by Optics } the Translatory Motion of the Sola?* System. 497 



III. 



In a lecture given on October 6, 1860, to the Royal Scien- 

 tific Society of Upsala, I explained a method of determining 

 the motion of the solar system by observations on the interfer- 

 ence-bands of a glass grating. I then showed that if we assume 

 the propagation of the undiffracted rays, passing through the 

 openings of the grating, to be uninfluenced by the motion of 

 the instrument, the same must be true of the formation of the 

 interference-bands on both sides ; consequently, also, that when 

 a telescope is used in the observations the customary aberration 

 must ensue, and be proportional to the ratio between the motion 

 of the telescope, in a direction perpendicular to its axis, and the 

 velocity of light along this axis. 



Hence, the velocity of light being taken as the unit, if h be 

 the velocity of the instrument in the direction of the incident 

 light, then for an angle ©, under which, e. g., the D line in an 

 interference- spectrum is observed, the velocity of the telescope 

 perpendicular to this direction will be 



h sin ®, 



which accordingly must be the expression for the aberration. 



If the angle © were observed for two positions of the instru- 

 ment in which the velocities in the path of the incident rays 

 were h and h f , we should then have 



A® = (h- h!) sinO, (1) 



or, since 2® is the angle immediately given by observation, 

 A.2@ = 2(A-A')sin(H). 



Putting h(=—h') equal to the velocity of the earth in its 

 orbit, this equation gives 



A.20 = 81' / -6sine; 



and since, for the double line D in the fourth spectrum, 



26 = 62° 55' 44"-2, 

 we deduce 



A.2A=42"-6, 



a magnitude capable of being readily observed. 



Two questions have here to be answered by observation. 

 The one has reference to the actual existence of the phenomenon, 

 and may be most readily answered by applying the method to the 

 known orbital motion of the earth ; the other has reference to 

 the employment of the method, when proved to be accurate, to 

 the determination of the translatory motion of the solar system. 



The experiments hitherto made cannot in any respect be con- 



