378 Prof. Snell's Instrument to illustrate 
into a horizontal position, presses the rods forward, and releases 
them from the upper cams. While they are placed thus, all the 
lower bars lie in a horizontal plane, and the upright ones in a 
vertical plane, so that the balls are in a straight line, represent- 
ing the particles of ether at rest. If the crank is turned, both 
systems of cams revolve, but produce no motion of the balls. 
If the lower support is barnal down, the horizontal bars fall 
upon their respective cams, the balls assume the arrangement of 
vertical waves, and on turning the crank, the wave-motion 18 
seen in a vertical plane alone, representing a plane-polarized pencil 
of light. Now let the lower support be raised again, and the 
upper one be turned down; then the vertical rods are held back 
by the elastic cords against the upper system of cams, while the 
pivots at the bottom are in a horizontal line. If the erank is 
now turned, the lower cams produce no effect, but the upper 
system, revolving by the band, gives the balls a wave-motion 12 
a horizontal plane, thus representing a pencil of light polarized in 
a plane at right angles to the former, Finally, if the lower sup- 
port is once more let down, and the crank turned, the vert 
waves coexist with the horizontal ones, and some resultant 
of polarization is represented, the precise character of ie . 
i e con- 
are the 
(1) Plane polarization, the vibrations being in a plane inclined 
45° e right 
tinted . 
(2.) The same, the plane inclined 45° to the left. ie 
(3.) Circular polarization, the atoms revolving from the righ 
over to the left. ; 
(4.) The same, the revolution being from left over to right, . 
(5.) Elliptical polarization, the transverse axis inclined 45 
the right, and the atoms revolving from right over to the left. 
_ (6.) The same, and the axis the same, but the atoms revolving 
from left to right. 
