ON THE PROSPECT OF OBTAINING RADIAL VELOCI- 

 TIES BY MEANS OF THE OBJECTIVE PRISM. 



By frank SCHLESINGER, M.A., Ph.D. 

 (Read April 20, 1912.) 



Among the many problems that confront the workers in stellar 

 spectroscopy at the present time, three stand out by reason of their 

 importance. First, the classification of stars by means of their spec- 

 tra. Second, the determination of absolute radial velocities for the 

 purpose of determining the sun's way, and the relation between 

 stellar spectra and stellar motions; and of throwing light upon vari- 

 ous hypotheses regarding star-streaming. Third, the determination 

 of the orbits of spectroscopic binaries. 



In the second and the third of these problems the need of extend- 

 ing our determinations of radial velocities to faint stars has become 

 very pressing. At the present day a comparatively large number of 

 observatories are equipped with apparatus that, without involving 

 unduly long exposures, will yield radial velocities for stars down to 

 the fifth magnitude ; but what is greatly desired is an extension 

 to stars at least as faint as the eighth magnitude. It is futile to 

 hope to attain such an extension by erecting more powerful tele- 

 scopes and by attaching to them slit spectrographs of even the most 

 approved design. Experience has amply shown that in work of this 

 kind the gain that comes with increasing the size of the telescope is 

 only very slight. The principal reason for this is the loss of light 

 at the slit, which under favorable circumstances in the case of tele- 

 scopes of the largest size must frequently be as much as ninety per 

 cent. ; and under certain atmospheric and instrumental conditions 

 this percentage of loss may be considerably increased. With smaller 

 telescopes the loss of light at the slit is not so serious, and for this 

 reason, as well as for others, it is a fact (to cite somewhat extreme 

 cases) that the spectrographs attached to certain telescopes having 



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