of the prism next the window, the panes of glass are greatly decreased in length, but not ) » i ^ A i\ VISION, AND COLOURS. 96 in breadth. If I change the poaition of the prirfiu from horizontal to perpendicular, with the thin edge of the prism to myself, and the back to my right, I can then, by turning the face of the prism to my right, increase the panes of gla*« in breadth, but not in length, and by turning the face of the prism back to the left, I decrease the panes of glaas in breiulth, but not in lonjith. It should be understood, every colour that is seen by the use of a prism, is lirst brought into readim-ss to bo seen by the projection of a shiidow, and each particile of a colour, fine and transparent us :. is, is, of course, enlarged by the shadow so attached to it, und as it is the shadow of a fluid, it will at the same time partake of the colour of the parti- , cle that projects it ; still, in this stage, it cannot be seen as a I colour by the naked eye, the light of the eye obliterates the I colour, and the sliadow alone can be seen ; as a shadow, the light of the eye can perceive ;us much of the shadow jis it ^ does not obliterate, but it cannot porceivo a substance so fine OS a colour, but obliterates it; nevertheless, in this state, it is in readiness to be viewed by the prism, whos«j capabilities will not only lower the brilliancy of the eye, but, at the same time, magnify each and all the particles of both the colours and their shadows. I Perhaps, there is no better place for a porson to com- n in «ueli a position, hut t!>c amount of brilliancy of tin; universe that surpasHPs the amount of brilliancy of the eye, not even the blue ether that can bv seen otherwise as blue sky nor the white atmosphere that ha» HO often and ho erroneously been termed white light. The blue ether «!an be well seen in a cle.ir day, by the naked eye, by a lon«,' range of vision looking through the atmosphere, until a sufficiency of blue is before the cyo to render it visible, and the wiiite atmosphere may also be seen by 'he naked eye, by looking through a window with one eye closed and the other partially closed, or by i -o^irc at the moon on a fine light night, with eyes so partially closed, when a long stream of white atmosphere is seei both upwards and downwards from the moon. ft should b.i noticed, that the light coming in at a >.in. dow, projects several shadows into the room, and thest shadows generally reach farther into the room than most persons are aware of, for the light of the eye destroys or obliterates all the extremities of them, and also reduces the appearance of the more substantial part of thoso sha- dows ; but, if a person wishes to gratify his curiosity iu this respect, let him hang up a tine curtain so as to cover the upper part of the window, and to produce a dimness in the part so covered, and then with his eyes so partiallv closed, as I have mentioned, he will then perceive all the dark sluidows, that are perpendicular to the eyes, to be still more darkened, and, by opening the eye, that is viewing them to the full extent, its brilliancy partially obliterates them, and by partially closing the eye again, the shadows appear more darkened as before, so by alternately viewing VISION. AND COLOURft, 97 thvhti pcrjiUiH'icular to tho eye, ttist witli a bright eye, then with u diui eyo, wc have (ccular t them constantly reduced by obliteration. It is well understood! how a dark sha(h)W is projected by an opaqur substance, and that no intermediate shadow can be se.u. witliin it, and that its own colour of blue cannot b^ di^corncd witliin, unless by vhe help of some linht on cue of iti. surfaces, or on the contrary side to the eye, for as the colour is cnli.i^htcncd, so the dark is obliterated, and another colour becomes visible ; this shadow, when shown upon by H<,^ht, produces violet, indiffo, and blue. It is not so well understood, that a shadow is projected on the sides of an opaf^ue substance, tind by what means, but such is a fact, and were it not for a shadow of this description, none of the three light colours could be seen; in the darkest part of this shadow the red colour can bo seen, and next the orange, and then yellow, and ^trreen which is generally seen between yellow and blue. The colour blue is seen in the lightest grade of the dark shadow, and the colour that is seen in the darkest grade of the light shadow, is red. A^ the violet colour is the nearest to where no other I f A TREATIHl ON MOnT. cdour can he soon, bccauw of darkncf^, and the yellow in the ncaren to whero no other colour can bo Hoen. bocause of lipht, the order of the colours ought to be. yel.'ow orange, red, blue, indigo, and violet. i yellow in , yel.'ow, VISION. AND roLOuaa. SECTroN xvir. l'r.Ml.u-.i.,n .r tw.> ShHl,.w,._The r„,br« .,,.1 ,|,« |'«„,unl.rA.-I|ow i»«r.v,.,..|.- Ohier»»ii..i.. ..n fh.- S», .low. a„.| )'rl«m.-Tho Kiihil>uW.-T».« *)!« goo,,. trum. "^ The following is a ,lm'riptioM of ho-y the lii^ht nhuJow is produced : — All lijrht. when iiut intercepted by some oparjue aub Htance, ha« a tendency, m part of its nature, for the dif- forent particles to iwsist each other in all directions, by a system of reciprocity of shining upon or through euch other ; but if an opaque substance intervenes, not only u a dark shadow projected in its rear, but all the light is ip>- mediately, on both sides of the opaque substance, debarred of the additional light that it would otherwise receive by reciprocity, if the opaque substance were not there; con- sequently, a faint shadow is produced on both sides of all opaque subr'tancos. The dark shadow, in the rear of an opaque substance, may be termed the uuibra; and the light shadow, on the sides of an upa(|Uo substance, maybe t^-^rmed the penumbra. Both tlu'so shadows are visible to the nak uru«ivablo tni each of tho?k? surfucos ; bul a buttor vi«w of the lii^iit mIiu'Imw luuy bo b.i«l by the? Uf»u of •oiU'jili'u;; .siii'iMth. UH the oili^i; of a knife, or thti cdu'O'* of the two bltifldH of ;i p tir of MoirtHor?* ; thu luttor, if tb«j arrt hol'l lip in u moilpruto li;^'lit with the bhulo'* u little «x- tondcil, ;i nhii'bi.v will bo Hoeii on thu cd'/o (»f each blaHt\ un'l lit tlii! junction of thu two bLulos. the two slnaoWH, by ihcir junction v ith ouch other, will fonn a con»[M)un ^ton, there all the colours and ut- moHphcrc uro oblitoruted, und they together form couimon IlKht. Thoru are four llglit iK.«nuiubni Mhadows, and four dark umbra nhaduws ; the liKht ol' the vyc cuIh oIV, or obliterato.% :i portion of tdl thcxc nhadown, bv its briliiftnoj, but thn priwni, by it« dimn«h«, '•cirttorfs it a^uin. A« it apponrs U- ho li^ht of the vya and the lij^ht of the universe, by tl. j. finoncw or briiliunoy, are 'r.pablc of obliterating', and do obliterate all shadows and idl diitino- tions of colours bf .seen thoni. ho, it ifl highly cMontial to tho tliscernnient of shadows or colour«, contained ill »Uv »tmo .sphere, that the light of the universe Hhould I., lowered in its brilliancy by virtue of some whadow, and that the light (.f the eye should be h»wered in its brilliancy by looking through a crystallized prism, by which means we have the light of the eye brcught down near enough to a lovcl (but not^exactly so), but to a fit state of taking .jguizancc of the colours contained in the shady places of the aunoaphere. that is, to such a state that neither of tliose lights diould possGSB the power of passing through those colours without a touch. It is now necessciry to take a view of the position of the shadows, and ol. ^rve how they have been fV;rmed, particu- larly the light penumbra shadow, as it is formed by the light in front of ihe sash-bar ; its breadth will bo greatest towards the inside of the window, and, consequently, every particle of colour will point itu shadow in the same direc- tion, that iSj inwards and downwards. h2 >>.. 102 A TREATISE ON LlQUr, At the aamc time, it is necessary to take a view at tUo pasition of tke prism, and observe how it directs the evo- si^'it upwards to this shadow ; here, we may perceive tha^. the raagnifyinj; power of the prism doeH not fall immedi- ately upon the apex or point of each particle of colour in -the shadow, but somewhat on one side, so that it strikea the particle and pasaoa through it diagonally, thereby mag- nifying both particle and its shadow, both in density and size, at the same time ; it thereby finds the three decrees of shadow in the colours of red, orange and yellow. With respect to the dark umbra shadow, not much need be said ; the light from the prism rising upward, m«rely slides orei- the surface, and, together with the light of the universe, dips as deep into the shadow as is necessary to form the three dark coloiira, violet, indigo and blue ; and, with respect to the green, it is a mixture or compound of yellow and blue. Enough has been .^aid to give an idea how colours, that reside in the atmosphere, are brought into visibility by the lue of the prism. I have demonstrated, by looking through a window — and it is necessary to understand that if so much is well considered a.pA understood, very little more can be added — that the same theory which applies to ap- pearances produced by a prism upon a window, applies to the numberless appearances that amuse the observer wlien looking over the horizon by the help of the prism ; every cloud, plant, house, building, or animal, produces its sha- dows, and the prism and the eye observe them in the same manner as they made observation upon the window. The rainbow that has amused the multi'-ades by its splendid appearance, and aroused the curiosity of thousand* VISION AND COLOURS, 103 I . . to explore and explain what irt called its phenomena, haa never been explained or undcratood, the cause of which had been the same that ha8 prevented the advance of know- ledge in the other departments of light, which is, the idea that objects sent images of themsclveg to all eyes that viewed them. The rainbow is formed by what I term a natural prism, being and composing a section of the atmosphere that is sufficiently darkened to lower the brilliancy of the eye. in the same manner that the prism does. The sun is shining on the back of an observer, whilst he is looking at the bows, the great light of the sun, with the addition of the eye-light of the observer, meet at an angle and obliterate all shadows, hence the space between the bows. The light from this space is gradually darkened, upwards and down- wards, and thereby the necessary shadows are formed, and the light of the eye. by the direction of the magnifying power of the prism, finJs its right place for seeing the colours, in the curvilinear f hapo. The fact that the bows are formed by the magnifying power of the prism, in union with the circular arch of the eye, may be easily understood by a knowledge, that if a number of persons are standing in a row, with their backs to the sun, whilst the rain forms the necessary cloud for making a bow in their front ; then each of those persona will have a bow, respectively ; and if that number of per- sons stand at equal distances from each other, so will the arch or upper section of their bows be at equal distances from each other. It therefore follows, that all parts of the cloud, with right height and right density, possesses all the necessary materials for rainbow making ; and the light of 104 A TREATWK ON LIOUT, the universe and the li.L'ht of the eye huvo nothint^ more to perform than to meet each otlier, and, under the directions of the maj^'nifying power of tlic prism, unite and form junctions upon the several grades of slmdes, above and below the illuminated spaee. The solar .spectrum about which iso many curious notions have been formed, (all of which, instead of explaining, arc calculated to confound the understanding) is as simple as the English alphabet, if considered rightly. It is pro- duced by letting a portion of strong light through an aper- ture into a dark room, and by placing a prism in the aper- ture for the sun to shine through. A screen of white paper may be held up at aomo little distance from the prism iu the dark room to receive the light ; the light, of course, is strongest in the middle, and from thence it decreases to- wards the top and bottom, and accordingly the colours arc seen from thence, varying in their tint towards the top and bottom, each colour in its respective grade of light ; that towards the bottom being yellow, orange, and red; that towards ';he top being blue, indigo, and violet, and that re- maining in the centre being green, is a compound of yellow and blue. If rightly ci»nsidered, the theory for the rainbow and the spectrum, is the same in every respect but one, that ig, the shape of the bows, for, when looking through the arti- ficial prism at the rainbow, the eye-light or eye-sight forma the bows, as it does by looking at a rail or any other pole of timber lying horizontally and parallel with the eye ; but whilst looking at tfie spectrum, the eye-light does not pass through the prism at all, so, consequently, no bows are formed, but if the prism is taken from the aperture and 1' 1' VISION, AND COLOURS. 105 . placed before tlio cyen, and loukiti;:^ towards the aperture, wo there 8ee all the colours revcrncd and the bowR ibrmcd as upon other occasions, because tlie lijrht of the eye Is now doing what the light of the universe had done before. Having sliown that all those seven colours have their residence in the atmosphere, and also shown how they are uiagnified into visibility by the use of the prisn), assisted by shadows, that each colour re((uires a shade peculiar to itself to enable it to be seen, and that where the light of the eye and the other light of the universe, either meet or unite, they obliterate all the shadows that arc essential to the exhibition of the different colours respectively, and that when all the shadows that exhibit the different colours are obliterated, all distinction of colours is also obliterated, and they together become common light ; enough has been done to enable any observer of those colours to form a cor- rect opinion for himself on the ground of what has been said of the pane of glass with its four light colours, and four dark colours, it and they may bo considered as a specimen of all that can be seen, when looking through the prism at the clouds or over a landscape, to see more, would only be to see the same over again in a different position*. The white atmosphere, as I have before stated, may be seen by certain contrivances by the naked eye, better with- out than with a prism. As it appears that the different colours are affected by their proximity to each other, under various circumstances whereby thoy produce a mixture of appearances, it may be a gratification to an observer to witness the real cause of the same. I wou I observe, that any person may gratify his curiosity by procuring a pasteboard, and proceed in some 10« A TRKATIMK OS I.IOHT, rc-ular wa>- to cut interstices throuc^h it, some bi,i;an