1912.] PICKERING— THE OBJECTIVE PRISM. 565 



that of the entire Hght entering the telescope, less than one per 

 cent, reaches the photographic plate, when a slit spectroscope is 

 used. The proportion of light transmitted by the objective prism 

 must be at least fifty times as great. In fact, the principal loss of 

 light is from the absorption of the objective. Consequently, far 

 fainter stars can be photographed with an objective prism than with 

 a slit spectroscope, the difference amounting to several magnitudes. 

 Another great advantage of the objective prism is that the spectra 

 of all the stars in the field of the telescope can be photographed 

 simultaneously, while with a slit spectroscope only one star can be 

 taken at a time. With the Harvard 8-inch doublet as many as three 

 or four hundred spectra are often photographed on a plate, includ- 

 ing all stars of the ninth magnitude and brighter, in a region ten 

 degrees square. 



A comparison spectrum cannot be used with an objective prism, 

 and it is accordingly difficult to obtain absolute wave-lengths, 

 which are needed to determine the motion of stars in the line of 

 sight. This constitutes the principal objection to the objective 

 prism. Various plans have been proposed to remedy this difficulty, 

 and how far they are successful will be described by another 

 speaker. This does not afifect the ordinary measures of wave- 

 lengths, as hydrogen lines are present in the spectra of nearly all the 

 stars, and since these lines are afifected by the motion, other lines 

 can be referred to them. 



The first photograph of the lines in the spectra of the stars 

 was taken by Dr. Henry Draper of New York. In 1886, Mrs. 

 Draper established, at the Harvard College Observatory, the Henry 

 Draper Memorial, to prosecute the study of stellar spectra. The 

 objective prism has been used almost exclusively in this work. Two 

 photographic doublets of eight inches aperture have been mounted, 

 one at Cambridge, the other at Arequipa, Peru. With these the 

 entire sky has been covered many times. On one plate more than 

 a thousand spectra were classified. The late Williamina P. Flem- 

 ing, Curator of astronomical photographs, from an examination of 

 these plates, discovered several thousand objects having peculiar 

 spectra. In fact, probably few bright objects of this class escaped 



