360 PROCEEDINGS OF THE ACADEMY OF [1893. 



glass plate T beyond the second decimal place, since the value of n, 

 or the index of refraction, cannot be measured beyond the third 

 decimal place. While an error of 2 mm. in the measurment of D 

 will affect R to an extent only of 0*016 mm., the same amount of 

 error in d will affect R as much as the 0'032 mm. An error of 0'02 

 mm. in T will involve only an error of O'Ol mm. in I, but an error 

 of - 01 mm. in n will change I by as much as - 02mm. Finally as 

 an error of 0'02 mm. in I will change the value of R by as much as 

 0*07 mm. it follows that of these different values that of n, or the 

 index of refraction, is the most important, the value of n affecting 

 that of R through I. 



Resume of 50 experiments ivith the ophthalmometer. 



