336 Proceedings of Royal Society of Edinburgh. [sess. 
must place the calcareous spar between two wedges of a medium 
having a greater action on light. 
If a plate of spar be placed between two wedges of glass, having 
its index of refraction just equal to the index of calcareous spar for 
ordinary light, no ordinary ray would be intercepted ; and there 
would remain the question — so to form these wedges as to exclude 
the extraordinary pencil, as also to determine the manner in which 
the slice of spar should be cut from the crystal. 
Supposing a glass obtained with the refractive power - = 1*655 ; 
a 
it is obvious at once that the best direction for the slice of calcareous 
spar is across the axis of crystallisation. The angle at which the 
extraordinary ray interior to the glass would suffer total reflection 
would have -5 for its cosine; hence, that angle would be 26°. . 10 '. . 19" 
Let, then, AB represent a plate of Iceland Spar cut at right angles 
to the axis: ABD, ABC, two wedges of glass ind. ref. = 1*655. 
Make ABD = 26° . . 10' . . 19", and a ray of light between AB and 
DB would send through only the ordinary pencil ; beyond DB both 
would pass. Make, again, the angle BDA such that its cosine 
is a, that is, make it 52°. .50'.. 35", and, of course, BAD = 
100° . . 59' . . 06" ; and then, while no ordinary ray is intercepted, 
no extraordinary one is suffered to pass. 
When *the refractive power of the glass employed is not the 
inverse of a, the computations become more intricate ; but they 
resemble so closely those of the first part of this paper that it is 
needless here' to go over them. 
The importance of the eye-piece as an instrument for experimental 
research, entitles it to a strict and minute analysis, that we may 
call into action the full development of its powers, and thus make 
sure of losing none of the benefits which it promises to confer. 
ZOth January 1837. 
