90 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. 
117. Seleaiite §tage. This instrument, which was invented 
by Mr. Darker, is shown in Fig. 51. It consists of a plate of 
brass, three or four inches long, 1^ inches broad, and J of an 
inch thick, having a piece of raised brass screwed to it, against 
which objects may rest when the body of the microscope is 
inclined. 
Fig. 51. 
In the centre of the brass plate there is a hole, one inch in 
diameter, into which is fitted a ring of the same metal, with a 
shoulder on its under side to receive certain cells, into which 
plates of selenite are fitted ; this ring can be revolved either to 
the right or the left of a central index or dart, by means of an 
endless screw S. P A J, P A |, P A represent three brass 
cells, into each of which are burnished two plates of thin glass^ 
having between them films of selenite of difierent thicknesses. 
The dart P A, denotes the direction of the positive axis of the 
selenite, and the figures •}, f , J, denote the parts of a vibration 
retarded by each disc, which, by their superposition and varia- 
tion in position, by means of the endless screw motion, produce 
all the colors of the spectrum. 
118. Polarizer with RevoSviaig §eleiiiitc Carrier. In 
order to afford the greatest facility of revolving the selenite 
plate, and for convenience of using it, a revolving selenite 
carrier is attached to the polarizer, as shown in Fig. 52. The 
solid ring is attached to the stage in the usual way by a 
bayonet-joint. A cylinder, with a milled head c, is supported 
by the ring d, which revolves in the ring a. Upon d rests the 
selenite carrier 5, covered by the cap e, so that the selenite plate 
is revolved by turning the milled head c. Within the cylinder 
J. & W. GRUNOW & GO’S ILLUSTRATED 
