512 POPULAR SCIENCE MONTHLY. 



the position of any ray produced by a substance is affected by no cause 

 but the motion of the substance. How and when this hypothesis may 

 fail is a very important question. It is found, for example, that the 

 position of a spectral ray may be altered by compressing the gas emitting 

 or absorbing the ray, and it may be inquired whether the results for 

 motion in the line of sight may not be vitiated by the absorbing atmos- 

 phere of the star being under heavy pressure, thus displacing the ab- 

 sorption line. 



To this it may be replied that, in any case, the outer layers of the at- 

 mosphere, through which the light must last pass, are not under pres- 

 sure. How far inner portions may produce an absorption spectrum we 

 cannot discuss at present, but it does not seem likely that serious errors 

 are thus introduced in many cases. 



These measures require apparatus and manipulation of extraordi- 

 nary delicacy, in order to avoid every possible source of error. The dis- 

 placement of the lines produced by motion is in fact so minute that 

 great skill is required to make it evident, unless in exceptional cases. 

 The Mills spectrograph of the Lick Observatory in the hands of Pro- 

 fessor Campbell has, notwithstanding these difficulties, yielded results 

 of extraordinary precision. Quite a number of investigators at some 

 leading observatories of Europe and America are pursuing the work of 

 determining these motions. The determinations have almost necessarily 

 been limited to the brighter stars, because, owing to the light of the 

 star being spread over so broad a space in the spectrum, instead of being 

 concentrated on a point, a far longer exposure is necessary to photograph 

 the spectrum of a star than to photograph the star itself. The larger the 

 telescope the fainter the star whose spectrum can be photographed. 

 Vogel, of Potsdam, who has made the most systematic sets of these 

 measures that have yet appeared, included few stars fainter than the 

 second magnitude. With the largest telescopes the spectro of stars 

 down to about the fifth magnitude may be photographed; beyond this 

 it is extremely difficult to go. The limit will probably be reached by 

 the spectrograph of the Yerkes Observatory, which is now being put 

 into operation by Professors Hale and Frost. 



THE MOTION OF THE SUN. 



When a star is found to be seemingly in motion, as described in the 

 last section, we may ascribe the phenomenon to a motion either of the 

 star itself or of the observer. In fact no motion can be determined or 

 defined except by reference to some body supposed to be at rest. In the 

 case of any one star, we may equally well suppose the star to be at rest 

 and the observer in motion, or the contrary. Or we may suppose both 

 to have such motions that the difference of the two shall represent the 



