482 PHOTOGRAPHY IN THE SERVICE OF ASTRONOMY. 



spectra, and the deviations thus established rest most frequently upon 

 impressions fading and very uncertain. Mr. Vogel has succeeded in 

 freeing himself from this difficulty, caused by the scintillations, by 

 photographing the stellar spectra at the same time with the spectrum 

 of a gas; the plates which he has published show with a surprising 

 sharpness the deviation of a ray common to several stellar spectra, and 

 which corresponds to the violet ray of hydrogen. It is established thus 

 for example that a certain star of the constellation of Orion recedes 

 from the observer with the velocity of 86 kilometers per second, a ve- 

 locity which is reduced to CI kilometers if it is referred to the sun. 



At Greenwich, where the spectroscopic study of the velocity of trans- 

 lation of stars has been pursued for fifteen years by the process of prim- 

 itive observation, contradictory results appear, which proceed without 

 doubt from the small amount of fixity of the images of stellar spectra.* 

 There is reason to hope that the photographic method, in causing this 

 source of error to disappear, will permit the making use of data of this 

 character on tjie same ground as the proper motions, perpendicular to 

 the visual ray, which modify the apparent positions of the stars. The 

 attempt has already been made to deduce from them the direction and 

 the velocity of the movement of translation of the solar system, and the 

 results agree very well with those which have been obtained by other 

 methods t. Finally, these are the only data for the present which we 

 can make use of in arriving at a more complete knowledge of the orbits 

 of double stars, for the usual observations reveal to us only the appar- 

 ent orbits in the way they are projected on the celestial sphere. These 

 projections are ellipses, and it is more than j)robable that the real orbits 

 which we see foreshortened are equally ellipses ; but it is not rigorously 

 demonstrated that the principal star occupies one of the foci. 



It follows that it can not yet be affirmed in an absolute manner that 

 the law of Newton, the law of universal gravitation, presides also over 

 the motions of double stars, although the generality of this law is ex- 

 tremely probable.f 



The photographic study of stellar spectra is also of a high interest 

 from other points of view, and above all for the comprehension of the 

 constitution of the universe. This is entered upon with ardor in 

 A merica. At the Cambridge Observatory, where is arranged a generous 

 foundation which the widow of Henry Draper made some years since 

 in memory of her husband, two refractors and two reflectors are devoted 



* The changes in the direction of the velocity of Siriiis, if they are real, can be ex- 

 plained by an orbital movement. 



tin taking the mean of numerous determinations, taken since W. Herschel, the 

 point towards which the sun is moving is found to be 267° in right ascension, and 

 31° of north declination. In resijcct to the velocity of this motion it has been esti- 

 mated at 25 to 30 kilometers per second ; this is a little less than the velocity of the 

 earth in its orbit. 



I Tieserand, " Treatise on celestial inecbauics," 1. 1, p, 42. 



