4$ Dr. Brewster’s experiments on the 
primitive rhomboidal base, as in the case of calcareous spar 
and topaz, and that there is a variation in the intensity of the 
light of the images E and F, during the revolution of the 
prism, we must consider the existence of two oppositely pola- 
rised images as no longer problematical. 
Hence it follows, that every substance which possesses the 
property of depolarising light in the second manner, must 
necessarily form two coincident or nearly coincident images 
polarised in an opposite manner ; or to speak more correctly, 
a pencil of common light transmitted through depolarising 
crystals, consists of a portion of light polarised like one of the 
pencils formed by calcareous spar, and of another equal por- 
tion polarised like the other pencil formed by calcareous spar. 
Depolarising substances, consequently, are not entitled to 
the name of doubly refracting crystals, when the two oppositely 
polarised pencils are not capable of being separated from each 
other, and till this separation is actually seen, we must con- 
sider the two pencils as produced by the same refractive 
power. 
We would therefore propose to designate all substances 
that form two separable images, such as calcareous spar, 
quartz, topaz, & c. by the name of doubly refracting crystals, 
and those which do not form two separable images, such as 
diamond, mica, heated glass, the human hair, &c. by the name 
of doubly polarising crystals. 
3. The third species of depolarisation is characterised by the 
substance having no neutral axes, and depolarising light in 
every position ; and is possessed by gum Arabic, caoutchouc , 
and many other bodies which are known to be formed by the 
successive deposition and induration of thin layers. 
