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. 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 white light besides the prism, of which one of the principal and most interesting to the photographer, is by _reflection_ from colored bodies. If a beam of white light falls upon a white surface, it is reflected without change; but if it falls upon a _red_ surface, only the red ray is reflected: so also with yellow and other colors; the ray which is reflected corresponds with the color of the object. It is this reflected decomposed light which presents the beautifully colored image we see upon the ground glass in our cameras. [Sidenote: LIGHT, HEAT, AND ACTINISM.] A sunbeam may be capable of three divisions--light, heat, and actinism; the last causes all the chemical changes, and is the acting power upon surfaces prepared to receive the photographic image. The accompanying illustration, Fig. 2, will readily bring to the minds of the reader the relation of these one to another, and their intensities in the different parts of a decomposed sunbeam. [Illustration: Fig. 2.] The various points of the solar spectrum are represented in the order in which they occur between A and B, this exhibits the limits of the Newtonian spectrum, corresponding with Fig. 1. Sir John Herschel and Seebeck have shown that there exists, beyond the violet, a faint violet light, or rather a _lavender_, to _b_, which gradually becomes colorless; similarly, red light exists beyond the assigned limits of the red ray to _a_. The greatest amount of actinic power is shown at E opposite the violet; hence this color "exerts" the greatest amount of influence in the formation of the photographic image. [Sidenote: COLORING WALLS BLUE.]