3 * 
depolarisation of light , &c. 
reous spar, and of topaz, have two neutral axes at right angles 
to each other, and two depolarising axes at right angles to each 
other, and forming angles of 45 0 with the neutral axes. 
In mica, the neutral axes coincide with the diagonals of its 
primitive rhomboidal base. 
In calcareous spar the neutral axes are coincident with the 
diagonals of any of its rhomboidal faces, while the depolarising 
axes are parallel to the sides of these faces; but when polarised 
light is transmitted along the short diagonal of the rhomb 
itself, there is no position in which it is depolarised, or in 
which the two images continue visible during the whole re- 
volution of the prism of calcareous spar next the eye. The 
evanescent image is no doubt restored by the interposition of 
the rhomb, but this only shifts the vanishing place of the 
images which will continue to disappear alternately in every 
quadrant. 
In topaz, the neutral axes coincide with the diagonals of the 
base of its primitive right prism, and the depolarising axes 
are parallel to the sides of any of its square faces. 
The minerals, in which I have not found the property of 
depolarising light, are muriate of soda, or rock salt, fluor spar , 
and a crystal of spinelle ruby. I have cut numerous plates in 
different directions from a cube of muriate of soda, and have 
tried many specimens of fluor spar, but in none of them could 
I discover the least trace of depolarising axes. 
The only mineral which depolarises light in every position, 
and therefore possesses no neutral axes, is the diamond. Out 
of fourteen specimens of this gem which I examined, seven 
depolarised light in every position ; four did not depolarise 
light at all ; one depolarised about a fifth part of the light, 
