i8 



NATURE 



{May 2, 1889 



fourth magnitude can be satisfactorily photographed with one 

 prism, the spectra obtained being about an inch long. Fainter 

 stars, if of a bkiish colour, give sufficiently distinct images, in 

 some cases good results being obtained with stars of the seventh 

 magnitude. For example, fourteen stars in the Pleiades are 

 well photographed with this apparatus. With four prisms 

 much lons;er spectra are obtained and many more lines are 

 visible. But certain differences in the character of the spectra 

 are better shown with the smaller dispersion. Numerous 

 photographs have been taken of the variable stars Ceti and )3 

 Lyrse. The changes in the spectrum of the latter star seem 

 to be undoubted, those of o Ceti, if any, to be slight. Various 

 peculiarities in the spectra of individual stars have been detected. 

 One photograph of i' Ursoe Majoris shows the K line distinctly 

 double, and others show it single. Many photographs will be 

 required to determine the law of its variation, if this is due to 

 changes in the star itself. Bright lines were detected in the 

 spectrum of <^ I'ersei, putting it in a class in which only two or 

 three other stars are known to fall. In the double star ,8 Cygni 

 the two components have spectra of different types, an important 

 consideration in the theories regarding their formation. The 

 brighter component is of the second type, the fainter of the first. 



Ordinary photographic plates are not sensitive to rays of much 

 greater wave-length than the F line, or 486. By staining the 

 plates with various coal-tar produc's the range of sensitiveness 

 may be greatly extended. With erythrosin the spectrum extends 

 to the wave-length 590. The sodium line D is distinctly seen 

 to be double in the photographs of o Bootis and a Aurigge. 

 Various experiments were also made with cyanin, but the plates 

 were not sufficiently sensitive to give good results. The entire 

 length of the spectrum with four prisms, including the portion 

 obtained by erythrosin, is about six inches and a half. 



A beginning has been made of the measures of the positions 

 of the lines in the spectrum. A scale of fortieths of an inch has 

 been ruled on glass, and the positions of the lines read off with 

 the aid of a magnifying-glass. Twelve of the photographs of a 

 Canis Majoris have been studied in this way. The spectrum of 

 this star is traversed by the hydrogen line-^, which are strong, 

 and by other lines which are so faint that they are only visible 

 when the dispersion is la-ge and the definition good. The 

 catalogue thus formed contains about 320 lines. The average 

 deviation of the measures of the same line on different plates is 

 about 0-05 of a millionth of a millimetre, or 0-05 cm. on the 

 scale of Angsti om's map. If the line occurs in the solar spectrum 

 these measures will generally identify it. In other cases the 

 exact position must be determined by a dividing engine. If a 

 line can he distinctly seen, its wave-length can probably be thus 

 determined with as great accuracy as that of the po-ition of the 

 solar lines on the map of Angstrom. In the spectrum of o 

 Bootis 140 lines are visible between the D and F lines. 



The classification of this large number of spectra is a matter of 

 no little difficulty. .Slight differences exist in many stars, and 

 certain stars apprar to hold an intermediate position, so as to 

 render a rigorous division into classes impossible. On the other 

 hand, many stars appear to have identical spectra. The first 

 step will be to arrange the stars in groups, and then compare the 

 best defined spectra of different groups. A minute discussion 

 and the measurement of wave-lengths will be necessary only in 

 the investigation of a comparatively small number of spectra. 



4. Faint SteUar Spcclra. — The 28-inch reflecting telescope 

 constructed by Dr. Draper was assigned to this work. During 

 the first six months of the year a careful study was made of this 

 problem, and the difficulties encountered bore evidence of the 

 skill of Dr. Draper in obtaining good results with this telescope. 

 'I he best method of using this instrument seemed to be a modi- 

 fication of the form first tried by Dr. Draper, — a slit spectro- 

 scope from which the slit had been removed. The rays from 

 the mirror were rendered parallel by a concave- lens which 

 replaced the o' jeclive of the collimator. As this lens had the 

 same focal distance as the objective of the observing telescope, 

 it was not necessary that either should be achromatic. After 

 long trials with this and other forms of apparatus, a spectrum 

 was at length obtained showing good definition. As the 

 results were not better than those described above, ard the 

 instrument, from its size, was slow in operation, the experiments 

 have not been carried further. 



5. Catalogue of Spectra of Bright Southern i'/arj.— The 8-inch 

 Bache telescope remained in California until February 2, 1889, 

 and was then sent to Peru to continue research No. i on the 

 southern stars. The sky from - 25° to the south pole will be 

 covered, and the resulting photographs sent to Cambridge and 



reduced, as in the case of the northern stars. The advantages 

 of discussing all stars from the north to the south pole according 

 to one system are very great, and are here secured for the first 

 time in so extensive an investigation. If no unforeseen difficulty 

 arises, the photographs will all be completed during the next 

 two years. 



6. Catalogue of Spectra of Faint Southern Stars. — Research 

 No. 2 will also be extended to the south pole simultaneously with 

 the observations required for No. 5. It is expected that these 

 photographs also will be completed in two years. 



The Bache telescope described above has proved an extremely 

 convenient instrument for various purposes. Besides the spectro- 

 scopic researches already mentioned, several other investigations 

 have been undertaken with it, some of which will be found in 

 the Memoirs of the American Academy, vol. xi. p. 179, and 

 the Harvard Observatory Annals, vol. xviii. Nos. iv. , vi., and 

 vii. Owing to its short focal lengih it possesses many advant- 

 ages over pho'ographic telescopes of the usual form. With 

 exposures of an hour and a half more stars were photographed 

 in the Pleiades than are given in the engraving accompanying 

 the Annual Report of the Paris Observatory for 1886, although 

 that work was ba^ed on photographs taken by the MM. Henry 

 with exposures of three hours and a telescope having an 

 aperture of 13 inches. Nearly twice as many stars were 

 photographed in this region as were visible with the 15-inch 

 telescope of the Harvard College Observatory. The short focus 

 of the telescope also gives it especial advantages for photo- 

 graphing nebula.'. Twelve new nebulae were thus discovered 

 in a region where but eighteen were known before. Various 

 other investigations, such as a determination of the law of 

 atmospheric absorption, have been undertaken with the aid of this 

 telescope. It has been so persistently used in spectroscopic work 

 that the other researches have been neglected, especially those in 

 which very long exposures were required. Its removal to Peru 

 now cuts it off for .'ome time from such use on the northern 

 stars. Acco'dingly, Mrs. Draper has procured a similar len«, 

 which is now in the hands of the firm of Alvan Clark and Sons 

 for retouching and mounting. Several important researches will 

 be undertaken with this instrument. Photography is now used 

 in so many departments of astronomy that a general investigation 

 of the photographic brightness of the stars seems desirable. A 

 plan has been proposed by which a single plate will contain 

 photographs of a number of regions one degree square, but in 

 different portions of the sky. Thus a series of standard faint 

 stars will be photographed, which can all be measured, and re- 

 duced to the same scale. One or more photographs of the 

 vicinity of the north pole will be taken on each plate, and thus 

 serve to correct the results obtained on different plate-. It is 

 proposed in this way to secure a series of standards of stellar 

 magnitude at intervals of about five degrees. A third lens of 

 similar form, having an aperture of four inches, will be attached 

 to the telescope, with which photographs on a smaller .'cale, but 

 five degrees square, will betaken simultaneously. These photo- 

 graphs w ill cover the entire sky, and it is proposed to measure 

 the photographic brightness of all stars of the seventh mag- 

 nitude, or brighter, which are represented on them. This 

 investigation will have a special value in connection with the 

 photometric measures of the spectra described above. It is 

 hoped also to photograph the entire northern sky by means of 

 the 8-inch telescope, with exposures of an hour. Each plate 

 covers a region nearly ten degrees square, of which the images 

 in the central five degrees square are satisfac'orily in focus. One 

 of the regions containing standard stars will appear in the centre 

 of each plate. By such a series of plates the photographic bright- 

 ness of any stars brighter than the fifteenth magnitude can be deter- 

 mined on a uniform scale. The faintest stars photographed will 

 be nearly a magnitude fainter than the limit proposed by the 

 .\strophotographic Congress, so that all plates included in that 

 work can 1 e reduced to a uniform system. The advantages of 

 such plates for studies of the distribution of the stars and other 

 similar investigations are obvious. 



From the ;.bove description it appears that the field of work 

 of the Henry Draper Memorial, as now extended, is almost 

 boundless. The problems to be investigated relate to the fun- 

 damental laws rfgulating the formation of the stellar system. 

 Questions of such importance should be discussed on a sufficiently 

 large scale, or the results of the discussion will soon be super- 

 seded by a repetition of the work. The liberal provisir n made 

 for the Henry Draper Memorial permits the investigations to be 

 planned on a scale which is likely to avoid such undesirable 

 duplication of work. 



