166 



Indices of Refraction for the Electric Ray. [Dec. 12, 



It will be observed that the final results obtained from the two sets 

 are freed from, many of the unavoidable errors. 



I give below the results of two sets of experiments each extending 

 through eleven observations. The receiver was in an unusually 

 good condition for nearly an hour, and during that time I took six 

 observations with P to the front and six more with Q in the same 

 position. As the receiver continued to remain in a fairly responsive 

 condition I took five more for each set. As I have said already, 

 greater weight should be given to the first two sets of six readings, 

 as being taken under the most favourable conditions. 



In the first two lines are given ten successive differences of the mean 

 readings, taken with the cylinders P and Q. 



P .... 71 70 70-5 70-5 70-5 70 69-5 69-5 69-5 70 = 701 

 Q .... 70 70 71 71 70 71 71 70 70 69-5 = 703-5 



Mean 702 25 



i = 702 1 25 = 35-11, ft = 1-738. 

 2X10 



The following are the readings in degrees for the first six sets of ob- 

 servations with P or Q. 



P. 



a. b. Mean. Difference. 



1 216 144 188 ?1 



2 144 74 109 7Q 



74 4 39 70 . 5 



4 (360) +4 293 328-5 7Q . 5 



5 293 223 258 7(>5 



6 223 152 187 5 - 



352 5. ... A. 



Q. 



c. d. Mean. Difference. 



1 308 360 + 18 343 7Q 



2 18 88 53 7Q 



3 88 158 123 n 



4 158 230 194 71 



5 230 300 265 7Q 



6 300 360 + 10 335 



352-0.... B. 



Mean of A— B = 352'25. 



i = 352 ' 2f L = 35 22, fx = 1-734. 

 2X5 



