314 Dr. Brewster on the structure of the 
-.s-H > yA iyi f ’P •.. : ; Y" 
I now removed the capsule of the lens, so as to let out the 
semifluid matter which it enclosed, and having rubbed off 
the very soft exterior coat, I immersed the diminished sphere 
in Canada balsam, but I could never observe the exterior sec- 
tors, 5,6, 11, 12, all the other appearances being exhibited as 
before. I next removed the middle coats of the crystalline, 
and replacing the nucleus, which was now reduced to one- 
eighth of an inch in diameter, in the glass parallelopiped, I 
observed the central sectors l, 2, 7, 8, without any of the 
middle or exterior ones. By pressing the nucleus between 
two plates of glass, or by allowing it to indurate gradually, 
the depolarised tints ascended in the scale of colours, as in 
the case of animal jellies. 
If we now take a plate of sulphate of lime which polarises a 
blue of the second order, and combine it with the crystalline 
lens, so that its axis may be parallel to the line 6, 10, 2, 1,9,5, 
the white tints 9,-10, will ascend to a green of the second 
order, while those at 1, 2, 5, 6, will descend to an orange red 
of the first order. In like manner, if the axis of the plate of 
sulphate of lime is parallel to the line 11,3, 7 , 8 , 4, 12, the 
tints 3,4, will become green, and 7, 8, 11, 12, an orange red. 
Hence, it follows, that the nucleus 1, 2, 7 , 8, and the exte- 
rior coat , 5, 6, ti, 12, have the same structure as one class of 
doubly refracting crystals , while the middle coats 3, 4, 9, 10, 
have the structure of the other class. 
In order to compare these different structures with those 
of glass crystallized by heat and by pressure,* I took a polished 
sphere of crown glass, and, having brought it to a red heat,, 
I cooled it by rolling it quickly in every direction over a 
smooth surface. When it was immersed in Canada balsam, 
* See Phil. Trans. i8;6, p, 46, 156. 
