I9I2.] SCHLESINGER— RADIAL VELOCITIES. 177 



to have occurred to Pickering, who suggests and dismisses it in a 

 single sentence, on page xxi, Vohime 26, Annals of the Harvard 

 College Observatory. A few years later the same method was inde- 

 pendently proposed in a somewhat different form by Orbinsky^ and 

 also by Frost.^ None of these suggestions seems to have been taken 

 up by astronomers, and so far as I know there is no record of any 

 actual experiment dealing with the length of spectrum from the point 

 of view of radial velocity. 



Whatever may have been the promise held out by these proposals 

 when they appeared twenty years ago, I should like to point out that 

 their chance for leading to valuable results may be greatly increased 

 by the use of modern photographic plates. Recently several investi- 

 gators have shown us how to prepare plates that are sensitive through- 

 out a far greater range in wave-lengths than was hitherto the case. 

 Formerly, only the region to the violet side of the F line (A 4,860) 

 could be photographed without necessitating very long exposures ; 

 but now we have at our disposal comparatively rapid plates that will 

 yield spectrograms of nearly uniform density from the K line at 

 A 3,933 to 'the D lines at A 5,895, or even to the C line at A 6,563. 

 With former spectra the shift in the lines in the violet region could 

 be ascertained by referring them at best only to lines in the neigh- 

 borhood of A 4,800, which themselves share the same shift to a con- 

 siderable extent. But the yellow and red portions are so closely 

 crowded in prismatic spectra, that lines in this region would show 

 very little shift due to velocity, and hence would form excellent 

 points of reference for lines in the blue and violet. 



To carry out this plan, an ordinary objective, whether corrected 

 for photographic or for visual rays, would not answer, since it is 

 necessary to have in focus at the same time lines in both regions. 

 Cooke in England has successfully put upon the market an objective,' 

 made up of three different kinds of glass, which brings into good 

 focus the entire range of spectrum from A 3,800 to A 6,000. An 

 objective of adequate aperture of this type would serve the present 



^ Astronomische Nachrichten, 138, 9, 1895. 

 ^ Astrophysical Journal, 2, 235, 1895. 

 ^ Designed by Mr. Dennis Taylor. 



