eee 
452 E. L. Nichols—Character and Intensity of the 
Ss 
osin(¢ —7r) (4) 
where @ denotes the thickness of the plates, 
¢ denotes the angle of incidence, 
r denotes the angle of refraction. 
Eliminating r the equation becomes, 
sin*é Sites sin?¢ cos? 
8/1 = asin (4/1 SE 88) (5) 
where n is the index of refraction. : 
The use of the opthalmometer offers great practical advan- 
tages over other micrometric methods, in that the angles corres- 
ponding to any given distances are independent of the distance 
of the object measured, and in that any slight unsteadiness of 
the object does not affect the accuracy of the determination. 
The two end-points a and } (fig. 5) of the bit of wire to e 
measured, were, as already stated, about 45™ apart ; and $0 “it 
especial contrivance was therefore necessary to bring both 
them at once into the field of the opthal mometer. A system 
cacy “made use of a biconvex lens of as great mag? awe 
er as the case permitted. There was a limit to the poss a 
