prism. Our light comes to us from circumscribed bodies; and the sun, which chiefly produces our prismatic appearances, is itself only a small, circumscribed, luminous object.
307.
We may, however, consider every larger opening through which the sun shines, every larger medium through which the sun-light is transmitted and made to deviate from its course, as so far unlimited that we can confine our attention to the centre of the surface without considering its boundaries.
308 (197).
If we place a large water-prism in the sun, a large bright space is refracted upwards by it on the plane intended to receive the image, and the middle of this illumined space will be colourless. The same effect may be produced if we make the experiment with glass prisms having