g68 Dr. Wollaston s s Method of examining 
The advantages which this method possesses above the usual 
mode of examining refractive powers, are greater than they may 
at first sight appear. The usual practice has been, to form two 
surfaces of the substance under examination, so inclined to each 
other that the deviation occasioned by them might be measured. 
The inclination of these surfaces to each other must also be 
known; and thence the refractive power might be computed. 
But, in the method here proposed, it is sufficient to have only one 
surface, and the result is obtained at once, without computation. 
The facility of determining refractive powers, is consequently 
such as to render this property of bodies a very convenient test 
in many philosophical inquiries. For discovering the purity of 
essential oils, such an examination may be of considerable utility, 
on account of the smallness of the quantity requisite for trial. 
In oil of cloves, for instance, I have met with a wide difference. 
The refractive power of genuine oil of cloves, is as high as 
1,535 ; but I have also purchased oil by this name, which did 
not exceed 1 ,498, and which had probably been adulterated by 
some less refractive oil. 
For such purposes, the refractive power of opaque substances 
may often be deserving of inquiry, which could not be learned 
by any means at present in use. For, in the usual mode, a 
certain degree of transparency is absolutely necessary ; but, for 
trial by contact, the most perfect opacity does not occasion the 
least impediment. 
Among other instances in which I have taken advantage of 
this circumstance, I may mention a substance that had been found 
in one of the islands of the North Pacific Ocean, which, to all 
outward appearance and by various trials, seemed to be perfect 
bees-wax, although it is supposed that there are no bees in the 
