99 
crystallized bodies on homogeneous light. 
struction of the apparatus I employed is as follows. AB is a 
brass tube, within which are fitted, ist, a fixed diaphragm, 
a abb, carrying the first plate of tourmaline in its centre; 
2dly, a diaphragm, ccdd, moveable freely in its own plane 
by means of the pin g passing through a slit in the side of 
the cylinder A B, which occupies an arc of about 120° of its 
circumference. This is destined to receive the crystallized 
plate dd, while a cylinder, hheejf, made to slide and turn 
smoothly within A B, carries the second tourmaline,//. It 
is essential that the tourmalines employed for this purpose, 
and especially the posterior one, should be perfectly free from 
all flaws and blemishes ; but large plates not being required, 
this condition is easily satisfied. The plates so arranged, 
and brought as near together as possible, the extremity A, 
of the cylinder A B, is fitted to slide somewhat stiffly on the 
brass tube P Q, furnished with a lens L, of about two inches 
focus, and a screw PP, by which it can be adapted to the ap- 
paratus usually employed for reflecting a sunbeam into a 
darkened chamber. The sliding motion of the cylinder A B 
allows the focus of the lens to be adjusted so as to fall exactly 
on the first surface of the posterior tourmaline/, while its 
rotation suffers the axis of the anterior one to be placed per- 
pendicular to the plane of reflection. By this arrangement 
two advantages are gained. The reflector employed (though 
metallic) always polarises a more or less considerable por- 
tion of the reflected beam, which in any other position is 
partially, or totally extinguished by the first tourmaline, and 
a great loss of light ensues, which it is of the utmost con- 
sequence to avoid : moreover, by this disposition, the action 
