31ft Dr. Brewster on the structure of the 
isinglass; but it derives this property from a bluish white 
membrane which covers the outside of it, for when this is 
removed, it loses the doubly refracting structure. If the 
sclerotic coat is boiled, it is capable of receiving the structure 
of doubly refracting crystals by mechanical compression and 
dilatation. In its natural state, it possesses the same pro- 
perty, but in an inferior degree. The cornea is also capable 
of having its doubly refracting force increased by compres- 
sion or dilatation. 
From these experiments the following conclusions may be 
deduced. 
I. All the parts of the crystalline lens of fishes correspond- 
ing to the two dark concentric circles, exercise no action upon 
polarised light. The outward spherical shell which acts upon 
light like one class of doubly refracting crystals, and also the 
solid nucleus which exercises a similar action, are in a state 
of mechanical dilatation, while the middle spherical shell which 
acts upon light like the other class of crystals, is in a state of 
mechanical contraction.* 
II. The structure of the crystalline lens in fishes is not 
symmetrical, as has hitherto been supposed, consisting merely 
of a number of coats of different densities ; but it has a dis- 
tinct relation to that diameter of the sphere which is the axis 
of vision. 
III. The variations of density which produce the doubly 
refracting structure, are not related to the centre of the crys- 
* When the crystalline lens is examined by common light, there is an obvious 
appearance of a rapid change of density at the line which separates the middle and 
the exterior sectors. This is probably the boundary of the fluid coat adjacent to the 
capsule. 
