as exhibited in its propagation along plates of glass. gy 
When polarised light was transmitted through the flat sides 
of the glass ABCD, Fig. 39. (PL IV.) four white spots were 
depolarised as shown at 1,2, 3, 4. All these spots have the 
same bluish white tint, but those marked i, 2, have their axis 
at right angles to that of the spots 3, 4.* 
The preceding phenomena as explained by the reasoning 
in Proposition XIX. furnish us with a complete explanation 
of the appearances exhibited by oil of mace , and described in 
a former paper.-f The dark and luminous sectors are obvi- 
ously produced by circular groups of crystals, having their 
axes directed to the same centre, and the halo, or nebulous 
image must be caused by the crystals having a form approach- 
ing to that of a sphere. This species of circular grouping is 
actually seen in a particular kind of adipocire , which I have 
noticed in the Paper already quoted. The axes of the crystals 
of adipocire, however, are not directed to the same centre, 
and therefore do not exhibit the same phenomena as oil of 
mace, 
Schoijum. 
The results contained in the Proposition, afford the most 
satisfactory explanation of the optical properties of Prince 
Rupert's drops described in a former Paper. (See Phil. Trans. 
1815? p. 1.) The cleavages which they exhibit in lines con- 
verging to the axis of the drop, and in lines concentric with 
the outer surface, are necessary consequences of the radial 
crystallization explained in the Proposition, and may be re- 
garded as an ocular demonstration of its truth. 
* These spots are the diagonal fringes described in Prop. XXIX. 
f See Phil . Trans. 1815, p. 38, and 49. 
MDCCCXVI, 
o 
