a66 Dr . Wollaston’s Method of examining 
the object ceases to be seen by refraction, is about 39 0 10'; but, 
when the object has been clipped in water, and brought into 
contact with the glass, it continues visible, by means of the 
higher refractive power of the water, as far as 57"2 incidence,, 
When any kind of oil, or any resinous cement, is interposed, 
this angle is still greater, according to the refractive power of 
the medium employed ; and, by cements that refract more 
strongly than the glass, the object may be seen through the 
prism, at whatever angle of incidence it is viewed. 
In examining the refractive powers 01 fluids, or of fusible 
substances, the requisite contact is easily obtained ; but, with 
solids, which can in few instances be made to touch to any great 
extent, this cannot be effected without the interposition of some 
fluid, or cement, of higher refractive power than the medium 
under examination. Since the surfaces of a stratum so interposed 
are parallel, it will not effect the total deviation of a ray passing 
through it, and may therefore be employed without risk of any 
error in consequence. 
Thus, resin, or oil of sassafras, interposed between plate glass 
and any other prism, will not alter the result. 
If, on the same prism, a piece of selenite and another of plate- 
glass be cemented near each other, their powers may be com- 
pared with tire same accuracy as if they were both in absolute 
contact with it. 
For such a mere comparison of any two bodies, a common 
triangular prism is best adapted; but, for the purpose of actual 
measurement of refractive powers, I have preferred the use of a 
square prism, because, with a very simple apparatus, it shows 
the sine of refractive power sought, without the need of any 
calculation. 
