THE POLAKISCOPE. 
159 
means of the handle, and which projects through a slit extending 
some way round the outer tube ; c is the crystal to be examined, 
which may be fixed in the centre of a disc of metal, or between 
two very thin glass plates ; t' is another plate of tourmaline 
fixed in the cap cn, which can be revolved so as to bring the 
axis of the tourmaline into any desired position. The colours 
and curves produced by the crystal are thrown on a screen, 
placed in front of c n : A b may be about 1 in. diameter. The 
plates of tourmaline are cut parallel to their axis. Tourma- 
line is a uniaxal crystal possessing the property of very 
rapidly absorbing the ordinary ray. A very thin plate will 
absorb all the light in the ordinary ray, and allow the other 
only to pass, furnishing a ray polarized in one plane. Plates cut 
and polished can be readily obtained of the optical instrument 
makers. 
Nitre and arragonite among biaxal crystals, and Iceland spar, 
some varieties of uniaxal apophylite among uniaxal crystals, 
produce the coloured curves spoken of very well. 
The appearances produced with a uniaxal crystal as the ana- 
lyzer is turned round are : — In one position we have beautifully 
coloured circular rings, divided by a black cross. The black 
cross becomes white, and the colours of the rings are changed to 
the complementary colours on revolving the analyzer through 
half a circle. With biaxal crystals the coloured rings are much 
the shape of the rings seen on a wooden table when the wood 
is cut across where two knots are close together. These rings 
are traversed by two dark hyperbolic brushes.* 
Another apparatus for producing polarized light, known as 
Nicols’ prisms, is very convenient for certain purposes. Two 
prisms cut from Iceland spar, of the same angle, are cemented 
together with Canada balsam, and mounted in corks fitting short 
brass tubes. The edge of one prism is parallel and of the 
other perpendicular to the axis of the crystal. Of the two rays 
into which a ray entering such a combination is divided, one is 
refracted by the first prism, so that on reaching the boundary it 
is internally reflected and lost on the blackened surface of the 
rhombohedron formed by the two prisms. The other ray is 
refracted, so that it passes both prisms and comes out in a 
direction parallel to that in which it entered the first prism. 
The emergent ray is, of course, plane polarized. Two Nicols’ 
prisms form convenient polarizers for the microscope. The one 
serving as analyzer may be fitted directly above the object glass, 
and as near to it as possible. It may screw into the tube con- 
taining the object glass, or on to the extremity of the erector, 
* For good figures of these curves see Pouillet-Miiller ^^Lehrhuch der 
Physik und Meteorologie,” vol, i. 
