. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. MICROSCOPE. 335 towards the object, the aberration is 4| times the thickness of the lens. Hence, when a plano- convex lens is employed, its convex surface should be turned towards a distant object, when it is used to form an image by bringing to a focus parallel or slightly-diverging rays; but it should be turned towards the eye, when it is used to render parallel the rays which are diverging from a very near object. The single lens having the least spherical aberration is a double convex, whose radii are as 1 to 6. When
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. MICROSCOPE. 335 towards the object, the aberration is 4| times the thickness of the lens. Hence, when a plano- convex lens is employed, its convex surface should be turned towards a distant object, when it is used to form an image by bringing to a focus parallel or slightly-diverging rays; but it should be turned towards the eye, when it is used to render parallel the rays which are diverging from a very near object. The single lens having the least spherical aberration is a double convex, whose radii are as 1 to 6. When the flattest face is turned toward parallel rays, the aberration is nearly 3f times its thick- ness ; but when the most convex side receives or transmits them, the aberration is only TJjths of its thickness. The spherical aberration may be still further diminished, however, or even got rid of altogether, by making use of com- binations of lenses so disposed that their op- posite aberrations shall correct each other, whilst magnifying power is still gained. For it is easily seen that, as the aberration of a con- cave lens is just the opposite of that of a con- vex lens, the aberration of a convex lens placed in its most favourable position may be cor- rected by a concave lens of much less power in its most unfavourable position; so that, although the power of the convex lens is weak- ened, all the rays which pass through this com- bination will be brought to one focus. This is the principle of the uplunatic doublet proposed by Sir J. Herschel, consist- ing of a double-convex lens of the most favourable form, and a meniscus with the concave of longer focus than the convex.* A doublet of this kind may be made of great use in the mi- croscope, as we shall hereafter show. But the spherical aberration is not doublet. the only imperfection with which the optician has to contend in the construction of micro- scopes. A difficulty equally serious arises from the unequal refrangibi
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