1887.] lb J [Barker. 



photographic telescopes, 15,729 spectra having been already photographed 

 and measured. Third, he decided to carry on a more careful study of the 

 spectra of the brighter stars. For this work, the Draper eleven inch cor- 

 rected refractor was specially used, a suitable observatory having been 

 erected for it in Cambridge. Four prisms, each having a refracting angle 

 of 15°, were constructed, of which three had a clear aperture of nearly 

 eleven inches, the fourth being somewhat smaller. These four prisms 

 with their mounting weighed more than a hundred pounds and occupied 

 a cubic foot of space. 



The original negatives have been enlarged by a novel process which 

 gives most excellent results. A cylindrical lens is placed close to the 

 enlarging lens with its axis parallel to the length of the spectrum. In the 

 apparatus actually employed the length of the spectrum and with it the 

 dispersion, is increased five times, while the breadth is increased nearl}'- 

 one hundred. This arrangement has the great advantage that it greatly 

 reduces the difficulty arising from the feeble light of the star. Until re- 

 cently, the spectra in the original negatives were made very narrow, since 

 otherwise the intensity of the starlight would have been insufficient to 

 produce the proper decomposition of the silver particles. The enlarge- 

 ment being made by daylight, the vast amount of energy then available 

 is controlled by the original negative, the action of which may be com- 

 pared to that of a telegraphic relay. The copies therefore represent manj^ 

 hundred times the original energy received from the stars. 



It was with the apparatus above described and under these conditions, 

 that the photographs were taken that I have the honor of exhibiting to the 

 Society. Although the earliest satisfactory results were obtained in Octo- 

 ber, 1886, yet it is evident that the full meaning of these photographs can 

 be discovered only after they have been carefully measured, and after 

 these measurements have been reduced and thoroughly discussed. Some 

 points of interest, however, appear on simple inspection of them. This 

 photograph of « Cygni for example, which was taken November 26, 

 1886, shows the H line to be double,* its two components having a differ- 

 ence of wave-length of about one ten-millionth of a millimeter. This 

 photograph of o Ceti shows the lines G and h as bright lines, as well as the 

 four ultra violet lines which are the characteristic of spectra of the first 

 type ; to which Dr. Iluggins gave the letters a, /?, ;' and S. In this spec- 

 trum, however, the H and K lines are seen to be dark ; showing that they do 

 not belong to that series of lines. The spectrum of a Tauri shows a mul- 

 titude of lines and bands, massed in the more refrangible region ; thus ac- 

 counting for the ruddy color of the star. The spectrum of Sirius shows, 

 besides the well-known broad lines characteristic of this brilliant star, a 

 large number of fainter ones. The spectrum of a Canis minoris, taken 

 with four prisms, shows the solar dark lines G, h, H and K. 



This entire research is entitled " The Henry Draper Memorial." The 



* According to Young, the line H in the solar cliromosphere, which is bright, is donbla 

 also. 



PROC. AMER. FHILOS. SOC. XXIV. 125. V. PRINTED .JUNE 4, 1887. 



