depolarisation of light, &c. 51 
polarised light. After it begins to crytallize, however, a small 
portion of light is at first depolarised. This portion gradually 
increases, and the image is not completely restored till the 
crystallization has pervaded the whole mass. The experi- 
ments with oil of mace furnish us with another proof of this 
explanation, and exhibit a case in which a part of the substance 
is permanently crystallized, while another part of it is perma- 
nently uncrystallized. 
Hence we may conclude, that bodies which depolarise only a 
portion of light , either consist of permanently crystallized and un- 
crystallized portions , or are in a state of approach to a perfect 
crystalline structure , the crystallized portion being always propor- 
tional to the quantity of depolarised light. 
6. The sixth kind of depolarisation is exhibited in the curi- 
ous phenomena of oil of mace, which sometimes depolarises 
four sectors of nebulous light. 
Those plates of oil of mace, in which the luminous sectors 
are alone depolarised, have obviously two structures, namely, 
that which forms the bright, and that which forms the nebu- 
lous image. The structure which forms the bright image has 
no more action upon light than a mass of water, as it does 
not in the slightest degree alter its polarity ; but the structure 
which forms the luminous halo possesses a peculiar character. 
If we suppose the halo to be divided, as in PI. V, fig. 1 ; into 
eight sectors 1, 2, 3, 4, 5, 6, 7, 8, every alternate sector 1, 3, 
5, 7, is polarised in the same manner as the incident light, 
while the other sectors 2, 4, 6, 8, are polarised in an opposite 
manner. Now, if this halo consisted only of one nebulous 
image, the evanescence of every alternate sector would take 
place without applying the calcareous spar, and merely by 
H 2 
