In the figures referred to in the above descriptions, only one eye-glass is represented to avoid complexity; but in most reflecting telescopes, the eye-piece consists of a combination of two plano-convex glasses, as in fig. 67, which produces a more correct and a larger field of view than a single lens. This combination is generally known by the name of the _Huygenian eye-piece_ which shall be described in the section on the _eye-pieces_ of telescopes. The following rule has been given for finding the magnifying power of the Gregorian telescope:--Multiply the focal distance of the great mirror by the distance of the small mirror from the image next the eye; and multiply the focal distance of the small mirror by the focal distance of the eye-glass; then divide the product of the former multiplication by the product of the latter, and the quotient will express the magnifying power. The following are the dimensions of one of the reflecting telescopes constructed by Mr. Short--who was long distinguished as the most eminent maker of such instruments, on a large scale, and whose large reflectors are still to be found in various observatories throughout Europe. The focal distance of the great mirror 9.6 inches; or P _m_, fig. 67, its breadth FD 2.3; the focal distance of the small mirror L _n_ 1.5--or 1-1/2 inch--its breadth _g h_ 0.6--or 6/10 of an inch; the breadth of the hole in the great mirror UV, 0.5--or half an inch--the distance between the small mirror and the next eye-glass LR, 14.2; the distance between the two eye-glasses SR, 2.4; the focal distance of the eye-glass next the metal, 3.8.; and the focal distance of the eye-glass next the eye, S _a_ 1.1, or one inch and one tenth. The magnifying power of this telescope was about 60 times. Taking this telescope as a standard, the following table of the dimensions and magnifying powers of Gregorian reflecting telescopes, as constructed by Mr. Short, has been computed. [Illustration: _figure 67._]