If a triangular prism of glass be placed on the inside of the dark room, so that the beam of light falls upon it, it no longer has the same direction, nor does it form a round spot, but an oblong painted image of seven colors--red, orange, yellow, green, blue, indigo, and violet.

This is called the solar spectrum, and will be readily understood by reference to the accompanying diagram, Fig.

1.
{133} To those who are unacquainted with the theory of light (and for their benefit this chapter is given), it may be a matter of wonder how a beam of light can be divided.
[Illustration: Fig.

1 (amdg_1.gif)] This can be understood when I say, that white light is a bundle of colored rays united together, and when so incorporated, they are colorless; but in passing through the prism the bond of union is severed, and the colored rays come out singly and separately, because each ray has a certain amount of refracting (bending) power, peculiar to itself.

These rays always hold the same relation to each other, as may be seen by comparing every spectrum or rainbow; there is never any confusion or misplacement.
There are various other means of decomposing {134} white light besides the prism, of which one of the principal and most interesting to the Daguerreotypist is by reflection from colored bodies.