124 PHYSICS. 
of this character must consist of prisms of two different substances, crown . 
and flint glass for instance, whose dimensions are so calculated that the 
violet rays of the one coincide with the red of the other, or vice versa; 
nevertheless, a perfect achromatism cannot be obtained in this way. The 
possibility of producing a perfect achromatism was long doubted: Euler, 
Clairaut, and D’Alembert instituted many experiments on the subject. Hell, 
in 1733, constructed achromatic telescopes, but Dolland was the first to 
publish them, in 1755. Even at the present time, when so much progress 
has been made in practical and theoretical optics, the construction of good 
achromatic instruments is one of the most difficult problems. 
A simple lens has actually ditferent foci for the different colored rays, the 
focus of the red lying at a greater distance than the violet. The result of 
this is, that the images of simple lenses are surrounded by colored borders 
and consequently appear impure. If, now, lenses be composed of different 
kinds of glass, as a concave lens of flint glass united to a convex one of 
crown, the two rays may be so related as that the foci of the differently 
colored rays shall accurately coincide, and the object appear free from all 
colored edges. A lens of this character is called achromatic, and is repre- 
sented in fig. 48. (It is wrongly marked 43 in the plate, standing 
immediately to the left of fig. 42, the polyzonal lens.) In the preceding 
instance, both lenses combined produce no colored dispersion at all; as, 
however, the flint glass has a greater dispersive power, a concave lens of 
flint glass capable of destroying the dispersion of a convex lens of crown 
glass, will not be able entirely to overcome the convergence of rays 
to p, produced by the convex lens of crown glass; the two combined 
will therefore act as a convex lens, and at the same time be achromatic. 
e. Of Sight. 
The sensation of sight, or the perception of light and color, depends upon 
the affection of certain nerves, whose delicate extremities are distributed 
and expanded in the eye as a nervous membrane, called the retina. It is 
upon this retina that rays of light proceeding from the objects of the external 
world, fall. The organ of sight is nevertheless very differently constructed 
in different classes of animals, and two essentially different kinds of eyes are 
distinguished—the mosaic composite eyes, as possessed by most insects and 
crustacea, and the simple eyes provided with convex lenses, possessed by 
man and the other vertebrata. 
A mosaic composite eye (fig. 49), is so arranged that a great number of 
transparent truncated cones stand perpendicularly on a convex retina. 
Those rays alone can reach the bottom of one of these cones which fall 
along the direction of its axis; all rays coming sideways are absorbed 
by the dark pigment clothing the sides of the cone. In fig. 49, let 
febg be a section of the convex retina, with the transparent cones set upon 
it. Rays passing from the luminous point A, can strike this retina only in 
cb, the base of the truncated cone, abcd. Any other luminous point, B, must 
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