434 
MR. R. T. GLAZEBROOK ON DOUBLE REFRACTION 
and in fx of the value 
This is not greater than 
x is not greater than 
2* cos (X—i/j) f-~—\ 
W H 
x cos (X — \ 
W ft' 
1 1 
—s—(—i-5 COs2 ( X ” 
/V \/V /*! 
PV aJ(—— 
w /v 
r 
2 
Term neglected is not greater than 
^ 2 W 
In the three cases considered y is less than 14'. 
The circular measure of 14' is ”004. 
^ = ”000008 
Using Rtjdberg’s values of /x l5 /x 3 we have 
fjL-[ is less than 1 ”7 
—r—^ is less than ”1 
AV AG 
[A-i 3 is less than 5 
Therefore, greatest difference is less than 
”5 X -000008 
or ”000004 
Hence neglecting y or supposing the plane of each of the prisms I., III., and IV. 
to coincide with the plane z x will never produce any change in the fifth decimal 
figure in the value of /x. 
In the case of prism II. the value of y is nearly 30', and we may have to take 
account of the obliquity. 
For prisms I., III., and IV. the value of 6 is given by formulae of the form 
6=\±<t>' 
</>' having the meaning attached to it in the results of experiment. 
