498 • M. A. J. Angstrom on a Method of determining , 



sidered as quite decisive. Last midsummer the weather was 

 unfavourable to my observations, and at the end of October the 

 latter were not sufficiently numerous to furnish an answer even 

 to the first of the above questions. 



I should not in fact have alluded to the subject had not M. 

 Babinet, in the Academy of Sciences, proposed a method of deter- 

 mining the translatory motion of the solar system identical with 

 the one which, two years ago, I submitted to the Royal Scientific 

 Society of Upsala. 



A small difference exists, however, in our calculations. I had 

 assumed the motion of the grating to have no influence on the 

 angle ©, whereas Babinet introduces, on this account, the cor- 

 rection 



h{\ — cos ©) tan ©. 



The truth of this formula may in fact be readily established by 

 help of the adjoining figure, in which 

 e sin © denotes the distance traversed 

 by light during the time that the gra- 

 ting describes the distance — he$m% 

 in a direction contrary to that of the 

 incident ray s . The differen ce of p ath 

 for the two interfering waves will 

 consequently, through the motion 

 of the grating, be diminished by 



fa?(l~cos©) sin©, 



a magnitude which, when equated to 



— ecos©^©, 

 gives 



d@= — h{\ — cos ©) tan ©. 



The value of d% will, of course, be positive when the instru- 

 ment moves in the same direction as the light. 



The expression thus obtained, added to the one in the formula 

 (1), gives for the total variation of the angle © the value 



A© = (A-A') tan ®; 



and if, moreover, h— — A'=20"*4, and 



2©=62°55'41", 

 then will 



A2©=49"-8. 



Hence in the special case under consideration, the variation of 

 the angle 2© is increased by 7 n '2 in consequence of the motion 

 of the grating. 



The observations on which the numerical values of Table I. 



