339 



On re-examining the adjustments, after taking these two 

 sets, I found the glasses had got slightly moved. It is probable 

 that this had occurred in taking the measures of er and rp 9 

 and therefore would not influence the others ; but I have not 

 calculated them. 



On May 22nd, I obtained the following : 



Angle $iy, fig. 1. = 19° 59' 39" 

 ef — = -70650 

 ii fig. 2. = -0897189 

 er — = 12-08 

 rp — = 20*25 



whence ab = -1500612; and when jx = 1*492, 



D,-D -00335378 

 when the refractive index p is taken = 1*495. 

 When we take p = 1*492, we find 



d-d,_ __ 00570817 = 



D,-D -00336775 



The reason of my having used two values of the refractive 



index is this. The circle being engaged with the other part of 



t^e experiments, and as I possessed no other divided instrument 



2X2 



