460 Prof. M. A. Cornu on the Distinction between 

 relative velocity v, the true wave-length being X, viz. :■ 



<*-v> 



This principle may be applied directly to the light emitted by 

 the solar disk at the two extremities of an equatorial diameter. 

 The absolute velocity v of a point on the solar equator is 

 very sensibly 2 kilometres per second, that of the velocity of 

 light, V, 300,000 kilometres per second. Hence we shall have 

 a variation of wave-length equal to 



2 X 



AA '-- X 300,000 -±150,000' 



+ or — according as we take the same radiation at the eastern 

 or western end of the solar equator. 



If we wish to know numerically the magnitude of the dis- 

 placement of a spectral line corresponding to this variation of 

 wave-length, it is sufficient to substitute for X the numerical 

 value which defines the region which we wish to observe. 

 Consider, for example, the two D lines (X 1 = 588*40, X 2 = 588*89). 

 The displacement of a line having X=589 will be 



589 

 AX ~ * 150,000 ' 

 If we wish to compare this displacement with the distance 

 between the two D lines {i. e. with X 2 — X 1 =0 , 49), we shall 

 have as the relative displacement, 



AX 589 1 



^2~\ - "49x150,000 124-8 



2 1 



The double displacement will therefore be . . „ or -w^j of 



the distance between the two D lines. 



This total displacement, small as it is, is perfectly appre- 

 ciable with spectroscopes of very high dispersion, such as the 

 prism-spectroscope of M. Thollon, and the instruments with 

 diffraction-gratings constructed by Prof. Kowland of the 

 University of Baltimore. 



With the magnificent grating presented by Prof. Rowland 

 to the Ecole Polytechnique, the double displacement may 

 become sensible almost with all the points of the solar contour ; 

 that is to say, even with those which are far from giving the 

 maximum separation. 



The experimental method consists in causing the images of 

 the two extremities of the solar equator to fall alternately on 

 the slit of the spectroscope. For this purpose the solar beam 

 is received on a condensing-lens, which produces in the plane 

 of the slit a sharp image of the solar disk. The substitution 



