resultant vibrations in Polarized Light. 377 
number, of circular section, and arranged in a regular helix, 
making two revo- 1. 
lutions from end 
axis is placed near 
the front of the 
OX, and on each fig hgh gens = Seen armas Sa arannerane aaa ea ae aT 
cam rests a horizontal rod or bar of wood, eighteen inches long, 
free to revolve in a ver- 
guides, which prevent ~ 
lateral motion. On the front extremity of each bar there 
another bar, upright, fifteen inches long, the two being pivoted 
the 101 
air o 
tends by its mostneale to hold back the vertical rod against its 
cam on the upper axis. Inserted in the extremity of each ver- 
tical rod is a black iron wire, reaching through a slit in the top 
of the box, having a white ball upon it, which thus appears in 
Sight, when all the mechanism is concealed from view.* The 
whole vertical rod, from the hinge to the ball, is eighteen inches 
long, the same as the horizontal bar. Fig. 1 presents one of the 
Tesultant forms of wave, when the rods are in contact with their 
Tespective cams on both axes. : 5 
Back of the lower axis, and below the horizontal bars, lies a 
Strip of board, which can be turned up on hinges, so as to lift 
all the bars off from the cams; a similar piece Is hinged below 
the upper axis, and behind the vertical rods, which, being turned 
* The top piece containing the slits is not represented in the figure. 
Am. Jour, Sci.—Srconp Srrims, Vou. XXXII, No. 96.—Nov., 1861. 
48 
