1887.] • -*^^' [Barker. 



made by the Clarks, but wliich, although of slightly less aperture — half an 

 inch — was provided with a photographic correcting lens. The large amount 

 of work done with these instruments appears from the fiict that at the time 

 of his death in 1883, he had taken more than a hundred stellar spectrum 

 photographs, the later ones having a comparison spectrum upon the same 

 plate. The methods employed both by Dr. Huggins and by Dr. Draper 

 in all their later work were in general the same. The light of the star 

 was concentrated by the object glass of a large teles,cope upon the slit of a 

 spectroscope placed at its focus. In consequence, a narrow slit was neces- 

 sary in order to obtain good definition, and very perfect adjustment of the 

 driving clock was required to keep the image of the star upon the slit. 

 The spectra thus obtained were of course quite minute ; being about half 

 an inch only in length, and only one sixteenth or thereabouts in width. 

 Dr. Huggins made use of a cylindrical lens placed in front of the slit to 

 obtain the necessary width to the spectrum. But Dr. Draper secured this 

 end by throwing the image of the star slightly out of focus. 



The first attempts to obtain photographs of stellar spectra which were 

 made at the Harvard College Observatory were undertaken in May, 1885, 

 by the aid of an appropriation from the Ruraford fund of the American 

 Academy of Arts and Sciences. In these experiments an entirely new 

 photographic method was adopted. The prism was placed in front of the 

 object glass of the telescope ; a plan originally suggested for eye observa- 

 tions by Fraunhofer in 1823* and employed subsequently, practically, by 

 Secchi and Respighi. The advantages, for photographic purposes, of this 

 method are twofold: First, the loss of light is extremely small; and 

 second, the stars over the entire field of the telescope will impress their 

 spectra upon the plate. Hence while previous observers could photo- 

 graph but one star at a time, and this satisfactorily only with stars of the 

 first or second magnitude, more than one hundred spectra have now been 

 simultaneously obtained on a single plate, many of them of stars no 

 brighter than the seventh or eighth magnitude. The earliest photographs 

 obtained at the Observatory were taken by placing a prism whose refract- 

 ing angle was 30° in front of a Voigtlander photographic lens of two 

 inches aperture and about seven inches focal length. No clock-work was 

 used, the spectra being formed of the trails of the stars. In the spectrum 

 of the Pole-star thus taken, over a dozen lines could be counted ; and in 

 the spectra of a Lyne and a Aquila?, the characteristic lines were shown, 

 even when the time of exposure was only two or three minutes. 



In the autumn of 1885, an appropriation was made from the Bache fund 

 of the National Academy of Sciences for the purpose of continuing these 

 investigations. A prism of 15° refracting angle and eight inches in clear 

 aperture was employed, placed in front of a Voigtlander photographic lens 

 having an aperture of eight inches and a focal length of about forty-five 

 inches. The details of the method are thus described in the report of Prof. 

 Pickering :t 



*See Schellen's Spectrum Analysis, English ed.. 1872, p. 462, ct seq. • 

 t Memoirs Am. Acad. Arts and Sci., xi, 209, 188C. 



