80 POLARIZED LIGHT AND ITS APPLICATION TO THE MICROSCOPE. 
are constantly in a state of alternate contraction and expansion, 
and are capable of communicating tlieir own motion to a very 
subtle ether pervading universal space, (and even solid bodies 
themselves,) through which vibrations are propagated like 
waves through water, but with immensely greater velocity. 
105. Illustrations of wave motion. Let a rope, made 
fast at one end, be stretched in a horizontal direction, while the 
other end is held in the hand ; if this end is moved quickly 
upwards and downwards at regular intervals, an undulatory 
motion will be propagated through every part of the rope, by 
a series of tremors or waves passing along from one end to the 
other. The vibratory motions will all take place in a perpen- 
dicular plane, the motion of each particle being at right angles 
to the general direction of the rope. This vibration of the 
rope may represent the vibrations of one of the polarized rays 
into which common light is separated by double refraction. 
Let another similar rope be agitated in the same manner 
from right to left, the particles of this latter rope will vibrate 
in a plane perpendicular to the vibrations of the other rope, 
and will serve to illustrate the vibrations of the other ray of 
light, polarized at right angles to the first, by the action of the 
doubly refracting medium. 
If we suppose a single rope agitated successively in an infi- 
nite number of planes, varying through every degree of the 
circle, the diflferently inclined vibrations, following each other 
at infinitely short (but successive) intervals, while the vibra- 
tions would take place in every possible plane, the successive 
waves, by which the vibrations would be propagated, would 
advance like the coils of a spiral, or the threads of a screw. 
Let us suppose this rope, whose waves are propagated in a 
spiral direction, gradually restrained by the approach of two 
plane resisting surfaces, parallel to each other, the spiral motion 
would be gradually obliterated, and vibration would be con- 
tinued only in a single plane, as was supposed in the case 
of the first rope. This may serve to illustrate, somewhat, the 
action of the tourmaline, which transmits light polarized or 
vibrating only in a single plane. 
J. & W. GRUNOW & GO’S ILLUSTRATED 
