POLARIZATION ILLUSTRATED BY RESULTANNT MOTIONS. 81 
106. Polarization illustrated by resultant motions. 
Every student of meclianics knows, that two forces at right 
angles to each other may combine and form a resultant force 
represented in direction and intensity by the diagonal of a 
parallelogram, the sides of which represent the direction and in- 
tensity of the original forces ; and that a single force, repre- 
sented in direction and intensity by the diagonal of a parallel- 
ogram, may be resolved into two forces at right angles to each 
other, which will be represented in direction and intensity by 
the sides of the parallelogram. 
Applying these principles 
to illustrate the polarization 
of light, let O, Fig. 43, 
represent the centre or axis 
of a ray of common light 
passing in a direction per- 
pendicular to the plane of 
the paper. Let A B, G H, 
D C, F E and I J, represent 
transverse sections of the 
planes in every direction in 
which the ray of light causes 
the luminiferous medium to 
vibrate, we can always se- 
lect two planes, as A B, C D, at right angles to each other, 
which shall correspond with the planes of polarization in which 
the light vibrates after double refraction. The vibrations in all 
the other planes, in which ordinary light is supposed to vibrate, 
may be resolved into vibrations in the planes A B and C D. 
Thus the vibration O G will be equivalent to two vibrations 
represented by O a and O d ; OF will be equivalent to O 5 
and O d' j O H will be equivalent to O V and O cy O E will 
be equivalent to O aJ and O g\ and so on. AV can thus resolve 
the vibrations in any number of planes into others in the 
planes A B and C D. 
Vibrations O I, very nearly coinciding with one of the planes 
C D, will give a resultant intensity in the direction of that 
CATALOGUE OF ACHROMATIC MICROSCOPES. 
6 
