88 POLAKIZED LIGHT AND ITS APPLICATION TO THE MICEOSCOPE. 
ble difference of density of their various parts, and these differ- 
ences may thus be distinguished and traced out much more 
satisfactorily than by common light. Polarized light may be 
compared to a new sense given to the student of nature, by 
which he is enabled to see things wholly invisible by ordinary 
light. 
Where the doubly-refracting properties of the tissue are too 
feeble to produce sufficient difference of color, the effect may 
be increased by placing the object over a plate of selenite or 
mica, of such a thickness as to give to the light any shade of 
color required, 
115. Tiie Cawse of Colors produced toy selenite or mica, 
when polarized light is transmitted through them, is easily 
understood by reference to the undulatory theory of light. In 
all doubly refracting substances, (of which selenite and mica 
are examples,) the ordinary and extraordinary rays move with 
different velocities, and consequently, when the two rays are 
again blended, unless the retardation amounts' to a certain num- 
ber of entire waves, the two rays will, by the interference of 
their waves, produce some change in the color of light. If the 
retardation equals any number of entire vibrations, the result 
will still be white light, the two rays conspiring to increase 
their mutual intensity. If one ray is retarded an odd number 
of half vibrations, they will mutually destroy each other and 
produce darkness, just as if two waves of the sea meet in such 
a state that the phase of elevation in one coincides with the 
phase of depression in another, the two will produce a level, or 
mutual obliteration results. Such a result in the case of light, 
would require the most exact adjustment of the thickness of 
the crystal, and would not often occur. 
The interference produced by selenite and mica, are, in 
general, similar to the results which would be obtained by 
placing one prismatic spectrum upon another, in a reverse 
position, but not exactly superimposed upon it. The amount 
of overlaping would determine the resultant color. 
116. Meltood Varying llie CoSors. When a film of 
selenite, of uniform thickness, is placed between the polarizer 
J. & W. GRUNOW k CO’S ILLUSTRATED 
