386 Dr. Wollaston on the oblique Refraction, &c. 
By calculation,* the angle which this conjugate makes with 
the perpendicular is 6° 7^'. But, by the following measurement, 
it appears to be 6° 16'. 
Exp.g. A piece of spar that measured 1,145 Inch * n thick- 
ness, was laid upon a line, and showed two images that were 
removed from each other of an inch. Then, as 1,145: 
0,126 : : radius : tang, of 6° 16'. 
The different results deduced from theory and from observa- 
tion, will be seen at one view in the following statement. 
In Exp. 2d, observed 1,518; calculated 1,5215. 
3d, 1,537 ~ — — l > 539 - 
4th, — 1,571 — 1,5736- 
9th, angle observed 6° 1 6' - - 6°7|-'. 
The angle observed differs from that obtained by computa- 
tion, in a greater degree than any of the former measures ; but, 
when the difficulty of measuring this angle with accuracy is 
considered, and also the greater effect of any incorrectness in the 
data from' which a semiconjugate is computed, I think the result 
of this, as well of the preceding comparisons, must be admitted 
to be highly favourable to the Huygenian theory; and, al- 
though the existence of two refractions at the same time, in the 
same substance, be not well accounted for, and still less their 
interchange with each other, when a ray of light is made to pass 
through a second piece of spar situated transversely to the first, 
yet the oblique refraction, when considered alone, seems nearly 
as well explained as any other optical phenomenon. 
* (Fig. 5.) CS : CP ; : tang. PCG : tang, /CO or co-tang. PCQj 
then CP ; CS tang. PCQ_: tang. PCM ; 
and LCP - PCM — MCL. 
