OPTICS. 123 
spectrum are simple. is shown from the fact, that if one be isolated and passed 
through a second prism, no further decomposition takes place. 
-As white light is resolvable into the colors of the spectrum, so these 
latter may be combined again to produce white light. Let the spectrum 
be received on a convex lens, / ( fig. 45) ; this lens will unite the differently 
colored rays into a single point, f, and if a screen of paper or of ground 
glass be placed here, the solar image again appears perfectly white. If the 
screen be removed to a greater distance than the focal length, the spectrum 
will again appear, but inverted, r’w’. If, instead of the screen, a mirror be 
placed at f, the reflected rays will again form a colored spectrum, 7’’u’’. 
The following is another experiment, illustrating the re-composition of light. 
If a disk of paper be divided into seven sections, and painted so as to 
resemble, as nearly as possible, the prismatic colors in their natural order, 
then, on giving’ the disk a rapid rotation, a whitish hue will be perceptible 
instead of the colors. The disk would appear perfectly white if the 
prismatic colors could be represented perfectly pure, and of their 
proportional spectral breadth. All the seven colors, properly combined, 
thus produce white; if, therefore, one of these colors be suppressed, or its 
proportions altered, any other tint may be obtained. If, for instance, red 
be omitted, a bluish tint will be perceived; on adding the red this will 
disappear, and white again be exhibited. ‘T'wo colors, which, when mixed 
with each other, produce white light, are said to be complementary to each 
other. Thus, violet is complementary to green, yellow to blue, &c. 
Not white solar light alone, but also the natural colors of bodies, can be 
decomposed by the prism. For this purpose small strips of the color should 
be cut off and examined through the prism. Paste, for instance, upon black 
paper (jig. 46). a series of very small strips of colored paper, about half a 
line in width, of the following colors, beginning at the !eft :—white, yellow, 
orange, deep red, green, blue. If these be examined by a prism whose 
axis is parallel to the direction of the strips, they will appear, not only 
displaced, but their colors decomposed. The colored image of the white 
paper is complete; that of the yellow is wanting in blue and violet; that 
of the orange in blue, violet, and green; the image of the red paper contains 
only a little orange in addition to the red: in the green and blue papers the 
red rays are wanting almost entirely. 
If the colors produced by prisms of different material be examined, it will 
be seen that the single colors, while following each other in an invariable 
order, yet differ in proportional breadth. This difference in different bodies 
is determined by the difference of the refracting indices of the red and violet 
rays, and is called the dispersion. ‘Thus, flint glass has in general a greater 
dispersive power than crown glass; and this than water. By the dispersive 
power is to be understood the quotient arising from dividing the dispersion 
by the index of refraction of the mean rays, minus unity. 
If two prisms, A and B, be so combined that the refracting edges are 
directed in opposite directions ( fig. 47), the one neutralizes more or less the 
action of the other. If the compound prism thus formed produce a refraction 
of light without a decomposition, it is called achromatic. A compound prism 
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