by means of Indices of Refraction* 



17a: 



indices which are at once sufficiently exact and sufficiently nu- 

 merous. In addition they have the great advantage of having 

 heen made with reference to lines in the solar spectrum the wave- 

 lengths of which had been measured by the same observer. 

 There can therefore be no question of identity. As a first ex- 

 ample of the method, I give a determination of the wave-length 

 of C, taking B as one of the lines exterior to C, and taking in 

 succession seven other exterior lines more refrangible than C, to 

 combine with B. Formula (2) was therefore employed, and with 

 the following data and results : — 



B , , 



1-61079 



687-48 







C . . 



1-61252 



656-56 







D . , 



1-61436 



628-11 



656-70 



+ 0-14 



11 . 



1-61537 



61396 



656-71 



+ 0-15 



13 . 



1-61560 



610-52 



656-56 



o-oo 



14 . 



1-61728 



589-56 



656-71 



+ 0-15 



16 , 



, 1-61973 



561-80 



656-76 



+ 0-20 



17 . 



1-62064 



553-19 



656-79 



+ 0-23 



19 . 



1-62143 



545-83 



656-87 



+ 0-31 



Mean of the errors . . +0*17 



In this Table the first column gives the designation or num- 

 ber of the line, the second its index of refraction as determined 

 by a Steinheil prism of 60°, the third the corresponding wave- 

 length according to Van der Willigen, and the fourth the wave- 

 length as found by formula (2) by combining B with each line 

 after C in succession. 



The mean of the seven values of the wave-length of C thus 

 found is 656*70, which is in excess of Van der Willigen's own 

 determination of the value of C by 014. From this it appears 

 that the method may be applied with a tolerable degree of ap- 

 proximation, even in the case of a flint-glass prism of high dis- 

 persive power, and for indices of refraction which refer to lines 

 at considerable angular distances from each other. The increase 

 in the computed values of C as the intervals between B and the 

 second line of comparison are increased, however, will clearly 

 appear from the Table. The following results were obtained with 

 the indices of a second Steinheil prism, No. 2, of 46° 52' 25"'8, 

 also of flint glass : — 



B and 8a 656-21 -035 



B and 11 656-33 —0-23 



B and 13 656*31 -0-25 



B and 16 65638 -0-18 



B and 17 65647 -0'09 



B . . 



1-60521 



C . , 



1-60694 



8a . 



1-60872 



11 . 



1-60973 



13 . 



. 1-60998 



16 . 



. 1-61408 



17 . 



. 1-61495 



