122 PHYSICS. 
of a spherical segment, a, and several rings, 6, c,d, surrounding it, exhibited 
in section by fig. 41. Their curvature is so calculated that their foci shall 
coincide with that of the segment, a; if now a light be placed in this latter 
point, all rays from it, incident upon the lens, will emerge parallel. This 
ean only take place in the common lens at a very small aperture, while in 
the polyzonal lens, the aperture may amount to 40°, and the desired end be 
still attained. It is evident that the light at f can be rendered visible at a 
great distance, as this kind of lens sends out, in one and the same direction, 
nine times as much light as the common lens. 
d. Color. 
White solar light is composed of variously colored rays, as may be shown 
by means of a prism (pl. 21, fig. 28) in the experiment already referred to. 
In fig. 43, let m be a mirror attached to the shutter of a darkened room, 
casting the rays received from the sun through the opening, o, into the 
chamber; let p be a prism, and ¢ a wall receiving the images. Before 
applying the prism, there is seen at g a round white solar image. After 
attaching the prism, an elongated colored image, ru, will be perceived, of 
equal breadth with the direct solar image, g ( es 44). This colored image, 
called the solar spectrum, is of equal breadth with the natural solar image ; 
its elongated length depends upon the refracting angle and the refracting 
medium. The relation of the material of the prism to the length of the 
spectrum, other things being equal, is called its dispersive power, which is 
sreater as the length of the spectrum is greater. A hollow prism, filled with 
water, will give a spectrum of different length from the same prism filled with 
sulphuret of carbon, or other liquid substance. Prisms of flint-glass have a 
ereater dispersive power than those of common glass. 
When the white band in the centre of the spectrum is destroyed by 
elongating the spectrum, seven principal colors will be distinguishable in 
the latter. These are, in the ascending order, red, orange, yellow, green, 
blue, indigo, and violet. These are called the colors of the rainbow, 
prismatic, or spectral colors; the latter, on account of their not being 
further separable into other colors. The red rays always appear near to 
where the white image stood before the application of the prism. It follows, 
therefore, that the different rays are of different refrangibilities, the red 
being least, and the violet most refracted. All media do not transmit the 
colored rays with equal facility ; if, for example, the hollow prism (fig. 30) 
be filled with a solution of sulphate of indigo, and the circular aperture in 
the window be viewed through it, we shall observe only two separated 
images of the bright disk, a blue and a yellow. A solution of chromate of 
lime gives a red and green image. Hence it follows that the entire 
spectrum consists of circular solar images, as shown in fig. 44, which cover 
each other more or less. _ The less of this superposition of individual 
images, the more distinct will be the colors. That the colors of the 
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