refractive and dispersive Powers. 3 6g 
island from which it was brought. On placing it by the side of 
a piece of bees- wax, in contact with a prism, the perfect equality 
of their refractive powers afforded a strong confirmation of the 
opinion before formed of their identity. 
For the examination also of media of which the refractive 
density is not uniform, the general method of trial by deviation 
wholly fails ; on the contrary, by placing a varied medium in 
contact with a prism, all its gradations of density, from greatest 
to least, become at once the object of mere inspection. An in- 
stance of this may very readily be seen with a piece of gum, 
the surface of which has been moistened for a few minutes ; 
when, by close application to a prism, a refractive power may 
be discerned, varying from that of the water on the surface, 
1,336, to nearly 1,51, the refractive power of gum arabic. 
I should not so much insist on this advantage, were it not 
for the opportunity hereby afforded of examining the crystalline 
lens of the eye, which is now known to be generally more dense 
in the centre than at its surface. 
Mr. Hauksbee, who was not aware of this difference, has 
estimated the refractive power of the crystalline lens, by forming 
it into a wedge by plates of glass, somewhat higher than I find 
it to be ; but, with his accustomed accuracy, he remarked the 
apparent enlargement of an object, occasioned by the variations 
of its density, which he was unable to explain. 
In the table that follows, I have set down, not only the limits 
of refractive power in a crystalline lens of an ox, ascertained by 
trial, but also an average, computed from the refractive density 
of a dried crystalline of an ox, of which the weight had been 
first taken in the recent state, and the quantity of water lost by 
drying also measured. 
