LIGHT.
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number of circular crystals will be formed with radiat¬
ing forms between them. These circular crystals may
be made larger and regularly disposed by touching the
mass with a fine needle point when crystallization
begins. Such ones will form about each point touched.
Magnify such objects so much that these circular
crystals will appear a foot in diameter upon the screen.
The Nicol’s prism will show each one with four arms
that will turn about the centre of the crystal when the
prism is rotated, while the radiating crystals will show
as red, yellow, or purple brushes sweeping over the
screen.
By inserting a sheet of transparent mica or thin
selenite between the reflector and the object, a colored
field will appear as a kind of background, upon which
the minute crystals, such as chlorate of potash and
oxalic acid, will appear more highly colored. The effect
is usually to heighten the color.
THE DOUBLE IMAGE PRISM.
Fig. 103•
With a large lens project the image of the aperture
upon the screen, the light being polarized by the black¬
ened reflector. At the focus of the lens place the
prism. Two images of the aperture will appear and