LIBRARY 
 
 OF THE 
 
 UNIVERSITY OF CALIFORNIA. 
 
 GIF"T OB" 
 
 C/ass 
 
THE 1900 SOLAR ECLIPSE 
 EXPEDITION 
 
 OF THE 
 
 ASTROPHYSICAL OBSERVATORY 
 
 OF THE 
 
 SMITHSONIAN INSTITUTION, 
 
 BY 
 
 S. P. LANGLEY, Director, 
 
 Aided by C. G. ABBOT. 
 
 (No. 1439.) 
 
 WASHINGTON: 
 
 GOVERNMENT PRINTING OFFICE. 
 1904. 
 
THE 1900 SOLAR ECLIPSE 
 EXPEDITION 
 
 OF THE 
 
 ASTROPHYSICAL OBSERVATORY 
 
 OF THE 
 
 SMITHSONIAN INSTITUTION. 
 
 BY 
 
 S. P. LANGLEY, Director, 
 
 Aided by C. G. ABBOT. 
 
 ( No. 1439.) 
 
 WASHINGTON: 
 
 GOVERNMENT PRINTING OFFICE. 
 1904. 
 
,OO 
 
 Y 
 
 

 THE SOLAR ECLIPSE EXPEDITION 
 
 OF THE 
 
 ASTROPHYSICAL OBSERVATORY OF THE SMITHSONIAN INSTITUTION, 
 
 MAY, 19OO. 
 
 CHAPTER I. 
 
 OBJECTS, AND PREPARATION FOR THE EXPEDITION. 
 
 The path of the total solar eclipse of May 28, 1900, was unusually accessible 
 from Washington. Preliminary observations by the United States Weather Bureau 
 indicated a fair prospect of good observing weather along the middle portion of the 
 belt of totality within the United States. As the conditions on this occasion were so 
 favorable to observation, it seemed desirable to send observers from the~8mithsonian 
 Astrophvsical Observatory; and Congress was therefore asked to appropriate the sum 
 of 84,000 to defray the expenses of the expedition, and this sum was so appropriated 
 and made immediately available." 
 
 1. OBJECTS OF THE EXPEDITION. 
 
 STRUCTUKE OF THE INNER CORONA. 
 
 Although many excellent photographs of the corona had been taken in recent 
 years, notably by Schaberle and by Campbell of the Lick Observatory, none of them 
 had exhibited adequately the very curious and interesting structure which the writer 
 and others had observed close to the sun's limb during the eclipse of 1878.'' In the 
 hope of obtaining material for a study of this peculiar structure of the inner corona, 
 its photography upon a greater scale than any hitherto attempted was decided upon 
 as a prominent object of the expedition. 
 
 "Observation of eclipse of May twenty-eighth, nineteen hundred: For cost of apparatus, transportation of 
 observers and equipment, subsistence, reduction of observations, printing and publishing of results, not exceeding 
 one thousand five hundred copies, and employment of such temporary aid as may be required, including all 
 necessary field and other expenses, four thousand dollars. Deficiency act, February 9, 1900. 
 
 bSee Report of S. P. Langley, Washington Astronomical and Meteorological Observations, 1876, pt. 2, Vol. XXIII, 
 p. 203. 
 
 3 
 
 130164 
 
4 THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 Before procuring the apparatus for this purpose, several astronomers of great 
 experience were consulted. It was found to be the consensus of opinion that a lens of 
 both larger aperture and longer focus than that used by Campbell in 1898 would be 
 required, and I was upon the point of ordering such an one of 8 inches aperture and 
 60 feet focus, when Prof. E. C. Pickering, the director of Harvard College Observatory, 
 very generously offered me the loan of a lens of 12 inches aperture and 135 feet focus, 
 together with other auxiliary apparatus. So welcome an offer could not but make 
 the attainment of this object of the expedition paramount over all other considerations, 
 and when in addition, through Prof. C. A. Young and the trustees of Princeton Uni- 
 versity, the use of a 5-inch lens of 38 feet focus was secured, similar to that used on 
 several occasions by Lick Observatory eclipse expeditions, it was apparent that in 
 optical equipment for photographing the inner corona the expedition would be well 
 provided. 
 
 VISUAL OBSERVATIONS. 
 
 To supplement this special photographic study of the inner corona, the writer 
 desired to have eye observations made in order to see if the filamentary appearance 
 would be as well marked as in 1878. He therefore felt great gratification in accepting 
 the loan through Prof. S. J. Brown, astronomical director of the United States Naval 
 Observatory, of the identical 5-inch telescope which he had used in the observations 
 on Pikes Peak in 1878. Two other visual telescopes of 3 and 7 feet focus were also 
 used. 
 
 PHOTOGRAPHY OF THE OUTER CORONA. 
 
 To photograph the outer corona several cameras of moderate focal length were 
 provided. Long exposures were intended for these, so as to reach the extremest 
 limits of the corona which could be photographed at an eclipse of such short duration. 
 
 BOLOMETRIC WORK. 
 
 The long experience of this observatory in delicate bolometeric measures on the 
 solar spectrum suggested the advisability of an attempt to detect the heating effect 
 of the coronal radiations and, if possible, to determine the distribution of their 
 energy in the spectrum; for if this latter point could be determined, light would be 
 thrown on the still disputed question of the nature of the corona. It will be 
 remembered that different sources of radiation emit very unequal proportions of 
 energy at different wave lengths, so that, for example, if the corona owes its light 
 to reflection of the photospheric radiations by particles of dust about the sun an 
 energy spectrum generally similar to that of the sun itself would be formed; but if 
 the light is, on the other hand, due to incandescence of such dust particles, owing to 
 their proximity to the sun, a coronal spectrum relatively richer than the sun's in 
 
 a The expedition is indebted to Mr. Saegmuller, of Washington, for the loan of a mounting for one of these. 
 
THE 1900 SOLAR ECLIPSE EXPEDITION. 5 
 
 infra-red rays would be found, owing to the temperature of the particles being less 
 than that of the sun itself. Bolometric study of the corona was therefore included 
 among the objects of the expedition. 
 
 PHOTOGRAPHIC SEARCH FOR INTRA-MERCURIAL PLANETS. 
 
 It was learned after preparations for the eclipse were somewhat advanced 
 that Prof. W. H. Pickering, of Harvard College Observatory, intended to make 
 a photographic search for as yet undiscovered planets near the sun by the aid 
 of a special apparatus of his own devising. It was further learned that Professor 
 Pickering, and also the eclipse committee of the Astronomical and Astrophysical 
 Society of America, desired, if possible, that a search with similar apparatus should 
 be made by other eclipse parties. This observation was with some hesitancy added 
 to the programme, for I had no strong expectation that anything but negative 
 evidence would be secured. 
 
 PHOTOGRAPHY OF THE "FLASH SPECTRUM." 
 
 In loaning the great lens above referred to, the director of the Harvard College 
 Observatory suggested the use of an objective prism with it at the beginning of 
 totality for the purpose of photographing the "flash spectrum," and offered to supply 
 the prism and an automatic apparatus for exposing the plate each second to the 
 spectrum. This proposal was accepted, and led to the only attempt at photographic 
 spectroscopy included in the programme. 
 
 TIMES OF CONTACT. 
 
 It was arranged that the observation of the times of contact should be made both 
 by the usual method of eye observation and by a photographic apparatus. 
 
 GENERAL SUMMARY OF THE OBJECTS OF THE EXPEDITION. 
 
 Briefly, the eclipse expedition was sent out from the Smithsonian Astrophysical 
 Observatory for the following purposes: 
 
 1. To photograph and to observe visually the minute structure of the inner 
 corona and the prominences. 
 
 2. To photograph the outer extensions of the corona and possible intra-mercurial 
 planets or other objects near the sun. 
 
 3. To measure the heating effect of the inner coronal radiations, and, if possible, 
 to determine the distribution of their energy in the spectrum by the aid of the 
 bolometer. 
 
 4. To photograph the flash spectrum. 
 
 5. To determine the times of contact. 
 
6 THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 2. PREPARATION FOR THE EXPEDITION, 
 
 (a) SELECTION OF SITE. 
 
 The most accessible point on the belt of totality was Norfolk, Virginia, and here 
 also the duration of totality was a few seconds longer than at points farther west. 
 But, on the other hand, the special observations of the Weather Bureau, extending over 
 several years," had showed that the chances of good observing weather at points near 
 the coast were inferior to those in the inland districts, and rose to a maximum for 
 portions of the States of Georgia and Alabama. Nevertheless the reports from a few 
 stations in North Carolina came well up with the best Georgia records. 
 
 The advantages of accessibility, duration of totality, and height of sun during 
 totality, all weighing in favor of eastern stations, with only slight and not very certain 
 chances of better weather as an inducement to go farther to the south and west, I 
 decided to send Mr. Abbot, aid acting in charge of the Astrophysical Observatory, 
 to look over the ground and report recommending some station in North or South 
 Carolina. He crossed the State of North Carolina, and being convinced that there 
 would >>e no appreciable gain by going into South Carolina, he recommended as 
 alternatives the towns of Wintou and Wadesboro, North Carolina, situated respectively 
 in the northeastern and south central portions of the State. According to the Weather 
 Bureau bulletin the chances of fair weather at Wadesboro were almost 5 to 1, and 
 were somewhat better than those recorded for Winton. Besides this difference for the 
 towns themselves, the data for points in their near neighborhoods indicated a decided 
 preference for Wadesboro, and with this in mind I finally selected it. 
 
 The ground occupied was a sod-covered level field of several acres extent, on the 
 edge of the town, which was most cordially placed at the disposal of the Smithsonian 
 Institution and its guests by its owner, John Leak, esq., of Wadesboro. 
 
 Mr. Abbot visited Wadesboro early in April to lay off the ground, and arranged 
 with Mr. William Brasington, a local contractor, to erect a shed for the apparatus, 
 and to provide material required for piers and support of apparatus. Accommoda- 
 tions were engaged for the party at the National Hotel, Wadesboro. The proprietor, 
 Mr. Huntley, did his utmost to make arrangements for the unusual number of these 
 and other eclipse guests, and at all times cared for their comfort. 
 
 (6) PERSONNEL OF THE EXPp;DITION. 
 
 The staff of the Astrophysical Observatory available for the expedition consisted 
 of three persons besides the writer, namely: Mr. C. G. Abbot, aid acting in charge, 
 Mr. F. E. Fowle, jr., junior assistant, and Mr. A. Kramer, instrument maker. The 
 observing force was increased by special detail, invitation, and volunteering as follows: 
 Mr. T. W. Smillie, for many years photographer of the National Museum, was placed 
 
 "Bulletin No. 27., W. B., No. 209., U. 8. Department of Agriculture, Weather Bureau. 
 
THE 1900 SOLAR ECLIPSE EXPEDITION. 7 
 
 in cnarge of the photographic work, and Mr. De Lancey Grill, of the Bureau of Ameri- 
 can Ethnology, was detached to be photographic assistant to Mr. Smillie. Mr. C. W. 
 Smith, a carpenter, was sent from the Smithsonian Institution. At my request the 
 Superintendent of the United States Coast and Geodetic Survey detailed Assistant 
 Gr. R. Putnam (since placed in charge of the survey of the Philippine Islands), to 
 determine the latitude and longitude of the camp and assist in the orientation of appa- 
 ratus, in which respect his services were most acceptable, as well as to observe visually 
 on the day of the eclispe. The following five gentlemen joined the expedition by 
 invitation: The Rev. Father Searle, C. S. P , with the Rev. Father Woodman, C. S. P., 
 from the observatory of the Catholic University of America; C. E. Mendenhall, Ph. D., 
 then instructoi in physics at Williams College; Mr. R. C. Child, for several years 
 connected with the Astrophysical Observatory, but more recently employed in the 
 United States Patent Office ; and Mr. P. A. Draper, of Washington. On the day of the 
 eclipse two volunteers, Mr. Little, of Wadesboro, and Mr. W. J Hoxie, of Beaufort, 
 South Carolina, assisted Mr. Putnam in giving signals and recording observations, 
 while Mr. George Wells, of the Smithsonian Institution, recorded the observations of 
 the writer. 
 
 Mr. C. D Walcott, Director of the United States Geological Survey; Mr. R. 
 Rathbun, assistant secretary of the Smithsonian Institution; Mr. W. N. Bannard, of the 
 Pennsylvania Railroad; and Mr. J. E. Watkins, of the National Museum, accompanied 
 the writer to Wadesboro two days before the eclipse, and remained as interested 
 spectators and observers of the general, aspects of the phenomenon. 
 
 RELATIONS WITH THE YEKKES OBSERVATORY EXPEDITION. 
 
 Several months prior to the eclipse an invitation was extended to Prof. G. E. 
 Hale, director of the Yerkes Observatory, to take part as a member of the Smithsonian 
 expedition. This invitation he at first was inclined to accept, and provision was made 
 for his use of two beams of light from coelostat mirrors attached to a second axis 
 added to the great Grubb siderostat. A little later, finding that the Yerkes Observa- 
 tory itself would be able to observe the eclipse independently, Professor Hale with- 
 drew as a member of the Smithsonian expedition, but as the two parties were located 
 close together, he was still able to make use of the coelostat beams above mentioned. 
 Much benefit was derived from the counsel and friendly interest of the director and 
 members of the Yerkes Observatory expedition, thus so fortunately located near that 
 of the Smithsonian Institution. 
 
 (e) TRANSPORTATION. 
 
 The plans of the observations had so far outgrown what was intended when the 
 small appropriation was asked for that this proved hardly sufficient, with strictest 
 economy, to defray all the expenses. In these circumstances I felt justified in asking 
 
8 THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 the cooperation of the several railroads whose tracks should be traversed in the 
 journey. They generously responded, offering each and all free transportation 
 both for observers and freight. I take this opportunity, therefore, to acknowledge 
 the indebtedness of the expedition to the Pennsylvania, the Richmond, Fredericks- 
 burg and Potomac, the Atlantic Coast Line, and the Seaboard Air Line Railroad 
 companies for the free transportation of observers and equipment both going and 
 returning over the portions of the route included by their several lines. 
 
 (d) PRELIMINARY PRACTICE AT WASHINGTON. 
 
 There can be no doubt that one of the first essentials to success in eclipse 
 observations is adequate preliminary practice; for the time of totality is too short for 
 thinking, and the mind is too much disturbed by the feeling of necessity for haste to 
 do rightly anything which has not become a settled habit. In order that there 
 should be the greatest possible opportunity for practice in the use of the most 
 important pieces of apparatus, and a chance to make such modifications as actual 
 trial might suggest while all the mechanical facilities were at hand, I gave especial 
 attention to a field installation for rehearsal, which was made in April by setting up 
 the apparatus in the Smithsonian Park. 
 
 A certain day and hour was selected at which an imaginary eclipse was to occur. 
 The Brashear coelostat, the Grubb siderostat, the portable photographic house, the 
 12-inch lens of 135-foot focus, and the 5-inch lens of 38-foot focus, with the cloth 
 tubes leading from the lenses to the photographic house, were all adjusted for the 
 supposed eclipse. Signals were given in warning, and at the time of assumed totality 
 actual shifting of the plate holders, as if for the photographic exposures, was gone 
 through with. The assumed eclipse being more accommodating than a real one, it 
 repeated itself a few minutes later to give a further opportunity for practice on the 
 part of the observers. As a preliminary rehearsal the whole went off very well. 
 
 At the conclusion of the rehearsal, which, together with the preparations for it, 
 lasted for several days, the apparatus and accessories were immediately taken down 
 and packed for shipment to Wadesboro. A freight car filled to the roof with the 
 equipment started from Washington on May 4 and reached Wadesboro on May 9. 
 
CHAPTER II. 
 
 1. ESTABLISHMENT OF THE CAMP. 
 
 A few days after the shipment of the apparatus the advance guard of the party, 
 consisting of Messrs. Abbot, Fowle, Kramer, Draper, and Smith, went on, carrying 
 with them the galvanometer for the bolometric work, and the chronometer and other 
 delicate pieces of apparatus. Immediately upon its arrival the carload of equipment 
 was removed to the camp, shown in PI. II, which had meantime been made ready 
 for occupancy under the good auspices of Mr. Brasington. From then till after the 
 eclipse day and night were filled with preparation. 
 
 Mr. Putnam, of the Coast and Geodetic Survey, reaching Wadesboro May 10, 
 immediately established his meridian instrument, and the weather continuing fine 
 he was able to get good observations for latitude and longitude on sevei'al nights. 
 During the days he assisted to adjust the axes of the Grubb siderostat and the 
 Brashear equatorial and coelostat, and ran north and south and east and west lines, 
 and independently computed and ran lines parallel to the direction of the coelostat 
 beam and the azimuth of the 40-foot direct telescope axis, so that his work during 
 the few days that he remained at Wadesboro was most valuable. 
 
 Mr. Putnam returned shortly before the eclipse and made additional observations 
 for position. The final values obtained for the latitude and longitude of the coast 
 survey tent shown in the diagram of the camp (PI. Ill) were: 
 
 Latitude 34 57' 52" North. 
 
 ( 80 04' 27" ) 
 Longitude j ^ g, ^ o 8 > West of Greenwich. 
 
 On May 16 Messrs. Smillie, Mendenhall, Child, and Gill came, and the rest of the 
 observers arrived on Friday and Saturday, May 25 and 26. 
 
 Meanwhile members of the Yerkes Observatory expedition had arrived, and as 
 Professor Hale was to make use of two mirrors mounted on a second axis connected 
 with the Grubb siderostat, their camp was laid out immediately adjoining ours to the 
 west. 
 
 From the first Mr. Brasington, the contractor, entered into the plans of both camps 
 with hearty interest, and his aid was most valuable in supplying everything in his 
 line, from bricks and lumber for the piers and building to barrels for reservoirs and 
 108A 04 2 9 
 
10 THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 washing tanks. He must have given up all his other work and devoted himself 
 and his force of men wholly to the eclipse, for besides the Smithsonian and Yerkes 
 expeditions, Professor Young and his party from Princeton, and the English expedi- 
 tions of the Reverend Mr. Bacon, and Mr. Nevil Maskelyne, had all come to Wades- 
 boro and found in Mr. Brasington their mainstay and ready helper for all demands. 
 
 RELATIONS WITH THE TOWNSPEOPLE. 
 
 The townspeople of Wadesboro manifested a lively interest in the eclipse 
 preparations; and not only were they interested, but helpful, and they treated the 
 observers with marked cordiality. At one of the first preliminary visits of Mr. Abbot 
 Judge Bennett, a prominent citizen and ex-Representative to Congress, called upon 
 him and tendered any assistance in his power to give an example followed by 
 others. After the arrival of the parties a meeting was held by the townspeople and 
 a committee appointed to assist the astronomers in any possible way. As a further 
 act of hospitality several carriages were placed at the disposal of such of the guests 
 as chose to ride; and finding that in the press of preparation this kindly offer could 
 not be improved so much as the people could have liked, the carriages were made 
 to call regularly to convey the observers to and from the hotels to the camps, 
 morning, noon, and night. Along with this public cordiality, private hospitalities 
 were not wanting, and one and all of those who observed at Wadesboro must join 
 in acknowledgment of the courtesies received. 
 
 As some slight return one or more of the Smithsonian party was usually in 
 attendance at the 5-inch visual telescope on fair nights to show celestial objects to 
 such as wished to look, and quite a number of the people gathered there every night and 
 seemed to enjoy the opportunity. 
 
 Provision was made by the town authorities for effectually guarding the grounds 
 from intrusion on the day of the eclipse. The whole space was roped off, the nearest 
 street closed, and special constables prevented all approach until after the eclipse had 
 
 passed. 
 
 2. APPARATUS AND ACCESSORIES. 
 
 The location of the apparatus at the camp is shown in PL III. On the left of the 
 "wagon track" is the main portion of the Yerkes camp. All at the right of the "wagon 
 track" appertains to the Smithsonian expedition, with the exception of the places 
 marked "spectroscope," "concave mirror," and "bolometer house," and those marked 
 "shadow band screen" and "meteorological station." The first three of these were 
 under Professor Hale's immediate charge, and were supplied with beams of light from 
 the Grubb siderostat, while the two last named were occupied Toy Mr. Clayton of the 
 Blue Hill Meteorological Observatory. 
 
 Passing now to the Smithsonian camp proper, there is shown at the northwest 
 corner the Brashear equatorial and coelostat, and immediately in front the 12-inch 
 
THE 1900 SOLAR ECLIPSE EXPEDITION. 11 
 
 Harvard lens and objective prism through which the beam from the coelostat passed 
 into the two-branched camera tube. The direct beam through the lens came to focus 
 in the small dark room, within which the operator changed plates and made the 
 exposures by a cord operating a shutter at the lens. Immediately before and after 
 totality the prism was to be interposed just in front of the lens to catch the "flash 
 spectrum" 1o be thrown down the northerly branch tube to a photographic plate 
 moved each second by the automatic plate carrier. 
 
 On the right of the dark room is seen projecting easterly the inclined tube of the 
 5-inch Princeton lens. This lens and its tube were fixed permanently at an inclina- 
 
 \ 
 
 tion suitable to the position of the sun on the moment of totality; and the image was 
 received on a moving plate holder driven by a water clock within the dark room. 
 An observer within the dark room changed plates and operated the exposing shutter 
 by a cord. 
 
 Under a little shelter on the north of the dark room was a chronograph and 
 chronometer, from which electric circuits ran to the shutters of the 12-inch and 5-inch 
 lenses, to the plate carrier of the prismatic camera, to the automatic contact camera 
 (shown north and east of the dark room), to the Coast Survey tent (shown at the 
 south of the camp), where contacts were observed by Mr. Putnam, to the SJ-inch 
 telescope where contacts were observed by Rev. Fr. Woodman, and finally to the 
 Yerkes camp. 
 
 Nearly west of the photographic dark room was the main building containing the 
 bolometric and " intramercurial planet" photographic apparatus. The bolometric 
 apparatus was scarcely less thoroughly installed than when permanently in use in 
 Washington. It was contained within a double-walled inner chamber. The galva- 
 nometer rested on a mercury float, itself supported on a system of stone and rubber 
 blocks, which in turn rested upon a heavy and deeply bedded pier. At a later page 
 the bolometric arrangements will be more fully described. 
 
 Within the bolometer chamber is shown the clock used to drive the intra- 
 mercurial planet apparatus. This clock" is the one customarily used to drive the 
 spectro-bolometric train at Washington, and was placed within the bolometric chamber, 
 as shown, in order to utilize its outside gearing with hand driving for producing a 
 holograph of the coronal spectrum, if this proved practicable, at the same time that 
 the clock itself was driving the large camera. 
 
 The intramercurial planet apparatus in the eastern wing of the building consisted 
 of four cameras mounted upon one polar axis. Of these, two were of 3 inches aperture 
 and 1 1 feet focus, and designed to cover fields of about 1 hour in right ascension by 
 10 in declination east and west of the sun, respectively. The two other cameras 
 had Ross lenses of 4 inches aperture and 39 inches focus. 
 
 "See PI. XIII, Annals of Astrophyaical Observatory, Smithsonian Institution, Vol. I. 
 
12 THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 Between the main building and the Coast Survey tent is located the 5-inch equa- 
 torial, and in a line running northeast from it were stationed a 3 J-inch telescope with 
 rough altazimuth mounting, ?nd still farther in the same line, but beyond the dark 
 room, a 6-inch equatorial of 7 feet focus. Near the CoavSt Survey tent was located 
 the signal bell upon which warnings were struck at the time of the eclipse, as directed 
 by the observer within the tent. 
 
 Several of the separate pieces of apparatus are shown in Pis. IV, V, VI, VII, 
 VIII, IX, X, and XI, and these may be consulted in connection with the following 
 description of some of the instruments. 
 
 THE COMBINED EQUATORIAL AND COELOSTAT. 
 
 (Plate IV.) 
 
 As long ago as 1880 I employed a 12-inch plane mirror on the south end of the 
 polar axis of the equatorial at Allegheny Observatory to give a fixed solar beam, and 
 was accustomed to use it either as a polar siderostat sending the beam down the axis, 
 or as what is now termed the coelostat. In this latter use the mirror lay in the plane 
 of the polar axis, which was then caused to rotate once in forty-eight hours instead of 
 once in twenty-four hours, as usual. Suppose the mirror to be set so that an observer 
 stationed at the east sees the sun's face as it rises, he will obviously continue to see it, 
 because the mirror turns half as fast as the earth. I did not then know that this 
 simple arrangement for a fixed beam was in fact described in 1863 by von Littrow" 
 who refers its discovery to August, nearly thirty years earlier. 
 
 This instrument is so simple mechanically and so desirable in practice from the 
 fact that it gives a fixed field of view instead of one rotating about a fixed central point 
 as in other siderostats, that I determined to employ the coelostat in connection with 
 the long-focus lens. 6 It was desired also to have an accurately driven equatorial 
 mounting for a 6-inch photographic lens of 7 feet focus, and the two requirements were 
 combined in the instrument shown in PI. IV, which was constructed for the occasion 
 by Brashear. Two polar axes, one for the equatorial camera, the other for the 18-inch 
 coelostat mirror, are driven by a common clock. The observer is shown ready to 
 manipulate both the exposure of the camera and the setting of the objective prism for 
 the "flash spectrum," both of which duties were included in his charge. 
 
 O. von Littrow, "Wien. Berichte, XLVIII, II, pp. 337-348, 1863. 
 
 & As generally employed the coelostat reflects the beam in a horizontal direction to the east or west according 
 as the celestial object is east or west of the meridian; so that as thus used the instrument is least efficient when 
 the object is farthest from rising or setting, respectively. A second mirror has recently been introduced here in 
 connection with the coelostat in such a way as to produce an efficient universal instrument, which is now (1903) 
 proving useful in the work of the Astrophysical Observatory, and it is hoped may meet with favor elsewhere. The 
 second mirror is mounted above and south of the coelostat proper, upon a support provided with slides in both an 
 east and west and north and south direction, like the slide rest of a lathe. The beam is sent north in a horizontal 
 direction from the second mirror. This mirror occupies different positions on its north and south track, according to 
 the declination of the celestial object, and is sometimes shifted east or west to avoid interrupting the entering beam. 
 For the sun, and at this latitude, two 27-inch mirrors will at the most unfavorable time give a 20-inch circular beam. 
 
THE 1900 SOLAR ECLIPSE EXPEDITION. 13 
 
 THE LONG-FOCUS TELESCOPES. 
 (Plates II, V, and VI.) 
 
 The tubes for the 135-foot focus lens were 42 inches square, of black canton 
 flannel stretched by ropes running longitudinally at the four corners, and supported 
 on trestlework. They were provided with diaphragms at 10-foot intervals and were 
 covered over with a canvas in the form of a prolonged A tent. For the direct 
 photography 30-inch square plates were used in plate holders with roll shutters. All 
 the photographic plates, specially prepared by the Cramer Company, of St. Louis, 
 were double coated, extremely rapid, and of the so-called "isochromatic" preparation. 
 
 The 38-foot focus tube was composed of a conical spruce frame of square cross 
 section covered with canvas blackened within and was supported at the proper angle 
 on the tops of posts. . The lens with its adjustments was separately supported upon a 
 post 8 inches square and braced well from the sides. As has been said the exposures 
 of both the long-focus cameras were made by the aid of cords running from the dark 
 room to shutters at the lenses. A water clock with horizontal cylinder was con- 
 structed to control the movement of the 11 by 14 inch plates along inclined guides at 
 the focus of the 38-foot lens. 
 
 INTRAMERCURIAL PLANET CAMERAS. 
 
 (Plate VII.) 
 
 As already stated, it had been recommended by Prof. W. H. Pickering that several 
 stations should be equipped with 3-inch lenses of 11 feet focus, to be used to photo- 
 graph large fields of view about the sun, in the hope of discovering possible intra- 
 mercurial planets. In PI. VI the projecting box marked "1, 2" is the end of a pair 
 of cameras of this type with axes inclined so that together the cameras covered a field 
 of view extending from about one hour east to one hour west and from 5 north to 
 5 south of the sun. It was intended to use a nest of 8 by 10 inch plates in each of 
 these cameras, arranged on a curved surface so as to be in the best focus in all parts 
 of the field, but single plates 24 by 30 inches were substituted finally, although as 
 the result proved the focus was not the best all over their extent. Two other large 
 cameras, shown in PI. VII, had Ross portrait lenses of 4 inches aperture and 40 
 inches focus, and these were used with 30-inch square plates. All these four rather 
 heavy cameras were mounted on a single "home-made" polar axis, but this was so 
 well counterbalanced that the clock drove it with great ease and accuracy. 
 
 THE AUTOMATIC RECORDING CONTACT CAMERA. 
 
 In this instrument a 22-inch focus lens, with pin-hole diaphragm, formed the 
 solar image on a 15-inch circular celluloid photographic plate of little sensitive- 
 ness, which was rotated by a ratchet wheel of 168 teeth, driven with an electric 
 escapement. This escapement and a shutter in front of the lens were operated 
 
14 THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 once a second by the break circuit chronometer, but the impulses were so timed by aid 
 of relays that the shutter was always opened and closed before the plate was moved 
 along one tooth. An observer was charged with the duty of cutting off exposures 
 with a card at certain seconds after the minute break. In consequence of the fifty- 
 ninth impulse being dropped by the chronometer, only 59 images were made by the 
 instrument in a minute, and as there were 168 teeth on the ratchet wheel it is apparent 
 that more than two and one-half minutes were consumed in one rotation of the plate. 
 During that time the image of the sun would be carried so far by the earth's diurnal 
 motion that the successive rows of images would not overlap, but would be in a spiral. 
 
 THE BOLOMETKIC APPARATUS. 
 (Plates VIII and IX.) 
 
 The bolometric observations lay in a wholly untried field, and practically no 
 forecast could be made of the heating effect of the coronal radiations to be measured, 
 and hence provision was made for a considerable modification of the programme, 
 according to the results reached in the first seconds of totality. A 17-inch mirror, on 
 the Grubb siderostat, reflected the beam horizontally south through a cat's-eye 
 diaphragm, whose aperture could be controlled by cords running to a slider on a scale 
 at the left of the observer at the galvanometer. He could thus vary the intensity of 
 the beam several thousand fold. The beam was concentrated upon a slit 1 centimeter 
 high and 1 millimeter wide by a concave mirror of 50 centimeters aperture and 1 
 meter focus, controlled by the second observer, who, watching the image on the jaws 
 of this slit, adjusted the concave mirror so as to throw the desired part upon it. Two 
 small flat mirrors beyond the slit reflected the beam to a collimating mirror of 75 
 centimeters focus, whence it passed to the prism. 
 
 This was a compound objective prism, loaned by the Harvard College Observa- 
 tory, having two circular half prisms rotating about a common axis, so that their 
 combined angle could be varied from to 15. This feature was not used during 
 the eclipse, but previously the prisms were set at an angle judged proper for 
 dispersing the coronal spectrum, should this prove desirable. One of these prisms 
 was silvered on the back, the intention being that the combination could be used 
 simply as a plane mirror in the early part of totality to get an idea of the intensity 
 of the coronal radiation, but could later be turned round so that the collimated 
 beam would pass through the prism from front to back and return as a spectrum. In 
 either case the rays next fell upon an image-forming mirror of 75 centimeters focus 
 and were brought to focus upon the bolometer strip, which was 1 centimeter high and 
 1 millimeter wide. The warming of this strip was to be determined by the deflection 
 of the sensitive galvanometer, and it was arranged that this deflection should be read 
 visually and photographically recorded, in the manner described in Volume I of the 
 Annals of the Astrophysical Observatory. 
 
THE 1900 SOLAR ECLIPSE EXPEDITION. 15 
 
 Two observers were employed. One was to read the galvanometer deflections, 
 record all observations, and operate the cat's-eye diaphragm, while the other was to 
 adjust the position of the image upon the slit, set the prism to previously adjusted 
 stops and clamp it, and to turn the prism and recording plate by clockwork through 
 previously determined intervals for observing the energy spectrum of the corona. 
 He also had in hand a pasteboard shutter and a plate of glass, which he was to inter- 
 pose before the bolometer at certain times to give readings, corresponding to zero of 
 outside radiations upon the bolometer, and to give an idea of the amount of radiations 
 transmissible by glass, respectively. 
 
 3. ASSIGNMENT OF OBSERVERS AND PRELIMINARY REHEARSALS. 
 
 The disposal of the observing force was as follows : 
 
 Mr. S. P. Langley, Director, telescopic observations with the 5-inch equatorial. 
 
 Mr. C. G. Abbot, aid acting in charge, manipulation of the spectrobolometer. 
 
 Mr. T. W. Smillie, in general charge of photographic work, manipulation of the 
 135-foot focus camera, 
 
 Mr. F. E. Fowle, jr., manipulation of the 38-foot focus camera. 
 
 The Rev. Father Searle, C. S. P., in general charge of intra-mercurial planet 
 apparatus, manipulation of the 11 -foot focus cameras. 
 
 Mr. G. R. Putnam, determination of times of contact and direction of warning 
 signals. 
 
 Mr. C. E. Mendenhall, observing at the galvanometer. 
 
 Mr. R. C. Child, telescopic observations with 6-inch equatorial and in charge of 
 electric circuits, including especially the prismatic camera and the automatic contact 
 camera. 
 
 The Rev. Father Woodman, C. S. P., times of contact and general observations 
 with 3^-inch telescope. 
 
 Mr. Delancey Gill, manipulation of 7^-foot focus camera and the objective prism. 
 
 Mr. P. A. Draper and Mr. C. W. B. Smith, exposures of the two 40-inch cameras 
 
 Mr. A. Kramer, instrument maker, looking to movements of the siderostat and 
 the 5-inch equatorial. 
 
 Two volunteers, Mr. Little, of Wadesboro, and Mr. Hoxie, of Beaufort, South 
 Carolina, were assigned, respectively, to strike signals and to record times of contact 
 under direction of Mr. Putnam. 
 
 On the two days next preceding the eclipse, several full rehearsals were carried 
 through to familiarize all the observers with their duties, which were specified in 
 written directions furnished each observer, that nothing might be overlooked. 
 
CHAPTER 
 
 OBSERVATIONS AND RESULTS. 
 
 The day of the eclipse proved cloudless; the sky, while not of the deepest blue, 
 was yet more than ordinarily clear, and the observing programme was carried through 
 with general success. In what follows, the observations will be described under these 
 four heads: (1) General and telescopic observations; (2) Photography of the inner 
 corona and prominences; (3) Photography of the outer corona and outlying regions 
 of sky; (4) Bolometric observation of the corona. 
 
 1. GENERAL AND TELESCOPIC OBSERVATIONS. 
 (a) GENERAL OBSERVATIONS. 
 
 The degree of darkness during totality was considered by several to be about 
 the same as on a clear night at full moon, but of planets and stars only a few of the 
 brightest were noted. Shadow bands were seen by several observers, but not very 
 distinctly. Mr. Child reported a pronounced pulse of light and shade eight minutes 
 before totality. The cessation of the noises of birds and animals as the time of 
 totality drew near was remarked. The equatorial extensions of the corona on either 
 side of the sun were variously estimated at from 2 to 5 lunar diameters by naked-eye 
 
 observers. 
 
 (6) COLOR OF THE CORONA. 
 
 Mr. Child, an artist, noted a distinct apple-green tinge to the outer coronal light, 
 which held to within a half diameter of the limb, where the color began to grow more 
 yellow, the extreme inner coruna appearing to him of a pale golden hue. Others 
 likened the coloring to that of mother-of-pearl, but nearly all noted a distinct yellow- 
 ish tinge. One observer reported a ruddier color toward the outer end of the stream- 
 ers on the lower side of the sun, but several noted, on the contrary, a tendency 
 toward green at increasing distances from the limb. 
 
 (c) TELESCOPIC VIEW. 
 
 To the writer's view with the 5-inch telescope the inner corona was filled with 
 detail, but far less sharp and definite than he saw it on Pikes Peak in 1878. He 
 could not identify any connection between the coronal structure and the presence of 
 108A 04 3 17 
 
18 
 
 THK 1900 SOLAR ECLIPSE EXPEDITION. 
 
 prominences, while his impression was that the details contained more ogivai curves 
 than straight streamers. Having in mind the wonderful structure seen with the same 
 instrument in the clear mountain air twenty-two years before, * the impression was a 
 disappointing one. 
 
 Mr. Child and the Rev. Father Woodman noted the appearance of Bailey's beads 
 very distinctly, and the former made sketches from which he later prepared a repre- 
 sentation of the corona in pastel colors. 
 
 (d) TIMES OF CONTACT. 
 
 Times of contact were observed by Mr. Putnam and by the Rev. Father Wood- 
 man by eye and ear methods, and were also signaled to the chronograph. Plate X 
 shows the Coast and Geodetic Survey tent as adapted for the former's observations 
 on the day of the eclipse. Owing to the imperfect working of Mr. Putnam's circuit, 
 his chronograph record was lost, but Mr. Hoxie, recording for Mr. Putnam, noted the 
 instant of his key taps as well as the results of his eye and ear observations. The 
 following table gives the mean of these two records. Third contact, as given below, 
 was not observed in the telescope by Mr. Putnam, as he was hindered by giving 
 signals, but Mr. Hoxie recorded the flash of light on the chronometer face. Rev. 
 Father Woodman's observations were recorded by him with a pocket chronometer, 
 and his signals also appear on the chronograph sheet; and since there is a slight 
 discrepancy both records are given. The times given below depend on three chro- 
 nometers compared for several days with telegraphic noon signals supplied from 
 Washington. A further check was given by transit observations on the night after 
 the eclipse. 
 
 Number of contact. 
 
 Mean time for seventy-fifth meridian. 
 
 Mr. Putnam. 
 
 Rev. Father Woodman. 
 
 First 
 
 19" 36 19'. 7 
 20 45 15 . 5 
 20 46 44 . 1 
 22 05 37 . 3 
 
 Jly pocket chro- 
 nometer. 
 
 19 h 36 21" 
 20 45 16 
 20 46 47 
 22 05 26 
 
 By chrono- 
 graph sheet. 
 
 22'. 
 16.4 
 43.5 
 
 27.8 
 
 Second . 
 
 Third 
 
 Fourth 
 
 
 The record of the automatic contact camera proved of little value; for at first 
 and fourth contacts there is an uncertainty of ten seconds just when the indentation of 
 the image ceases; second contact was lost by failure of the electrical circuit, and five 
 seconds elapsed after third contact before the images became strong enough to be 
 distinguishable. The record is, however, so striking that a reproduction of the 
 negative which includes totality is here given. (PI. XI.) 
 
 a Washington Astronomical and Meteorological Observations, Vol. XXIII, pt. 2, p. 203, 1876. 
 
THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 19 
 
 A record of third contact is practically furnished by the chronograph sheet which 
 recorded the exposures of the 38-foot camera, for the negative corresponding to an 
 exposure terminating at 20 h 46 m 43". 7 shows a thin, bright crescent streak inside the 
 very black overexposed prominence layer. Therefore at the time noted a portion of 
 the sun had emerged which was sufficiently bright to produce a positive effect on the 
 
 plate. 
 
 2. PHOTOGRAPHY OF THE INNER CORONA AND PROMINENCES. 
 
 (a) THE 135-FOOT CAMERA. 
 
 Mr. Smillie secured five negatives during totality, which were given exposures of 
 |, 1, 4, 16, and 8 seconds, respectively, the last exposure ending a little after third 
 contact. Owing to a failure of the electric circuit, the exact times of exposure were 
 not recorded on the chronograph, as had been intended, but all the negatives are 
 excellent and show both the inner corona and the prominences with much detail. 
 The originals must be seen to be appreciated, for there is no way of reproducing on 
 the printed page what can be seen on looking through a glass negative against a 
 ground-glass surface. Nevertheless, where there is so much of interest in the original 
 a good deal remains in a good reproduction, so that the series of Pis. XII to XX are 
 given in the hope of conveying to the reader something of the structure of the inner 
 corona and prominences at the time of the eclipse. 
 
 Pis. XII to XIX, inclusive, show the whole corona as photographed near 
 midtotality with sixteen seconds exposure and PI. XX from an exposure of eight 
 seconds ending at third contact gives the best view obtained of the great prominences 
 on the southwest quadrant. The positions indicated are determined as follows : From 
 the center of the sun's disk radii were drawn, of which that which passed through the 
 north pole of the sun was numbered 0, and starting from north the numbering 
 increases in the direction NESW. 
 
 (6) THE 38-FOOT CAMERA. 
 
 Mr. Fowle secured eight good negatives during and immediately after totality. 
 The times of exposure as recorded on the chronograph were as follows: 
 
 No. 
 
 Beginning of ex- 
 posure. 
 
 End of exposure. 
 
 Interval. 
 
 1 
 
 20 11 45 m IT. 7 
 
 20" 45 18'. 7 
 
 1 nec. 
 
 2 
 
 20 45 27.7 
 
 
 1 sec. (?) 
 
 3 
 
 20 45 38.3 
 
 20 45 40.3 
 
 2. sec. 
 
 4 
 
 
 20 45 53 9 
 
 4 sec (?) 
 
 5 
 
 20 46 3.7 
 
 20 46 12.0 
 
 8. 3 sec. 
 
 6 
 
 20 46 23.4 
 
 20 46 31.8 
 
 8. 4 sec. 
 
 f- 
 i 
 
 20 46 40.1 
 
 20 46 43.7" 
 
 3. 6 sec. 
 
 8 
 
 20 46 51 . 
 
 20 46 52.0 
 
 1. sec. 
 
 a End of totality. 
 
20 THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 These negatives are excellent, though in detail they do not equal those secured 
 with the 135-foot camera, as was to be expected. PI. XXI is reproduced from the 
 exposure beginning at -20 h 46 m 23 s .4 and shows the coronal streamers somewhat 
 farther out than Pis. XII to XX because of its relatively greater exposure. 
 
 The original negatives of the inner corona, especially those secured with the 
 135-foot focus camera, are so full of detail that only confusion would result from an 
 elaborate inventory, but as years go by and future eclipses yield new points of view 
 in regard to the nature of the corona, it is believed that these plates will be of 
 increasing value for reference. In a general view one gets the impression that the 
 coronal streamers belong in two sets, respectively polar and equatorial in position. 
 The polar streamers, like the lines of force of a bar magnet, curve as if to run together 
 from the north pole to the south. The equatorial streamers start in a general way 
 radially and curve in such a manner as soon to stretch out farther and farther from the 
 sun in a direction roughly parallel to the plane of the ecliptic, and exhibit no tendency 
 ever to run together from east to west. Thus the polar streamers in numerous 
 instances appear to cross the equatorial streamers before becoming too faint to be 
 readily followed. 
 
 On closer inspection it is seen that the radial appearance of the equatorial 
 streamers as they emerge from the limb is by no means general. Near the large 
 prominences on the southwest limb some of the streamers are much curved, while one 
 group very remarkably exhibits what seems to be its origin. Beginning at the 
 corner of the large fan-shaped spreading prominence at position angle 265, two 
 sickle-shaped coronal rays, broad at the base, start out tangent to the limb and at 
 first run toward the equal or, but soon curve outward to join the general direction of 
 the equatorial rays, crossing over several radially directed rays in their path, while 
 two other noticable rays above the prominence seem to be directed toward the same 
 origin, but from the other side. Thus the four rays present the appearance of a 
 doubled-framed lyre with the center of its bow just outside the photosphere and in a 
 region of great prominences. Indeed, it may be said in general that disturbed regions 
 of the corona seem to be associated with large prominences. 
 
 3. PHOTOGRAPHY OF THE OUTER CORONA AND OUTLYING REGIONS 
 
 (a) The 1\ -foot focus camera. Mr. Gill obtained an excellent negative of 
 eighty-two seconds exposure, on which the coronal streamers are seen to extend about 
 to the image of the planet Mercury. Better results would have been reached by 
 reducing the aperture, as the sky was overexposed. No stars fainter than the sixth 
 magnitude are shown. 
 
 (6) The 40-inch cameras. A good coronal negative of about eighty-two seconds 
 exposure was obtained with each of these instruments, in one of which a color screen 
 
THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 21 
 
 was employed, but without marked effect. The coronal streamers are seen to extend 
 about three diameters, but no stars are shown fainter than the sixth magnitude. 
 
 (c) The 11-foot cameras. The two negatives obtained with these instruments 
 were of unequal merit. That covering the region west of the sun is very good; the 
 other unfortunately had numerous defects in the film, and is of little value. PI. I 
 (frontispiece) is taken from a careful enlargement by Mr. Smillie of the corona as 
 shown on the first mentioned of these two plates, though much of the fainter outlying 
 region is lost in reproduction. A glass positive from the enlarged negative shows the 
 whole region bounded by the northwest and southwest streamers to be filled with 
 faint coronal light quite out to the image of the planet Mercury, a sharp boundary 
 line separating this faint luminosity from the outlying sky to north and south. 
 
 In searching over this plate for possible planetary objects, 114 stars were found. 
 The faintest is of the 8.4 magnitude as given in Argelander's Durchmusterung, though 
 not all parts of the plate show stars as faint as this, owing to imperfect focus. The 
 other negative, covering the region east of the sun, showed only 13 stars, of which 
 the faintest was of the 6.3 magnitude. 
 
 In all, on botli plates, eight uncharted objects of starlike appearance were found. 
 Four of these are more or less uncertain in their appearance, and deserve little notice. 
 The other four can not be differentiated in appearance from the nearest stars, but 
 nothing is certain in regard to their character, because, so far as the writer is aware, 
 no other plates taken by any of the eclipse expeditions show stars as faint as these, so 
 that their reality can not be verified. 
 
 The positions of these objects as interpolated on the Durchmusterung charts of 
 1855 is as follows: 
 
 EPOCH 1855. 
 
 Right ascension. 
 
 Declination. 
 
 Magnitude. 
 
 Appearance. 
 
 3 h 40"' 22 
 
 20 08' 
 
 7.2 
 
 Doubtful. 
 
 3 47 55 
 
 20 21 
 
 5. to 5. 2 
 
 Good. 
 
 3 52 28 
 
 17 50 
 
 6.5 
 
 Good. 
 
 3 58 48 
 
 18 31 
 
 6.1 
 
 Good. 
 
 4 08 08 
 
 19 39 
 
 6.2 
 
 Good. 
 
 4 15 30 
 
 21 49 
 
 7.0 
 
 Doubtful. 
 
 4 31 00 
 
 19 50 
 
 4.5 
 
 Doubtful. 
 
 4 52 00 
 
 20 36 
 
 5. 5 to 6. 
 
 Doubtful. 
 
 PI. XXII, drawn from measurements of the original negative, shows the position 
 of all stars and starlike objects found in the region west of the sun. 
 
 The results obtained with the 11 -foot cameras make it certain that no new 
 planets brighter than the fifth magnitude exist, unless these were hidden by the sun 
 or the brightest coronal regions, though it is possible that there may be such planets 
 fainter than the fifth magnitude. If there be any brighter than the ninth magnitude, 
 they can be photographed in future eclipses with cameras of this type. 
 
22 
 
 TUB 1900 SOLAR ECLIPSE EXPEDITION. 
 
 4. BOLOMETRIC OBSERVATIONS OF THE CORONA. 
 
 Referring to PI. IX, and to the accompanying description of the bolometric 
 apparatus, the reader will recall that the beam entered the spectrobolometer room 
 through an aperture of variable size, that the image was adjusted upon the slit jaws 
 by moving the large concave mirror; that the prism was used sometimes with glass 
 face forward, sometimes with silvered face forward as a mirror; that a cardboard 
 screen was at times interposed in front of the bolometer to give the zero reading; that 
 a glass screen was sometimes thus interposed; and that by a common clockwork the 
 prism could be rotated and a photographic plate at the galvanometer could be moved 
 vertically so as to record automatically the proposed spectrum observations. 
 
 Recalling all these arrangements the following data of the observations may now 
 be considered: 
 
 BOLOMETRIC OBSERVATIONS. 
 
 [Current in each bolometer strip, 0.2 ampere. Time of single swing of galvanometer, 2.0 seconds. Observer at galvanometer, C. E. 
 
 Mendenhall. Observer at prism, C. G. Abbot.] 
 
 Aperture of cat's-eyc dia- 
 phragm. 
 
 Position of slit with 
 reference to image. 
 
 Prism acting as mir- 
 ror or as prism. 
 
 Screen. 
 
 Time of obser- 
 vation (76th 
 meridian mean 
 time). 
 
 Galva- 
 nometer 
 reading. 
 
 Deflec- 
 tion of 
 galva- 
 
 iiomt'lcr. 
 
 No. of 
 observa- 
 tion. 
 
 
 
 
 
 20 b 42" (?) 
 
 
 40mm 
 
 1 
 
 
 
 
 
 20 43 00 
 
 
 25 
 
 2 
 
 
 
 
 Cardboard insert- 
 
 Immediate 1 y 
 
 
 25 
 
 3 
 
 
 
 
 ed in front of 
 
 after above. 
 
 
 
 
 0~.8 X 0.5 = s .4 
 
 Nearly tangent to di- 
 
 As mirror 
 
 bolometer after 
 
 20 44 15 
 
 
 G 
 
 4 
 
 
 minishing crescent 
 
 
 each deflection. 
 
 Immediately 
 
 
 6 
 
 5 
 
 
 of sun's limb. 
 
 
 
 after above. 
 
 
 
 
 
 
 
 
 20 44 35 
 
 
 G 
 
 G 
 
 
 
 
 Cardboard screen. 
 
 20 45 15(?) 
 
 60 
 
 00 
 
 7 
 
 
 At center of moon 
 
 do 
 
 No screen 
 
 20 45 30(?) 
 
 42 
 
 -18 
 
 8 
 
 
 On inner corona 
 
 do 
 
 do 
 
 20 45 45(7) 
 
 47 
 
 -13 
 
 9 
 
 
 0.2 mm. beyond 
 
 
 
 
 
 
 
 17 x 17 = 289* 
 
 moon's preceding 
 
 
 
 
 
 
 
 
 limb. 
 
 
 
 
 
 
 
 
 At center of moon 
 
 do 
 
 do 
 
 20 45 55(7). 
 
 42 
 
 -18 
 
 10 
 
 
 On inner corona 
 
 . ...do 
 
 Glass plate 
 
 20 46 05(?) 
 
 51 
 
 9 
 
 11 
 
 
 Tangent to moon's 
 
 As prism for wave 
 
 No screen 
 
 20 46 15(?) 
 
 60 
 
 00 
 
 12 
 
 
 advancing limb. 
 
 length 0t. 40. 
 
 
 
 
 
 
 36x3& =1,225 
 
 ....do 
 
 do 
 
 do 
 
 20 46 30(?) 
 
 60 
 
 00 
 
 13 
 
 
 ....do 
 
 do 
 
 do 
 
 20 46 40(?) 
 
 60 
 
 00 
 
 14 
 
 
 ....do 
 
 do 
 
 do 
 
 20 46 46(?) 
 
 60 
 
 00 
 
 15 
 
 This part of the observations was, it will be remembered, in the immediate 
 charge of Mr. Abbot, whose notes written five minutes after totality are as follows: 
 
 The spectrum was intended to be turned from H through to Jf3, but the clamp screw was not 
 turned by me so that all spectrum readings were on H in the violet. The photographic plate 
 may or may not have been exposed by C. E. M. He has a vague idea that he raised the shutter. 
 It may pay to develop, but all deflections are superposed. C. G. A. interposed piece of card- 
 board in preliminary readings, but not during totality. The slit was a very little (about 0.2 
 millimeter) west" of limb of sun, i. e., beyond the advancing moon. The beam was a little too 
 low on the slit during the exposure, so that in the moon deflections about one-fifth of the strip 
 
 a "West" here refers to position in bolometric house. 
 
-. 
 
 - A " 
 
 ^ . 
 
 THE 1900 SOLAR ECLIPSE EXPEDITION. 23 
 
 was affected by the radiations from the polar coronal region. The moon's path was nearly at 
 right angles with the slit. C. G. A. could observe image on white asbestos paper. The corona 
 visible to him extended out about one-half sun's diameter in all directions. He could observe no 
 special structure or shape other than circular in the time he could spare to see. 
 
 Later during the same day an attempt was made by a holographic process to 
 calibrate the apparatus on the solar spectrum, but owing to accidental fogging of the 
 plate this work was lost. The plate supposed to be taken during the eclipse appears 
 not to have been exposed, as nothing showed on developing it. 
 
 The bolometric apparatus was employed in August, 1900, to measure the radia- 
 tions of the moon. Two days before full moon, with the apparatus in every way as 
 nearly as possible like that of experiment No. 9 of the preceding table, but with 
 a very humid and hazy indeed, almost cloudy atmosphere, positive deflections of 
 55 millimeters were obtained with the slit at the center of the moon's image, as 
 compared with a cardboard screen at the room temperature, and 86 millimeters as 
 compared with the dark zenith sky. 
 
 DISCUSSION OF THE BOLOMETRIC OBSERVATIONS. 
 
 The observations 1 to 6, inclusive, were made just before totality on the region of 
 sky where the corona was shortly to be observed and with an aperture of only 0.4 
 square centimeters. Reduced to the same aperture of diaphragm as that used during 
 the coronal observations 7 to 10, inclusive, the positive deflections in experiments 1 
 to 5 would range from 28,800 down to 4,300 divisions. During totality all the 
 deflections were negative. This was extremely disconcerting to the observer at the 
 galvanometer, for in anticipation of considerable positive deflections from the corona 
 the spot of light had been brought near the zero of the scale, and hence but little 
 room remained for negative deflections. Hence it was that the cats-eye diaphragm 
 was not opened wider in experiments 7 to 10, inclusive. 
 
 If we assume the temperature of the dark moon to be approximately the temper- 
 ature of space," we find on comparing the negative deflections observed in experiments 
 8, 9, and 10, that the radiation of the corona caused a deflection of five divisions 
 [ 13 ( 18)]. The brightest coronal radiation is then very feeble indeed as 
 compared with the radiation scattered by our atmosphere when illumined by the 
 narrow crescent of the photosphere even at a single minute before totality. 
 
 The meaning of the negative deflections deserves explanation. 6 It is well known 
 that in a room of uniform temperature bodies in general radiate the same amount of 
 energy they absorb, while a warmer body radiates more than it absorbs in its exchanges 
 with a cooler body. In the experiment number 7, just before totality, the interposition 
 
 See Memoirs National Academy of Sciences, Vol. Ill, p. 31, and also Vol. IV, Part 2, Third Memoir, pp. 158- 
 160. 
 
 6 See also Memoirs of the National Academy of Sciences, Vol. IV 7 , Part 2, Ninth Memoir, pp. 113-116. 
 
24 THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 of the screen of cardboard may be regarded as completing a constant temperature 
 chamber in which the bolometer was inclosed. If we disregard the slight warming of 
 the bolometer by the electric current, it may be said to have absorbed radiations from 
 the screen equal in amount to the radiations which itself emitted toward the screen. 
 But when the screen was removed in passing from experiment 7 to experiment 8, the 
 surface of the moon in effect replaced the screen and became a small portion of 
 the wall of the chamber inclosing the bolometer. Owing to the low temperature 
 of the dark moon, which, as the writer and others have elsewhere shown is indistin- 
 guishable from the temperature of space, less radiation now reached the bolometer 
 from this direction. Hence the rate of output of energy of the bolometer for an 
 instant exceeded the inflow, until with diminished temperature its radiation was 
 sufficiently reduced to establish a new condition of equilibrium. The corresponding 
 reading of the galvanometer was of course in a negative direction from the reading 
 corresponding to the card at room temperature which was taken as zero. 
 
 The interpretation of the negative deflections then is that the dark moon and 
 inner corona were effectively cooler than the room temperature, which was reasonably 
 to be expected in the former case, but was surprising in the latter, when the visible 
 intensity of the coronal radiations is considered. From a comparison of the results of 
 several observers of the eclipses of 1870, 1878, and 1898, it has been concluded that 
 the average brightness of the coronal region observed with the bolometer was about 
 equal to that of the full moon. But the bolometric experiments of August, 1900, as 
 given above, show that a deflection of eighty-six divisions was given by the nearly full 
 moon as referred to the dark sky, and a positive deflection of fifty-five divisions as 
 compared with the screen at room temperature. 
 
 To restate the matter in another form, it has been shown by the writer and others 
 that the radiation of the dark moon is practically indistinguishable in amount from 
 that of space; the reading on the dark moon may therefore be regarded as the zero of 
 the series," and the same which would have been obtained upon the sky in the direc- 
 tion of the inner corona had the latter been absent. Referred to this zero the radiation 
 of the inner corona produced a deflection of five divisions; that of a card at room 
 temperature produced eighteen divisions; that of the full moon produced eighty-six 
 divisions, and the diffuse reflection of the rays of the crescent sun in the five minutes 
 before totality from 4,300 to 29,000 divisions. 
 
 Why then should the coronal radiation of equal apparent brightness to that of 
 the full moon be comparatively so feeble in heating effect? It might be urged that 
 this is occasioned by the supposed rare gaseous structure of the corona, through 
 
 a The radiation of our own atmosphere depending upon its temperature, is superposed on that of the moon, 
 and also on that of each of the other sources measured, hut being small in amount, and as appears from the equality 
 of the two readings on the dark moon, presumably constant in the single half minute of the eclipse observations, it 
 disappears from the differences of the galvanometer readings, and is hence neglected. It should be noted that as 
 the sun is hidden, the diffuse reflection of the sky is not here in question. 
 
THE 1900 SOLAR ECLIPSE EXPEDITION. 25 
 
 which the radiations of the bolometer may pass readily and reach the cold space 
 beyond; and, furthermore, it might be said that the Milky Way, known to be 
 composed of numerous hot stars, would undoubtedly be indistinguishable by the 
 bolometer from the blackest sky. To these considerations it must be replied that the 
 amount of radiation of the bolometer outward is a function of its own temperature 
 only, and is independent of the nature of the objects it radiates upon; so that what is 
 different in the observations of the full moon and the inner corona is not the outward 
 radiation of the bolometer, but rather the amounts of radiation they send back to it. 
 But if these are to the eye the same, why should they be different in heating effect? 
 Obviously a natural explanation would be the comparative absence of some invisible 
 radiations in the case of the inner corona, which are present in the case of the moon, 
 such as would be due to an absence of infra-red rays. In the case of the Milky Way 
 cited above no such conclusion is to be drawn, because the Milky Way is as faint to 
 the eye in comparison with the full moon as It is feeble in heating effect upon the 
 bolometer. 
 
 The conclusion just reached from experiments 7, 8, 9, and 10, namely, that the 
 inner corona appears to be lacking in infra-red rays, depends mainly on two assump- 
 tions, first, that the experiments are substantially correct; second, that the average 
 visible brightness of the coronal region observed is nearly equal to the average bright- 
 ness of the full moon. Both require confirmation in future eclipses. 
 
 As regards experiment number 11, the result reached is irreconcilable with the 
 others unless the glass, which was small, failed of being interposed so as wholly to 
 cut off the beam, or unless an error of recording was made so that the reading should 
 have been 61, not 51. The result to be expected by the interposition of the glass was 
 that a small positive deflection would result, for glass is opaque to the radiations of 
 the bolometer, and would have the essential properties of a screen at the room temper- 
 ature, while at the same time it would transmit most of the feeble coronal radiations, 
 which would cause a slight positive deflection. 
 
 Experiments 12 to 15, inclusive, are in effect equivalent to a repetition of experi- 
 ment 7, for the coronal spectrum in the violet is evidently too feeble in heating effect 
 to cause a deflection, and the prism would act only as a reflector of the radiations from 
 the walls of the room to the bolometer. These experiments are then to be regarded 
 as furnishing four additional zero readings, and indicate by their agreement the excel- 
 lent behavior of the bolometric appai-atus. 
 
 SUMMARY AND CONCLUSION. 
 
 The expedition was generally successful in its observations. 
 
 The eclipse was attended with only a moderate degree of darkness. Telescopic 
 observations indicated considerable coronal detail, though less than had been observed 
 in 1878. Large prominences were present, and these appear to have been associated 
 108A (M 4 
 
26 THE 1900 SOLAR ECLIPSE EXPEDITION. 
 
 \ 
 
 with regions of coronal disturbance. The equatorial streamers were followed on 
 photographs to nearly 4 solar diameters, and were there lost by reason of 
 diminished intensity rather than as appearing to end. The polar streamers were 
 conspicuous, and curved in much the same way as the lines of force of bar magnets. 
 Excellent photographs filled with detail were secured with the long-focus cameras, 
 and these, it is hoped, will have greal; and increasing value for reference in future 
 years. The 135-foot focus lens proved of the highest value for the purpose of eclipse 
 photography. 
 
 In the bolometric observations the heating effect of the inner coronal radiations 
 was recognized and found unexpectedly feeble. The results seem to indicate a 
 comparative weakness of the infra-red portion of the coronal spectrum, alike incon- 
 sistent with the hypothesis that it radiates chiefly by virtue of a high temperature, or 
 acts chiefly as a reflector of ordinary sunlight. This, taken in connection with the 
 appearance of the corona, seem to support the hypothesis that the principal source of 
 its radiations is of the nature of an electrical discharge. The well-known polorization 
 of its outer portions, and the presence of faint dark lines in the outer coronal spectrum, 
 announced many years ago by Jaussen and confirmed by the photographs of Perrine 
 in the eclipse of 1901, prove that a small portion of the coronal radiation is due to 
 reflected photospheric light. But the photographs of the coronal spectrum by 
 Campbell in 1898, and Perrine in 1901, indicate that the principal part of the coronal 
 light is not reflected sunlight. Many are disposed to believe the main source to be 
 the incandescence of particles due to the proximity of the hot photosphere, but so far 
 as the writer is aware the spectroscopic evidence is equally in accord with the 
 hypothesis of a glow electrical discharge. An example of such a discharge is found 
 in the aurora of the terrestrial atmosphere, but while we can hardly deny the 
 possibility of its existence in the case of the sun the above observations do not seem 
 to the writer to be conclusive on the point. In any case bolometric observations at 
 future eclipses seem very desirable as furnishing a method of determining the real 
 nature of the coronal radiations. 
 
 The results of the search for intra-mercurial planets render it improbable that any 
 of these bodies exist brighter than the fifth stellar magnitude, but there may still be 
 some fainter than this. Several suspicious objects were found on one plate, but could 
 not be confirmed for lack of a duplicate photograph of equal merit. The 3-inch 
 aperture 11 -foot focus camera proved well adapted for the search, and in future 
 eclipses intra-mercurial planets may be photographed with similar apparatus, if any 
 such planets brighter than the ninth stellar magnitude exist. 
 
 In conclusion, I desire to express my gratitude for the aid furnished by so many 
 in furthering the interests of the expedition, and to each and all of the observers whose 
 untiring efforts in preparation and rehearsal, and whose skill and coolness during the 
 
 eclipse, insured the successful issue. 
 
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SMITHSONIAN OBSERVATORY SOLAR ECLIPSE EXPEDITION, MAY, 1900. 
 
 PLATE VIII. 
 
 THE BOLOMETRIC APPARATUS. 
 
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