376 Prof. Snell’s Instrument to illustrate 
Art. XLIT.—An Instrument designed to illustrate certain resultant 
vibrations in Polarized Light; by Prof. E. 8. SNELL, Amherst 
College. 
I HAVE lately designed and constructed an instrument for ex- 
hibiting the combinations of two rays of light, polarized in 
planes at right angles to each other, when they coincide in di- 
rection, but one is in advance of the other by any fraction what- 
ever of the wave-length. 
Iam aware that an instrument for presenting these resultant 
forms and motions has been invented by Plucker, a German 
physicist. But it appears to me that its mechanical difficulties 
must be so serious, as to render it, practically useless. There 1s 
also much inconvenience in attempting to show by it one polari- 
zation by itself, then the other, and finally some resultant of 
combined. Besides, after sending forward two or three 
waves at most, the process comes to an end, and there must be 
a backward movement, before another progression of waves can 
be shown. I have never seen one of Plucker’s instruments 12 
operation, but the description and engravings satisfy me, that 
unless the parts are almost absolutely free from friction, the r 
will bind on the surfaces and in the grooves, and stop all pro- 
gress, or become bent in the attempt to continue the motion.* 
The - aratus here described is free from these several objec- 
tions. e friction of the parts has no tendency to make them 
bind against each other, and thus check the motion of the-waves; 
the change can be made in a moment from each single polarized 
system to the other, and to any desired combination of the two 
systems; and finally, the series of waves, of whatever kind, 
passes along uninterruptedly, so long as the experimenter 
chooses. 
The general plan of giving motion to the rods which carry 
the balls is the same as that employed in the instrument ie 
showing the molecular motion in water-waves, an acne 
presents a view of one of these systems of cams, twenty-four 
_ * For a description and re gorge of Plucker’s instrument, see “ Introdue 
tion a la Haute Optique, par A. Beer, traduit par M. C, Forthomme,” pp- acl 
