168 Dr. J. Kerr on Measurements 



first and second lines contain potentials and optical effects as 

 formerly ; the third line contains a set of numbers which are 

 as the squares of the first five integers 1, 2, &c. — that is, as 

 the squares of the successive potentials. 



30 60 90 120 150 



13 53 117 206 321 

 13 52 117 208 325 



Dividing the squares of the potentials by the observed op- 

 tical effects, and keeping the quotients true to quarter units, 

 we obtain the five numbers 



69J -67f 69i 69J 70. 



Taking now the number 70 as divisor of the squares of suc- 

 cessive potentials, we find the following table of results (they 

 are arranged, line for line, as those given in the end of the 

 preceding article): — 



30 



60 



90 



120 



150 



12f 



51f 



115f 



205f 



321$ 



13 



53 



117 



206 



321 



\ 



14 



X 7 



If 



f 



~? 



The differences in the second and third columns are still 

 rather large (13). 



16. The two following measurements were made as one set, 

 in new circumstances, and with great care. The light em- 

 ployed was that of a paraffin-lamp ; and the distance of the two 

 conductors was about one eighth of an inch, which was a good 

 deal larger than in the former experiments. The potentials 

 were 90 and 120 ; and the quotients of square of potential by 

 optical effect were as follows: — 



902 -213 ^-215 



These numbers are satisfactorily near equality. Otherwise, 

 the calculated value of optical effect at potential 120 



= l;x 38 = 67^ 



which differs from the observed value by half a unit. 



17. Is there any regular difference, however small, between 

 observed and calculated values of the optical effect ? Such 

 was the question that I specially attempted to solve in some 

 of my latest experiments on this part of the subject. Some 

 changes of method were adopted. The lens D of (8) was 

 withdrawn, and its place was supplied by an ocular lunette ; 

 the second Nicol, deprived of its diaphragms, was moved close 

 up to the compensator ; the room also was well darkened, so 



