306 



NATURE 



[July 26, 1888 



THE PROGRESS OF THE HENRY DRAPER 

 MEMORIAL} 



'"THE additional facilities provided by Mrs. Draper have per- 

 mitted a considerable extension of this research during the 

 past year. The 11-inch refractor belonging to Dr. Draper, and 

 the 8-inch photographic telescope provided by the Bache Fund, 

 have been kept at work throughout every clear night. The 28- 

 inch and 15-inch reflectors constructed by Dr. Draper have been 

 moved to Cambridge, and the first of these instruments is placed 

 in a building surmounted by a dome constructed for the purpose. 

 Experiments are now in progress with it, and it will probably 

 soon be employed regularly. Four assistants take part in making 

 the photographs, one of whom comes to the Observatory every 

 clear night about midnight, and keeps the 8-inch and 11-inch 

 telescopes in use until interrupted by the morning twilight. 

 Five ladies have been employed in the measurements and 

 reductions. 



The various investigations now in progress are described in 

 detail below. The first three of these, including the photo- 

 graphic work of the 8- inch and 11 -inch telescopes, will be 

 finished in about a year. It is accordingly proposed in the 

 autumn of 1889 to send an expedition to the southern hemi- 

 sphere, probably to Peru, and there complete the work to the 

 South Pole. As only about one quarter part of the sky is too 

 far south to be conveniently observed at Cambridge, it is 

 expected that the photographs needed to cover this portion of 

 the sky could . be obtained in two years. Each investigation 

 could thus be extended to all parts of the sky upon the same 

 system. 



An important advance has been made by the recent improve- 

 ments in the manufacture of dry plates. The M. A. Seed 

 Company of St. Louis have endeavoured to comply with our 

 request for more sensitive plates, an 1 have gradually increased 

 their sensitiveness, so that they now furnish us with plates 

 measuring 27 on their scale, while a year ago the most sensitive 

 plates were only numbered 21. As a result, stars nearly a 

 magnitude fainter can be photographed, and the number of 

 objects which can be examined is nearly doubled. A careful 

 study will shortly be made, by the help of the instruments 

 described below, of the most sensitive plates obtainable. It is 

 hoped that makers of very sensitive plates will send specimens 

 to Cambridge for trial. The demand for increased sensitiveness 

 is so great not only here, but at all other observatories where i 

 stellar photography is carried on, that a real improvement would 

 be widely appreciated. 



Various improvements have been made in the methods of 

 detecting defects in the photographic processes. Each plate, 

 when it is taken from its box, is exposed to a standard light for 

 exactly one second. The portion of the flame of an oil lamp 

 shining through a small circular aperture constitutes the standard 

 light. The exposure is made for a second by means of a 

 pendulum, which allows the light to shine on the plate for this 

 interval through a small square aperture. When the plate is 

 developed, a dark square appears near its edge, whose intensity 

 measures the sensitiveness of the plate, and also serves to detect 

 any defect in its development. Passing clouds, or a variation in 

 the clearness of the sky, are detected by an instrument called 

 the Pole- Star recorder. It consists of a telescope with a focal 

 length of about 3 feet, placed parallel to the earth's axis. An 

 image of the Pole-Star is formed by it, and allowed to fall upon 

 a sensitive plate, describing an arc of a circle, which is inter- 

 rupted whenever clouds pass. The plate is changed every day, 

 and the instrument is closed automatically by an alarm-clock 

 every morning before the twilight begins. Much trouble is 

 experienced from the deposition of moisture on the objectives of 

 the photographic telescopes, on account of their exposure to a 

 large portion of the sky. The failure of some of the earlier 

 plates may be due to this cause. Moisture is now carefully 

 looked for, and, if detected, removed by gently heating the 

 objectives. Another te-t of the quality of the plates consists in 

 occasionally exposing a plate in the 8-inch telescope to the 

 circumpolar sky, first with and then without the prism. The 

 trails of the stars near the Pole and the spectra of the brighter 

 stars are thus photographed. A comparison of the intensity of 

 these images tests the condition of the air, the instrument, and 

 the plates. 



1 Extracted from t'le " Second Annual Report of trie Photographic Study 

 of Stellar Spectra conducted at the Harvard College Observatory," Edward 

 C. Pickering, Director. With 2 Plates. (Cambridge : John Wilson and 

 Son, University Press, 1888.) 



The various investigations will now be described in order, as 

 in the last Report. 



1. Catalogue of Spectra of Bright Stars.— The spectra of all 

 the brighter stars have been photographed with the 8- inch tele- 

 scope, giving an exposure of at least five minutes to each. Each 

 plate contains from two to four regions io° square. The plates 

 representing the region north of - 25 were divided into three 

 series, which may be distinguished as polar, zenith, and equa- 

 torial. Each region is contained on two plates, and the work 

 has been repeated in two successive years, so that at least four 

 photographs should be obtained of all the brighter stars. If a 

 plate proved poor, it was repeated, so that the very bright stars 

 will appear in several plates. The photographic portion of this 

 work was finished last November. If no plates had been 

 repeated, 36 polar, 72 zenith, and 72 equatorial plates would 

 have been required each year, or 360 in all. The actual numbers 

 of plates taken and measured were 46, 120, and 93, total 259, 

 the first year ; and 61, 209, and 104, total 374, the second year ; 

 or 633 in all. In the later work the number of zenith plates 

 was doubled, to avoid the confusion arising when several ex- 

 posures were made on a single plate. The numbers of speclra 

 measured on these plates were 2381, 3314, and 2618, total 

 8313, the first year; and 7199, 8217, and 4074, total I9,49°> 

 the second year. Two plates covering the immediate vicinity of 

 the North Pole contain 150 spectra. The whole number of 

 spectra is therefore 27,953. The measurement and identification 

 of this large number of spectra has occupied the greater portion 

 of the time of the corps of computers. Each plate to be mea- 

 sured was placed on a stand, and the light of the sky was 

 reflected through it by means of a mirror. The approximate co- 

 ordinates of each spectrum in turn were then read off, and a 

 careful description of the spectrum was given. Besides the usual 

 division into types, each additional line visible was recorded 

 both as regards its position and intensity. The photographic 

 intensity of the brightest portion of each spectrum was also 

 measured by means of a photographic plate, dark at one end and 

 light at the other, like a wedge of shade glass. When the 

 spectra show sudden changes in brightness, additional measure- 

 ments are made. This portion of the work is complete only for 

 the polar plates and about 62 of the other plates, including 

 1 2, 574 spectra. The identification of the spectra is effected either 

 by computation from its co-ordinates, or by laying the plate upon 

 the maps of the " Durchmusterung," the scale being the same 

 for both. All the plates have, however, been checked by the 

 latter method. The names of the stars are then taken from the 

 " Harvard Photometry," "Uranometria Argentina," or "Durch- 

 musterung," according to their brightness and declination. Their 

 places are next brought forward to 1900, the epoch of the final 

 catalogue. As the intensity of the photograph of a given 

 spectrum will vary greatly with the sensitiveness of the plate, 

 the clearness of the air, and the rate of the driving-clock, all 

 must be reduced to the same system. The scale of the " Harvard 

 Photometry " is adopted for this purpose. The most prevalent 

 spectra are those of the first type, in which the K line is too 

 faint to be visible. After applying a correction for the declina- 

 tion of the stars, the brightness of all such spectra on each plate 

 is compared with the photometric magnitudes. A correction is 

 thus derived for each plate, which is applied to all the spectra 

 upon it. The effect of colour, so far as it varies with the type 

 of spectrum, is thus eliminated. It is possible that, owing to 

 variations in temperature, or other causes, some stars may be 

 redder or bluer than others having the same type of spectrum. 



2. Catalogue of Spectra of Faint Stars. — Until the photo- 

 graphs required for the research mentioned above were com- 

 pleted, the time of the 8-inch telescope was mainly devoted to 

 them. Since then it has been used principally in photographing 

 the fainter stars. An exposure of one hour is given to each 

 portion of the sky, a region 10° square being included upon each 

 plate. Stars as far south as - 25 can be advantageously photo- 

 graphed at Cambridge, and the plan proposed covers this region. 

 The plates overlap, so that the region north of - 20 will appear 

 on at least two plates. The southern stars are only photographed 

 when the sky is unusually clear. Each plate is examined, and, 

 if unsatisfactory, the work is repeated. If all were good, 650 

 plates would be required. Thus far, 606 plates have been taken, 

 covering 339 of the desired regions. As the time of the com- 

 puters has been mainly devoted to the first investigation men- 

 tioned above, the greater portion of these plates have not been 

 measured or reduced. The total number measured is 105 plates, 

 containing 6931 spectra, of which 94 plates and 6293 spectra 



