Although the performance of telescopes chiefly depends on the goodness of the object-glass, or the object-speculum of the instrument, yet it is of considerable importance, in order to distinct vision, and to obtain a large and uniformly distinct field of view, that the eye-piece be properly constructed. The different kinds of eye-pieces may be arranged into two general divisions--_Astronomical_ and _terrestrial_.
1. _Astronomical eye-pieces._--The most simple astronomical eye-piece is that which consists of a single convex lens; and when the focal distance of this lens, and that of the object-glass of the instrument is accurately ascertained, the magnifying power may be nicely determined, by dividing the focal length of the object-lens by that of the eye-glass. But, as the pencil of white light transmitted by the object-glass, will be divided by the eye-glass into its component colours, the object will appear bordered with coloured fringes, and the distinctness of vision consequently injured. Besides, the spherical aberration, when a single lens is used, is much greater than when two or more glasses are employed. Hence astronomical eye-pieces are now formed by a combination of at least two lenses.
The combination of lenses now generally used for astronomical purposes, is that which is usually denominated the _Huygenian eye-piece_, having been first proposed by the celebrated Huygens, as a great improvement on the single lens eye-piece. The following figure (73) represents a section of this eye-piece. Let AB be a compounded pencil of white light proceeding from the object-glass; BF a plano-convex field-glass, with its plane side next the eye-glass E. The red rays of the pencil AB, after refraction would cross the axis in R, and the violet rays in V, but meeting the eye-glass E, the red rays will be refracted to O, and the violet nearly in the same direction, when they will cross each other about the point O, in the axis, and unite. The distance of the two glasses FE, to produce this correction, when made of crown glass, must be equal to half the sum of their focal distances nearly. For example, suppose the focal distance of the largest, or field lens, to be 3 inches, and the focal distance of the lens next the eye, 1 inch, the two lenses should be placed exactly at the distance of 2 inches; the sum of their focal length being 4, the half of which is 2. In other words, the glass next the eye should be placed as much _within_ the focus of the field-glass as is equal to its own focal distance. The focal length of a single lens, that has the same magnifying power as this compound eye-piece--is equal to twice the product of the focal lengths of the two lenses, divided by the sum of the same numbers. Or, it is equal to half the focal length of the field-glass. Thus, in reference to the preceding example, twice the product of the focal length of the two lenses--is equal to 6, and their sum is 4. The former number divided by the latter, produces a quotient of 1-1/2, which is the focal length of a single lens, which would produce the same magnifying power as the eye-piece; and 1½ is just half the focal length of the field-glass. The proportion of the focal lengths of the two lenses to each other, according to Huygens, should be as 3 to 1; that is, if the field-glass be 4-1/2 inches, the eye-glass should be 1-1/2; and this is the proportion most generally adopted. But some opticians have recommended that the proportions should be as 3 to 2. Boscovich recommended two similar lenses; and in this case the distance between them was equal to half the sum of their focal distances, as in the Huygenian eye-piece.