QB 543 -78 U45 UC-NRLF INSTRUCTIONS OBSERVING TOTAL SOLAR ECLIPSE 2C 1878. / ttENT ISSUED BY THE/U. S. NAVAL OBSERVATORY WASHINGTON: GOVERNMENT PRINTING OFFICE. 1878. INSTRUCTIONS OBSERVING THE TOTAL SOLAR ECLIPSE ULY 2(), 10J 818. ISSUED BY THE/U. S. NAVAL OBSERVATORY. * ' -_,- WASHINGTON: GOVERNMENT PRINTING OFFICE. 1878. '72 UNS OF CONTENTS. Page I. Observations with the Naked Eye _ II. To Photographers III. The Photoheliograph .!..'! !o IV. The Equatorial Camera V. Telescopic Observations VI. Spectroscopic Observations -_ 2O VII. Polariscopic Observations VIII. Photometric Observations IX. Therino-Electric Observations 76<) These instructions were, by my direction, written by Prof. William Harkness, United States Navy. Suctions I, II, V, and Vlll describe such observations as can be made with the apparatus usually possessed by amateurs ; while the other sections relate mostly to observations which can only be carried out by persons who are able to command expensive apparatus, and who are' skilled in astronomy and physics. JOHN RODGERS, Bear-Admiral, U. S. N., Superintendent. INSTRUCTIONS FOR OBSERVING THE TOTAL SOLAR ECLIPSE OF .JULY 29, 1878. The following suggestions are intended to direct the attention of persons who may witness the total solar eclipse of July 29, and who may desire to co-operate with the United States Naval Observatory, to some of the phenomena which, in the present state of science, it is most desirable should be carefully observed on that occasion. In writing these suggestions, much assistance has been derived from the instruc- tions issued to the government eclipse expeditions sent out by various nations during the last ten years ; and free use has also been made of the reports of these expeditions. SECTION I -OBSERVATIONS WITH THE NAKED EYE. i. At all places on the central line of the eclipse, the following phenomena will be seen during totality, if the sky is clear, viz: Just as the sun's last ray fades out, some glowing points of light will start into view, hanging upon the edge of the black moon and glistening like rubies ; while surrounding the whole will be seen a halo whose mild radiance reminds the beholder of the glories with which the painters of old adorned the heads of saints. The glowing points are the red prominences, and the halo is the corona ; both of which are now known to be solar appendages. The corona consists of a comparatively bright inner part lying close to the sun, surrounded by a much fainter mass of luminous matter of vast extent, and generally of most irregular form. Accurate drawings of it will be exceedingly valuable, and for- tunately, inaccuracy, such as affects the scientific value of the drawings, can be easily avoided. With this view, persons intending to make sketches should provide them- selves with a sheet of paper about 9 inches wide by 1 2 inches long, having upon it a black disk, i^ inches in diameter, to represent the moon, from the center of which straight lines are drawn at angles of thirty degrees, the whole being an exact copy of Plate I. Let a weight be suspended by a string in such a position that the observer can see it hanging over the sun's center ; and let the diagram upon which the draw- ing is to be made be so placed upon any convenient stand that the line marked " top " " bottom" shall be in the plane passing through the observer's eye, the string, and the sun's center, the end marked "top" corresponding to the top of the string. Also, let a lighted lantern be ready to illuminate the paper, in case the light of the corona is insufficient. Finally*, two or three minutes before the beginning of totality, let the observer close his eyes and cover them with a handkerchief, so as to make them as sensitive as possible. A friend must be ready to warn him when totality begins, and then, removing the handkerchief, he must rapidly sketch in the outline of the faint outer parts of the corona, taking special care to preserve their correct proportions s relatively to the moon, and above all being scrupulously particular to place the prominently marked branches and rifts in their proper positions. In doing this, -real assistance \\ill lie afforded by the line- drawn upon the diagram from the center of the black disk. Having completed the outer portions of the corona, the outline of the bright inner part should be sketched : and in doing this it will probably be best to view the eclipse through a spectacle glass of a light green tint. Finally, if any time remains, the whole drawing should be carefully revised; with the aid of an opera- Lj-lass. if possible.* It is not worth while to insert the red prominences. The duration of totality being only three minutes in the most favorable localities, it is useless to attempt making more than a single drawing: in fact, such an attempt would show that the work had been so hurriedly done as to deprive it of all value. Plate II is a drawing of the eclipse of April 16, 1874, given by .Mr. K. .). Stone, director of the Royal Observatory at the ( 'ape of ( iood Ilope.f It shows the style in which the drawings should be made : , //, the outline of the bright inner part, of the corona. Probably many parts of the outer corona fade away so gradually that it will be impossible to say precisely where they terminate. This uncertainty should be indicated by a serrated outline, as shown at a'. The original sketches must not be altered after the totality is over: but if the observer desires to make a finished drawing from memory, it may In- done upon another piece of paper. 2. Portions of the corona may probably be seen for a few seconds, both before and after the totality. Note the exact positions and appearances of these portions, observing particularly whether the parts seen before totality are the same as those seen after totality, and how long they are visible before and after totality. 3. Note the character of the extreme outer boundary of the corona. hoes it fade gradually away, or has it a definite edge f 4. The moon's shadow should be bordered by diffraction bands; and some ob- s( rvers claim to have seen them as dark shadows -living over the earth's surface imme- diately In-fore and immediately after totality. Probably they would be most \isible from an eminence commanding an extensive landscape, and it would be a matter of interest to look for them. 5. As the truth of Leverrier's discovery of an apparently unexplained motion of the perihelion of Mercury is now established beyond all doubt, it is important to renew the search for an intra-Merciirial planet or planets. With this view, the parts of the ecliptic near the sun should be carefully examined during the totality of the coining eclipse, and if an\ thing hitherto unknown is discovered, its bearing and distance from the sun should be accurately measured. The bearing can be defined by holding a watch between the eye and the corona, and giving the hour which points toward tin- object, it being understood that the XII is toward the zenith. The distance should be li'iveu in inches and parts of an inch, as measured by a rule held exactly eighteen inche s from the eye To insure the rule being i'i>,.ii< may MM- it i|iiiti- ilitl'rri-ntl.v. It i* I'M llir )>iir|iosi- of iliDiiuJHli- illj- till-. |IITSOM.-|]||\ tliat till- IIM- III' III,' ll|H-):l nla~^ I- HTIMIMIICIIlll'll. Mi-moils iil'lhr l.'\:il Aitroiiiiniiriil Soi-irty. Vol. I'.', p. 1:1. Mr. Slime uilvnratril t hr mi-tlioil nl' i-Ui-l. Inn.; whirl) .-; lii'i u ,1. -i-i ilii-il. there is a knot at a point just eighteen inches from the rule. By holding this knot between his teeth, and keeping the string stretched, the observer will be sure that the rule is at the proper distance from his eye, and then he has only to be careful to hold it at right angles to the string, lint one eye should be used in making the measures, and no attention need be paid to any object more than five and a half inches from the sun. Mercury will be quite conspicuous at a distance of seven and a half inches, and Mars will be seen at about five and a half inches. 6. Very good observations of the third and fourth contacts, the difference between which is the duration of totality, can be made by the naked eye; the only instrument requisite being a watch having a seconds hand. The importance of such observations and the method of making them are full}' explained in Section V. 7. Note what part of the sky is darkest during totality, the form of this dark part, and how the light varies from it to the brighter parts. SECTION II. TO PHOTOGRAPHERS. Photographs of the corona of very considerable scientific value can probably be made with the apparatus in ordinary use by photographers. With this view, the camera should be fitted with the largest and longest focused portrait lens at the operator's dis- posal, and then, the diaphragm being removed, it should be most carefully focused upon some distant object. The image will be well defined only over a small space in the center of the ground glass, but that is of no consequence. Two or three minutes before totality, some half-sized plates, prepared with the most sensitive chemicals ob- tainable, should be in readiness for immediate nse. The number of these plates should be equal to the number of half minutes in the predicted duration of totality at the place occupied by the operator. For example, if the predicted duration is 2 m 30", then five plates should be prepared. The camera should also be in readiness, directed to the sun, and held there in any convenient manner. As the sun will be high in the heavens, the ordinary camera stand may riot be available, but with a little ingenuity some rough contrivance can be made which will answer the purpose. As many of the plates as possible should be put in holders and laid beside the camera, so that they can be used without delay. Then, the focus of the camera re- maining as it was when adjusted upon the distant object, as soon as totality commences, and the operator has assured himself that the image of the corona is in the center of the ground glass, he should insert a plate and expose it. The ground glass should next be replaced, when, owing' to the diurnal motion of the earth, it will be seen that the image of the corona has moved away from the center of the field. It should be piit back, after which another plate should be inserted and exposed; and this routine should be continued till the totality is over, when the plates can be developed. With a quick-acting portrait lens, the first plate should be exposed three seconds, the next six seconds, the next three seconds, and so on, alternately. With an ordinary portrait lens the exposures should be alternately five and ten seconds; but it must be under- stood that diaphragms are not to be used in any case. The development must be car- ried far enough to bring out all the details ; but extreme care must be taken not to push it solar as to clog up the delicate outlying portions of the corona. On account of the 10 motion of the earth while the plate is liein^ exposed, the intake of the sun or rather of the moon, which covers the snn will he a little blurred, and somewhat oval in shape; and to prevent disappointment, it must lie borne in mind that the portrait lenso in ordinary use have such short foci that they can yive but very small pictures of the moon. The actual si/e in inches will be very nearly as follows, viz: With a 1-4 lens, 0.04; with a 1-2 lens, 0.07; with a 4-4 lens, 0.09 : and with an extra 4-4 lens, 0.14. The diameter of the extreme outlying portions of the corona may be four or five times these figures. It is hoped that photographers making pictures in this manner will send the original negatives to the Naval Observatory; but, if they prefer to retain them, it is earnestly requested that they will, at least, send positive copies on e. e\ eu more important than photographing the corona. The magnitude of its intensitv ratio enables it to depict taint objects rapidly, and the extent of its angle of \ie\v is such as to embrace a field of more than forty degrees. It. therefore, >eeius peculiarly fitted to assist in 13 the search for intra-Mercurial bodies, provided the illumination of the sky does not prove too great. Mercury's maximum elongation from the sun scarcely reaches twenty-nine degrees, and, if we postulate Bode's so-called law, the greatest elongation of the hypothetical intra-Mercurial planet or planets should not exceed fifteen degrees. Double this distance is the space along the ecliptic which must be searched; and to photograph it a plate 17.7 inches long will be required. Our lens will cover a plate measuring 20 by 22 inches, but it is very desirable to keep the apparatus as light as possible, and on that account it seems best to have a camera twenty inches square, and to use plates measuring i 7 by 20 inches, which will suffice to cover a space of thirty-three and a half degrees along the ecliptic. Assuming the adoption of an equatorial camera twenty inches square, provided with a lens whose intensity ratio is one-sixth and whose focal distance is about thirty- three inches, it yet remains to consider how this apparatus should be managed during a totality lasting only three minutes. As the illumination of different parts of the corona varies greatly, there can evidently be no certainty of getting all the details of the phenomenon unless a series of plates are taken, in which the exposures vary from the shortest possible up to the point where it is certain that an increase of time does not improve the picture. On. this account it will be desirable to take as many as six plates, the exposures being, respectively, 3 s , 5 s . io s , 2O S , 40", and 6o s .* The first four of these plates will receive such short exposures that it is unlikely they will show anything but the corona, and therefore their size should be 4^ by y/ 2 inches. With the last two plates the case is different. Their size should be 17 by 20 inches, because their longer exposures will probably suffice to bring out upon them any bright points which may exist within their field. A lens such as is here under consideration should depict an eighth-magnitude star. in about one minute, but of course the inten- sity of the sky-illumination during totality will determine the limit of brightness at which faint luminous points will cease to impress themselves upon the negatives, and what this limit may be it is impossible to predict, The necessity for at least two large plates is evident when it is remembered that the image of a small bright point could not be distinguished from an accidental blemish in the film, and it wquld only l)e by finding it upon both plates that its true character could be unmistakably recog- nized. It is exceedingly desirable to determine accurately the maximum exposure that the corona will bear with advantage, and it is hoped that on at least one of the large plates it will [trove to be over-exposed. For the management of the equatorial camera two persons will be required, desig- nated respectively as No. i and No. 2, and the following routine will be found conven- ient. The operations in the dark room must be so managed that the six sensitive plates shall be placed beside the camera, in readiness for instant use, a minute or two ' It is questionable whether or not all these exposures can be made in tbiee, minutes, but a few preliminary trials will settle that point. Experiments should also be made upon the stars in the evening twilight to ascertain if an ex- posure so long as sixty seconds is really required. In any event, it will probably be best to reverse the order given above, and to work oft' first those plates requiring most time, thus reducing to a minimum the damage which would accrue from the sun bursting out while an exposure was in progress. 14 tin- predicted time of Mvoiid contact. Six sand-glasses must also lie pro- vided, each adjusted to run for the number of seconds that the plate to which it belongs is to lie exposed. Then, the cap being on the lens, the first plate must he placed in the camera, and the slide must lie withdrawn from the holder: after which operator No. i will take position with his hand upon the cap covering the lens, and No. 2 will seize the proper sand-glass and hold it in such a way that the sand shall all he in the bottom of the glass. At the instant totality begins, No. I will say, " Ueady?" No. 2 will answer, "Ready." No. I will begin the exposure of the plate bv careful! v removing the cap from the lens, exclaiming as he does so, "Turn;" when No. 2 will instantly turn the glass. Moth operators will stand motionless while the sand is running, No. 2 watching carefully for the escape of the last grain, when he will exclaim, "Out," and No. i will instantly terminate the expos- ure by capping the lens. Without a moment's delay No. i will push the coyer into the plate-holder, remove the exposed plate, place the next holder, containing a fresh plate, in the camera, and draw the slide; being assisted in all this by No. 2, if neces- sary. Then No. i will again take position- with his hand upon the cap of the lens; No. 2 will seize the second sand-glass; and this plate will be exposed in the same manner as the preceding. This routine will be continued till all the plates have been exposed, and after totality is over they n'ill be developed in the usual way. With regard to the plate-holders, it may be remarked that the last two plates must necessarily he in holders as large as the camera, but the first four plates can he more conveniently handled if they are in half-sized holders. To render such an arrangement possible a reducing frame must be provided, which, upon being applied to the camera, will reduce its opening to that proper for receiving the small holders. As to the photographic process to be employed, if only small pictures are desired, dry plates, prepared with washed emulsion, are recommended; but if any large plates are to be taken the wet collodion process will be preferable. In either case it is specially important to use well bromized collodion, because the bulk of the coronal light has probably a wave length of 5315.9 (the 14/4 line), and to such light silver bromide is far more sensitive than either silver chloride or silver iodide. For wet plates there is reason to think that fineness of deposit and richness in details can be best secured by a free use of sugar in the iron developer. To facilitate the study of the red prominences, it has been proposed to photograph them on a scale of ten seconds of arc to a millimeter. The optical apparatus for the production of such pictures must have an equivalent focal distance of 2062.7 centi- meters, or 8 i 2. i inches, and if we take (' equal to O.OO2, which is probably very near 15 the truth,* the values of t for lenses of various apertures can be found from the formula already given, and will be as follows : V* CJ l _ i O o '+ *r-t o 2 - 3 H c c * 3 o F ( J in v " 3 . o '-* * . r/ V (? / 0. f 593 13-2 7-2 12 67.7 45S3 9.2 5-0 15 54-1 2927 5-8 3-2 2O 40.6 1648 3-3 1.8 26 31-2 973 1.9 I.O The last column of the table gives the moon's motion during an exposure of the length specified in the preceding column. A prominence one minute high is a large one, and yet it could scarcely be photographed with a six-inch objective, because twenty seconds of its height would be covered by the advancing moon before the exposure was over. In short, it does not seem possible to photograph prominences during eclipses on the scale here contemplated with an aperture much less than ten inches, and twelve woiild be far better. Under these circumstances the practicability of the scheme is questionable. SECTION V. TELESCOPIC OBSERVATIONS. Ti'lexcojH's. A proper telescope for observing the eclipse is very desirable, but any telescope at all is better than none ; and owners of small spy-glasses, or opera- glasses, should bear in mind that their instruments are capable of doing good service. ( )pera-glasses can be held in the hand with sufficient steadiness, but spy-glasses, how- ever small, will require a stand to produce the best effect. This need not be an elaborate affair, but may be made of wood in the simplest manner, provided only that it is capable of holding the telescope steadily pointed in any required direction. Some artificial support is absolutely necessary, and, in default of anything better, the tele- scope might even be lashed to a round post. Sh(if TelescojH'. When observing a brio-lit sun the eye end of the telescope soon becomes quite hot, and thus air currents an* set up in the tube which interfere seriously with the definition. To remedy this the tube must occasionally be cooled, either by shading the objective or by turning the telescope away from fly sun As it is important to have the best possible definition while observing the contacts and totality of the eclipse, special care must be taken that the temperature of the lube at these times is the same as that of the surrounding air. Tlic ( oittm-lt;. The first touch of the moon upon the sun's limb is technically called the first contact, the complete extinction of the last rav of sunlight, is simulta- neous with the second contact, the reappearance of the minutest possible portion of the sun marks the third contact, and tho disappearance of all trace of the moon from the sun's disk constitutes the fourth or last contact. The first and fourth contacts are sometimes called the external, and the second and third the internal, contacts. The observation of these contacts consists in noting the exact instants of their occurrence, and for that purpose a time-piece with a seconds hand and a telescope are usually employed, hut if the latter instrument is not obtainable, the internal contacts can be well observed with the naked eye. There is generally some uncertainty about the first contact, because the only evi- dence of its occurrence is a slight indentation in the sun's limb, which, unless the ob- server knows precisely where to look for it, naturally attains some si/e before it is seen, ami thus the contact is noted too late. If the telescope has a position-micrometer, its wire may be set at the computed point of first contact, and then there \\ill be no dilli- culty ; but in the absence of such an arrangement, the only way is to use a power of not more than sixty diameters, and to watch carefully a portion of the sun's circumference 17 large enough to include with certainty the point in question. The shade-glass enl^- ployed should be sufficiently dark to destroy all glare, and to permit of the sun's im- age being viewed without effort. The internal contacts are phenomena of such a definite character that the instants of their occurrence "can be noted within a small fraction of a second. For a minute or two before the predicted time of second contact the sun's thin, and now fast wan- ing, crescent should be carefully watched through a neutral tint or green shade glass as light as the eye can conveniently bear. Presently the crescent will become a mere thread of light, which will rapidly shorten, and suddenly disappear. This is the second contact, and there can be no mistake about the exact instant of its occurrence because the ruddy glow of the chromosphere, which remains at the point last covered by the moon, is totally different from the dazzling white light of the photosphere which has just disappeared. Through a red shade glass the contrast between these two kinds of light might be partly lost, but through a neutral tint or green glass it will be suffi- ciently apparent. Finally, there is yet another indication of the second contact, and that is the seemingly miraculous appearance of the complete outline of the moon, round and black, reposing upon the wondrous radiance of the corona. The approach of the third contact will be heralded by the rapid brightening of the chromosphere at the point of the moon's limb where the sun is about to reappear, and two or three seconds later a sudden burst of light will announce the contact itself and with it the termination of totality. This contact may be observed without any shade glass. The observation of the fourth contact is a very simple matter. The segment cut out of the sun by the retreating limb of the moon is carefully watched as it be- comes less and less, and the instant of its final disappearance is noted as the fourth contact. The shade glass used must be so dark as to destroy all disagreeable glare. To observe the contacts, a station must be chosen where there is an uninterrupted view of the sun, and the party must consist of only two persons, one of whom must have a time-piece and writing materials and the other must have a telescope. With regard to the latter instrument, it may be remarked that if it is a common one a spy- glass, for instance there will probably be no choice as to its magnifying power; but if it belongs to the better class, and has a battery of eye-pieces, the power chosen should be equal to fifteen or twenty times the diameter of the objective in inches. To secure the best results, the party must be entirely alone; and above all, it must not Ite within hearing of other observing parties. A minute or so before the expected time of contact the person with the time-piece will fix his eyes upon its seconds hand and begin silenth* to count the seconds, while the observer at the telescope will care- fully watch the sun. Upon the occurrence of contact the observer will cry "Mark," and the person at the time-piece will immediately write down, first, the second; then, the minute ; and, finally, the hour. Before beginning to observe, care must be taken to see that when the seconds hand points to 60 the minute hand is exactly over a minute mark; and in observing, if the seconds are near sixty, special care will be re- quired to avoid recording a wrong minute. For example, when the time is 4"' 58" the minute hand will be almost exactly over 5"' ; and special care must be taken not to 3 18 record 5 58* instr;id of 4 58'. Practice in calling and noting time previous to the eclipse will greatly diminish the risk of such mistakes. A little before totality a lan- tern should be lighted, because without it there may be difliculty in seeing the hands of the time-piece at the approach of third contact. Observations of the internal contacts can be utilized without a knowledge of the error of the time-piece ; but with the external contacts the case is dill'erent. They will be of no value unless the error of the time-piece is accurately known, either on local or on Washington time. To meet tliis difliculty, it is hoped that arrangements may be made by which the Western Union Telegraph Company will furnish, free of cost, a signal from the Naval Observatory, indicating the instant, of Washington mean noon, to all proper persons who may apply for it at any of their ollices on July 27, 29, and 30. The intending observer can best utili/e these signals by making sure that the minute and seconds hands of his watch arec with each other, as explained above, and then carrying the watch to the telegraph otlice on each of the days men- tioned and noting the hour, minute, and second indicated by it when the signal is re- ceived. No matter how the watch may run, it must not. be meddled with in any wav, either by moving its hands or touehing its regulator, between July 26 and 31. With- out this precaution, neither its rate, nor the degree of dependence to be placed upon it, can be determined. The final record of a set of observations of contacts should contain the following information : 1. The name of the station; including town, county, and State. 2. The date. 3. A description of the location of the station with reference to the nearest promi- nent landmarks; as, for example, its bearing and distance from the nearest court-house, railroad station, or church ; or its position with respect to the lines of the I'nited States land surveys; or the bearing and distance of a prominent mountain peak; or the bearings of three or more such mountain peaks; or, in the case of a citv, the street and number, together with the names of the nearest cross streets on each side. . 4. A description of the time-piece, and a statement of the aperture, focal length, and magnifying power of the telescope emploved. 5. The times indicated by the watch at the reception of the Washington noon signal on July 27, 29, and 30. 6. The time of each of the contacts; just as read from the face of the watch, without the application of any correction. 7. An estimate of the uncertainty to which these times are liable. S. Any remarks which may be thought necessarv. 9. The signatures of the two observers, thus: A. 1!., observer with telescope; (' 1 > .. time observer. It is particularly requested that the original pencil memoranda of the times of contact lie s,. n f to the Naval Observatory, inclosed in the above-described report. If several parties observe the eclipse in the .same neighborhood, it is quite likely that their recorded times may differ a little. Such differences occur even in the work of the most practiced astronomers; and thev must be allowed f<> remain. It is never 19 permissible to alter the record. A mere suspicion that sucli a thing has been done will insure the entire rejection of the observation. Limits of the Shadow Path. There is yet so much uncertainty in the best solar and lunar tables that in the eclipse of July next the British Ephemeris places the path of the moon's shadow, while crossing Colorado, about three and a half miles south- west of the position assigned by the American Ephemeris. It is therefore important to determine accurately the true position of the path, and this may be readily accom- plished by observing the duration of totality at points situated from one to ten miles within the shadow. As the duration of totality is the interval between the second and third contacts, it is determined by observing these contacts in the manner already described; except that for this purpose a knowledge of the error of the watch is unnec- essary, and the observations may be made with the naked eye if a telescope is not available. At places very near the edge of the shadow the totality may be so brief that the time observer would risk losing the third contact if he stopped to record the second. At such places it will be necessary to have a third person to make the record, and then both contacts can be readily observed, even if they follow each other at an interval as short as a single second. . Sketches of the Corona. In making a general sketch of the corona the magnifying power employed should not exceed from twenty to forty diameters, and some me- chanical means should be adopted to insure accuracy of outline. Owing to the faint- ness of the corona, it is doubtful if the camera lucida can be successfully used; and the most promising plan seems to be a system of squares, ruled on glass, in the focus of the eye piece of the telescope, and a corresponding system drawn upon the paper on which the sketch is .to be made. It is known that spider lines in a micrometer sometimes cease to be visible under the feeble illumination of the corona, and to avoid the possibility of this happening with the squares, the "lines defining them should be rather coarsely cut. More than once attempts have been made to use a telescope after the manner of a camera obscura, by drawing out the eye piece a little, and thus causing it to throw an enlarged image of the corona upon a piece of finely ground glass, or semi-transparent paper, placed a few inches beyond ; the whole being so arranged that the image could be seen through, and traced upon, the glass or paper. Owing to the faintness of the image, such attempts have not hitherto been very suc- cessful, but perhaps they might do better if for the plate of ground glass a thin sheet of gelatine, or mica, were substituted, and the drawing were traced upon it with a needle point. The proper mode of executing such sketches has been already described in Section I. Miit/i/c Examination of the Corona. With as high a magnifying power as the telescope will readily bear, examine the structure of the corona; noting particularly whether it is homogeneous, nucleated like a star cluster, or filamentous. If any nuclei are seen, describe their shape, size, color, and mode of distribution. If filaments are discerned, state their length, diameter, color, and mode of distribution ; whether they are straight, curved, or contorted ; in what direction they lie relatively to the sun ; and, if curved, specify how the curvature faces. Be sure to observe how the structure varies in passing from the sun to the outermost visible limit of the corona. 20 Since the spectroscope furnishes an efficient means of studying tin- red promi- nences at any time, it will be very umlesiraltle to waste a single one of the precious nioineiits of totality in examining them. Intr. It !> known that the prominences consist mainly of hydrogen; but do they ever contain large quantities of calcium .' In other words, are the // lines sometimes con- spicuous in them .' Instruments, .1, (', I). Probably the // line is produced only by very hot hydrogen. As giving a clew to the temperature of the prominences, look for it in their spectrum, and note how high above the photosphere it exists. Instruments, A, C, />. Look for new lines in the ultra- violet part of the spectrum. Instrument, Cor />, provided with a fluorescent eye piece. The Cm -(iiin. Direct attention specially to the following points : l>oes. the line 1474 stop short of the chromosphere, or does it enter that forma- tion f If the latter, does it broaden out as it nears the sun :' Instruments, .1, <\ I). Are there any parts of the corona which give a spectrum containing bright lines between //, and 14/4 ' In trying to answer this question use rather a wide slit, and, if any lines are found, locate the part of the corona giving them. Instrument, C or />. IJesides the well-known bright line 1474, the corona gives a faint spectrum, which most observers have thought to be continuous. Is it really continuous, or is it crossed by the Fraunhofer lines ? Look particularly for />, 7-,', 1>, and /'', and if any of them are seen note in what part of the corona. Instrument, (.' or I). Examine the dark rifts in the corona very carefully to make sure whether or not thev give any spectrum. Instrument, C or 1). Does the corona give any ultra-violet lines ? Instrument, C or I), provided with a fluorescent eye piece. Examine the brightest parts of the outer corona in order to determine how far from the chromosphere any spectrum is visible, and note also what that spectrum is. Instrument, C or />. Examine what part of the coronal spectrum is polari/.ed. This may be done by noting the effect produced upon the spectrum when a Nicol's prism placed before the slit is rotated ; but the results thus obtained will be complicated by the polarization due to the passage of the light through the dispersing prisms, and to eliminate this the slit must be placed at different angles to the vertical, and upon various parts of the corona. The following seems a more satisfactory mode of proceeding: If it is borne in mind that the polarization of the corona is partly radial to the sun and partly in the plane passing through the zenith, the sun, and the observer's eye, it will be evident that the parts of the corona cut by a vertical circle passing through the sun's center are polar- i/.ed only in the plane of that circle. Hence, if the >lit of the spectroscope is placed in that plane, and the image of the sun is allowed to fall upon it in such a wav that the sun's center coincides with the axis of the slit; and if the Nicol's prism is placed with its principal section at right angles to, and immediately before', or immediatelv behind, the slit, then it is evident that all verticaliv polari/.ed light will be excluded, and the spectrum visible will be due solely to the unpolari/.ed light of the corona. Examine the thermal properties of the coronal spectrum. Eor this purpose a delicate thermoelectric pile ami galvanometer will be required: and as glass is coin 23 paratively opaque to heat waves, a reflecting 1 telescope should be employed, the lenses of the spectroscope should be of rock salt, and the dispersion should be obtained, either by means of a rock-salt prism, or by means of a reflecting diffraction grating. As totality approaches, observe with an integrating spectroscope what rays fade away and then brighten ; and observe particularly whether all the Fraunhofer lines are reversed at the instant of totality During totality note the intensity of (7 relatively to F, and the intensity of 1474 relatively to F, C, 7> ;! , and b. Bear in mind that the relative intensities may change during totality, and pay attention to that point! Watch for the reversal of the Fraunhofer lines at the close of totality. Instrument, 7? or E. Examine the appearance of the eclipsed sun with a meteor spectroscope, having neither collimator nor slit. So far as the corona is monochromatic it will be distinctly seen notwithstanding the prism, while those portions of it which shine only by re- flected sunlight will be indistinct, their light being dispersed. The same object may be attained to some extent by merely looking at the corona through an ordinary prism, or through a direct-vision combination. Instrument, F. Photof/rajtliy. The duration of totality is so brief that it is exceedingly desirable to photograph the spectra then seen, in order that they may afterward be examined at leisure. Attempts to accomplish this have been made with two very different kinds of apparatus, but thus far with only moderate success. One variety of the apparatus, known as the prismatic camera,, belongs to the class of slitless spectroscopes, and con- sists of a photographic camera before whose objective a prism is placed, the whole being equatorially mounted and driven by clock-work. The adjustment of the prism to minimum deviation is effected by the aid of a collimator temporarily placed before it ; but on some accounts it would be advantageous to dispense with the prism entirely, and to use in its stead a diffraction grating placed behind the objective. The photo- graphs produced by this instrument exhibit a dispersed series of spectral images of the corona and prominences, each image being due to light of a definite degree of refrangibility. The other variety of apparatus consists of a spectroscope of the ordi- nary form, except that it is provided with a small camera instead of a reading tele- scope, the whole being either attached to an equatorially-mounted reflector of short focus, or else so arranged upon a table that an image of the sun may be thrown upon its slit by means of a suitable heliostat and lens. With the prismatic camera, pictures were obtained during the eclipse of April 6, 1875 ; but thus far it is believed that no results have been got from the other form of apparatus. The reason for this is not apparent, because the brightness of the image depends mainly upon the intensity ratio of the lens by which it is depicted upon the sensitive plate, and there is no reason why this ratio may not be the same in the two forms of apparatus. Theoretically, the slit- less form should work slightly the quickest, because it has fewest optical surfaces ; but, as an offset to this, there is great difficulty in determining the wave-lengths of the .somewhat complicated series of spectral images which it produces. A careful search in the library of the Naval Observatory failed to reveal any data upon which to found an estimate of the value of the exposure constant for the 1474 line. Under these circumstances, if spectroscopes with slits are used it will probably be best to expose a single plate during the whole three minutes of totality; and as prisms give brighter spectra than diffraction gratings, the former will be pref- erable for any moderate degree of dispersion ; unless, indeed, the object is to photo- 24 irraph tin- ultra violet r;ivs. If prismatic cameras are employed, the exposure constant should probably IK- from 0.234 to 0.469 at least, these were the values used l>y the Kuglish c\])edition to Siaiu in their work on the eclipse of April 6, 1875. SECTION VII. I'OLAUISCOPIC OBSERVATIONS. ( Hiservations of the polarization of the corona wore first undertaken at a time when it was still uncertain whether the corona was a true solar phenomenon, or was in some way produced l>y the earth's atmosphere ; and their primary object was to decide between these two hypotheses. Since then the spectroscope and photography have placed the solar origin of the corona beyond doubt; but the polariscopic obser- vations have proved so delicate that the results hitherto obtained from them cannot be regarded as final. Besides, the details of the polarization, if sufficiently well defined, may reveal something of the condition of the matter emitting the coronal light; and if photography can be substituted for eye-observations, information may even be ob- tained concerning regions further from the sun's surface than any of which we have at present cognizance. Dfsrrijition of fiifttntiiicittfi. The polariscope has assumed many forms, but those best adapted to the investigation now under consideration may be briefly described, as follows : Iiixti-nn'iit A. A Savart polariscope, consisting of a compound quart/ plate and a Nicol's prism or tourmaline: preferably the latter. This instrument indicates the presence of polarized light by exhibiting a series of bands, either parallel or perpen- dicular to the plane of polarization ; and it is so delicate that these bands become visible whenever the polarized light amounts to so much as one per cent, of the total illumination. To discriminate between the plane of polarization and a plane perpen- dicular to it, we must know whether the central band is black or white, and as that is a difficult question when the bands are faint, it will be desirable to make a small addi- tion to the apparatus. The Savart polariscope being attached to the eye-piece of a telescope in the usual way, a light-colored tourmaline should be inserted in the eve- piece in such a position as to cut, off a small segment of the field of view: the axis of the tourmaline being parallel to the edge or chord of the segment, and the bands being perpendicular to that chord. Of course the tourmaline must be so placed as to be dis- tinctly visible through the eye-piece, and it must also maintain its position relatively to the Savart when the eye-piece is rotated. With this arrangement, the plane of po- larization will be determined by noting whether the bands continue across the tour- maline or break off at its edge liixti-iniii'iit I'>.\ pair of l.abinet's quartz compensating wedges, placed at the common focus of the objective and eye-piece of a telescope, the eve-piece being pro- vided with an analyzer consisting of a Nicol's prism or tourmaline. This instrument indicates the presence of polarized light by exhibiting a series of bands, which make an angle of forty-live degrees with the plane of polarization. It nearly equals the Savart in delicacy, and excels it in the facility it affords for determining the plai f polarization, because the position of the middle band can be marked upon the quartz, and then a glance suffices to show whether it is black or \\hite. / l,',iiiii-iit ('. A biquartz, arranged in the same manner as the I'.abinet's wedge.- 25 just described. This instrument indicates the presence of polarized light by the colora- tion of the quartz ; which, however, does not become perceptible until the polarized light amounts to from 5 to 15 per cent, of the total illumination. If the biquartz is turned till its two halves are colored alike, both being of the purple hue known as the sensitive tint, or tint of passage, then the line of junction of the two pieces will be in the plane of polarization, and as a very slight rotation suffices to destroy the uni- formity of the tint, this position can be found with great exactness. Thus it appears that a biquartz is inferior to the instruments A or B in sensitiveness to feebly polar- ized light, but superior to them in the accuracy with which it will determine the plane of polarization. Instrument D. An Arago polariscope, which consists either of a double-image prism and a plate of quartz cut parallel to the axis, or of a double-image prism and a plate of selenite. It will be most convenient to mount the prism and plate of quartz, or selenite, in a brass cell, which can either be used alone, or can be inserted in one end of a tube about a foot long which contains at its opposite extremity a diaphragm of such size that the two images of it, seen on looking through the polariscope, may appear just in contact with each other. This instrument indicates the presence of polarized light by the coloration of the field of view, and has about the same degree of sensitiveness as the biquartz, but it is not well adapted to the exact determination of the plane of polarization. Instrument E. An achromatic double-image prism. Instrument F. A Nicol's prism, or a tourmaline. These instruments, as well as E, are specially valuable when the light is faint, but they will scarcely indicate the pres- ence of polarization unless its amount is at least 10 per cent, of the total illumination. A Polarimctcr will be necessary to determine the percentage of polarized light present. It should consist of four plates of glass, turning upon an axis lying in a plane parallel to their surfaces and at right angles to their length, and carrying an index moving over a graduated circle arranged to show the angle through which the plates are turned. The object whose light is to be tested is viewed through the polarimeter with a Savart's polariscope, so set that the bands seen in it are parallel to the axis of rotation of the plates ; then the whole instrument is turned until the bands are per- pendicular to the plane of polarization of the light to be tested; and, finally, the plates are rotated until the bands disappear. When this happens, the amount of polarization present is just equal to that produced by the plates, the amount of which is known from the reading of the index. With the exception of D and E, which are best used in the hand, all the above- described polariscopes should be attached to telescopes provided with positive eye- pieces magnifying about twenty diameters, and giving pencils of light sufficiently large to fill the pupil of the eve completely. Before using the telescopes their lenses should be examined by the aid of polarized light, to make sure that they are free from strain, and after putting the instruments together again special care must be taken to see that all reflections are stopped out in the tubes. Any neglect of these precautions may lead to serious errors. Points to fee investigated. On the whole,' the observations hitherto made seem to lead to the conclusion that the corona is strongly polarized in planes passing through 4 26 the sun's center ; or, in other words, radially ; anil that upon this radial polarization there is superposed a much feebler vertical polarization, which is nearly uniform over tin- whole corona. The radial polarization is probably due to reflections taking place within a few hundred thousand miles of the sun's surface, and Professor Pickering- lias shown that if it is produced in accordance with Fresnel's theory it should be strongest in the outer corona, and should diminish to nothing close to the sun's limb. The ver- tical polarization is probably due to the action of our own atmosphere. Theoretically there should be no difference between the indications of an Arago polariscope with a quartz plate and one with a selenite plate; and upon ordinary objects the results given by these two forms of the instrument ai m e certainly identical; but Professor Pickering thinks that his observations upon the eclipses of August, 1869, and December, 1870, show that there is some peculiarity about the corona which causes its light to behave differently toward a quartz plate from what it does toward a selenite one. From these considerations, it will be gathered that the questions demanding special attention are the following : 1. Is the light of the corona polarized radially or vertically, or in both these ways? 2. If radial polarization exists, does it extend with unabated intensity down to the sun's limb, or does it continually diminish, and finally vanish, in passing from the outer corona inward to the chromosphere ? 3. What is the percentage of polarized light present ? 4. In examining the corona with an Arago polariscope, do the results given by an instrument with a quartz plate differ from those given by an instrument with a selenite plate? MctJiods of Observation. If a beam of light polarized in a vertical plane is exam- ined through any of the polariscopes described above, the appearances presenting themselves as the instrument is rotated will be as indicated in the following table : 1<2 11 II a c .J. o c ' - 3 tc CU *O & 03 rt a c 3 J UO 3 O rn c U . !_, 3 O c Is. 1.2 8 o K e c in ts) c t: O C3 o M .2 o o H .fl o u HH "M a c g "^ W5 'Sea .s c "a uiBbcg. S S2P - c "3 ,: HI III ^ CJ 0> o > > 3 lali H -S J ^ o u i; DO V r ^ < o U n PQ o Maximum. Disappear. Violet. Red. Green. Maximum. Minimum. Maximum. 45 Disappear. Maximum. Green, Red. Colorless. Equal. 90 Maximum. Disappear. Yellow. Red, Green. Max., Min. Minimum. 35 Disappear. Maximum. Green, Red. Colorless. Equal. 1 80 Maximum. Disappear. Violet. Green. Red. Minimum. Maximum. Maximum. 225 Disappear. Maximum. Red, Green. Colorless. Equal. 270 Maximum. Disappear. Yellow. Green, Red. Min., Max. Minimum. 3'5 Disappear. Maximum. Red, Green. Colorless. Equal. 27 Of course the changes indicated in the table take place, not suddenly, but gradually and continuously, as the polariscope is rotated. With the Savart and Babinet instruments, if the bands are black centered at the first and third maxima they will be white centered at the second and fourth maxima, or vice versa. In describing the action of the biquartz and Arago instruments the colors red and green are mentioned; but it must be clearly understood that the two complementary tints actually given by any instrument are determined solely by the thickness of the quartz or selenite plate employed. At first sight the table seems to indicate that no two polariscopes are governed by the same law ; but such a conclusion would be a mistake. They all exhibit two perfectly distinct test phenomena, or reactions ; as for example, bands with a white center and bands with a black center, or a given tint and its complementary, or a maximum and a minimum of light ; and if in any given position of the principal sec- tion of the polariscope one of these reactions is at a maximum, then the same reac- tion will also be at a maximum when the position of the polariscope is 1 80 different, while for positions 90 or 270 different the other reaction will be at a maximum. Now, supposing the eclipsed sun to be viewed through a telescope armed with a polariscope, let us apply the principles just enunciated to the solution of the first of the questions proposed above. The corona will be brought to the center of the field of view, and the polariscope will be turned until it exhibits a maximum of one of the reactions. Then, if the polarization is vertical, this reaction will be exhibited over the whole corona ; but if the polarization is radical, the corona will appear divided into four quadrants, the first and third of which will exhibit one of the reactions while the second and fourth will exhibit the other. Next, the polariscope will be slowly rotated, while the corona is kept steadily in the center of the field of view. Then, if the polarization is vertical, the first reaction will gradually fade out and be replaced by the second, which in its turn will be again replaced by the first, and so on, as indicated in the table above ; but if the polarization is radial, the two reactions exhibited in the alternate quadrants of the corona will not undergo any change, except that they will rotate with the polariscope. The observations necessary for the solution of the second question proposed above may be made with almost any polariscope by noting whether or not the reactions just described continue undiminished quite down to the moon. It is probable, how- ever, that the results obtained from a biquartz will be most satisfactory. It should be set so that the normal to the moon's limb makes an angle with the line of junction of its two halves sufficiently large to cause their colors to contrast vividly. Then the observation will consist in noting whether this contrast continues with unabated force all the way to the moon's limb, or whether it gradually diminishes, and finally fades out. The amount of polarized light present in the corona can only be determined by measurements with the polarimeter, the method of using which has been already described. In dealing with the fourth question proposed above, it is desirable that the two Arago polariscopes employed should give as nearly as possible the same colors. The 28 method of proce durewill be as follows : Firstly, the corona will be examined through one of the polariscopefl, furnished with its tube and diaphragm, and the reactions, both upon the corona and upon the sky, will be noted; secondly, the tube and diaphragm will be removed from tin- brass cell containing the prism and plate, and the corona will be again observed. In this case, the images of two portions of the sky, distant from each other about t\vo and a half degrees, will be superposed, thus eliminating their polarization, and therefore it will only be necessary to note the reaction upon the corona itself; thirdly, all the observations will be repeated with the other polari- scope. In order to familiarize themselves with the appearances likely to be presented during the eclipse, intending observers will do well to practice beforehand upon an artificial corona radially polarized, such as has been described by Professor Picker- ing in the United States Coast Survey Report for 1870, page 167. The polariscopes which seem likely to give the best results are Babinet's compensating wedges and the biquartz.* Photography. Among the various forms of polariscopes the Nicol's and don hi e- image prism seem almost the only ones available under the peculiar conditions which surround attempts to photograph the corona. If a Nicol is employed, its aperture will probably be comparatively small, and on that account it should be placed behind the photographic objective, and near its focal point; but if a double-image prism is used, its aperture should be sufficient to permit its being placed immediately before the objective. On the whole, the simplest form of photographic polariscope, and that most likely to yield satisfactory results, seems to be an ordinary camera, provided with a photographic objective having an intensity ratio of about one-eighth and a focal dis- tance of about twelve inches, immediately before which is mounted a double-image prism with a clear aperture equal to that of the objective. With this apparatus wet collodion plates of ordinary sensitiveness should be exposed about thirty seconds, and after each exposure the prism should be rotated thirty degrees, the motion being in the same direction as that of the hands of a watch. SECTION VIIL PHOTOMETRIC OBSERVATIONS. - As there is now reason for suspecting great variability both in the size and in the brightness of tlie corona, exact photometric observations during total solar eclipses become of much importance ; and in making them, attention may be directed either to the general illumination of the atmosphere, or to the quantity of light emitted by the corona. For investigating the first of these points the apparatus devised by Prof. J. II. Eastman, and used by him in observing the eclipse of August 7, 1869, will be found For farther details consult the following: Report on Observations of the Total Solar Eclipse of December 22, 1870. By Prof. E. C. Pickering. U. 8. Coast Survey Report for'iS/o, p. 165. List of Observations on tin- I'ol:iri/.ation of the Corona. By Prof. E.G. Pickering. Journal of the Franklin Institute, 1871, vol. 61, p. 58. Instructions for Observers at tin- English GoviTimiriit Eclipse Expedition, 1871. Nature, 1871, vol. 5, p. 18. Suggestions to Observers of t lie Solar Eclipse of DcctMnber next. By A. C. Ranyard. Nature, 1871, vol. 4, p. 327. English Government Eclipse Expedition, 1875. Instructions to Observers. Nature, 1875, vol. n, p. 352. 29 / very satisfactory.* On that occasion the light during totality proved about equal to that on a clear moonless evening, at the time Avhen the sun has sunk so far below the horizon that third magnitude stars are just becoming easily visible. For measuring directly the amount of light emitted by the corona, some form of Bunsen's photometer will be required, and probably that adopted by Professor Pick- ering will be found as convenient as any.f It consists of a box 9 inches wide, 1 8 inches high, and 6 feet long, within which a lighted candle can be moved backward and forward by means of a long projecting rod. One end of the box is covered by a piece of thin white unruled writing paper, in the center of which is a greased spot about half an inch in diameter. The observation consists in pointing the box toward the corona, so that the rays of the latter may fall squarely upon one side of the paper while those of the candle fall upon the other; moving the candle by means of the rod until the greased spot disappears ; and then marking upon the rod by a stroke of a lead-pencil the distance between the candle and paper. A number of disappearances should be observed, and after totality is over the distances between the marks and the candle must be measured and recorded. During the eclipse of December 22, 1870, Professor Pickering found that, on the average, the candle had to be 18.5 inches from the paper to make the spot disappear, but the observations were much embarrassed by flying clouds. Owing to the difference of color between daylight and candle-light, no adjust- ment of the candle will suffice to make the greased spot disappear completely ; it can only be reduced to a minimum of visibility.! Care must be taken to have the interior of the photometer-box painted dead black with a mixture of lamp-black, shell-lac, and alcohol. The candle must be of wax or sperm, of the kind known as six to the pound, and experiments must be made to determine exactly how many grains it burns per hour ; or, better, the candle may be sent to the Naval Observatory and these expe- riments will be made there. Persons who are inexperienced in photometry should have some practice previous to the eclipse. The necessary apparatus will be, two lighted candles placed three or four feet apart, and a sheet of thin white writing-paper, having in its center a spot half an inch in diameter, rendered transparent by the application of a little grease. The practice will consist in holding the paper in, and at right angles to, the line joining the two candles, and then moving it backward or forward until the greased spot disappears, which it will do when the illumination on the two sides of the paper is exactly equal. When this experiment has become quite familiar, the photometer which is to be em- ployed during the eclipse should be taken into a room dimly lighted by daylight, and the intensity of the illumination should be repeatedly measured by making the spot disappear as completely as possible, and then noting the interval between the candle and the paper. Excellent practice may also be had by measuring the intensity of twilight in the early evening. * Reports on Observations of the Total Eclipse of the Suu, August 7, 1869, Appendix II to the Washington Observa- tions for 1867, p. 100. t See United States Coast Survey Report for 1870, p. 172. t The greased spot can be made to disappear completely by using paper which is blue on one side and white_on the other ; but this cannot be recommended, because it introduces serious theorectical difficulties. 30 SECTION IX. THERMO-ELECTRIC OBSERVATIONS. The principal points to be investigated are the total amount of heat emitted by the corona and chromosphere, and the relative temperatures of the prominences and of different parts of the corona. The galvanometer employed should be very sensitive, and it will be advisable to have some small resistances which can be inserted in the circuit to reduce the deflections of the needle in case the currents given by the pile prove unmanageably strong. For investigating the first of the points mentioned above, an ordinary thermo- pile, provided with the usual conical reflector, will be the best instrument ; and it should be used by pointing it to the corona without the intervention of any condensing lens or mirror whatever. After measuring the heat in that way, its so-called quality may be tested by noting the effects produced by the interposition of screens of glass or other transparent material of various known thicknesses. For investigating the relative temperature of the prominences, and of different parts of the corona, a thermopile placed in the focus of a telescope, which should be a reflector if possible, will be required; and as we are ignorant of the degree of heat to be expected, the pile should not be too delicate. A very suitable one can be made by taking a piece of iron wire about six inches long, soldering to each of its extremi- ties pieces of german-silver wire, and then bending the whole into the form of an M, the solderings being at the points forming the top of the M. The wire should not be more than one or two hundredths of an inch in diameter, and the whole should be so mounted in a piece of cork that, the junctions may project only a short distance above its surface. The currents given by this pile will be due solely to the differences of temperature of its two junctions, and, consequently, the galvanometer will be insensible to any flow of heat which affects both junctions alike. The mode of using the instrument will be to place one of its junctions upon the moon, or upon a promi- nence, and the other upon the point whose relative temperature is to be measured. One of the most important points to be ascertained is how much more heat comes from the prominences than from the neighboring portions of the corona. As soon as possible after the totality is ended, the constants of the apparatus should be most carefully determined, so as to furnish the means of converting the gal- vanometer readings into ordinary thermometric degrees. Plate 1. fioftont f - V .