_General remarks on Gregorian Reflectors._--1. In regard to the hole UV, of the great speculum--its diameter should be equal, or nearly so, to that of the small speculum L, fig. 67. For if it be less, no more parallel rays will be reflected than if it were equal to _g h_, and it may do harm in contracting the visible area within too narrow limits. Nor must it be larger than the mirror L, because some parallel rays will then be lost, and those of most consequence as being nearest the centre. 2. The small hole at _e_ to which the eye is applied, must be nicely adjusted to the size of the cone of rays proceeding from the nearest lens S. If it be larger, it will permit the foreign light of the sky or other objects to enter the eye, so as to prevent distinct vision; for the eye should receive no light, but what comes from the surface of the small mirror L. If the hole be smaller than the cylinder of rays at _e_ then some of the necessary light will be excluded, and the object rendered more obscure. The diameter of this hole may be found by dividing the aperture of the telescope in inches by its magnifying power. Thus, if we divide the diameter of one of Short’s telescopes, the diameter of whose large speculum is 2.30, by 60, the magnifying power, the quotient will be .0383, which is nearly the 1/25 of an inch. Sometimes this hole is made so small as the 1/50 of an inch. When this hole is, by any derangement, shifted from its proper position, it sometimes requires great nicety to adjust it, and, before it is accurately adjusted, the telescope is unfit for accurate observation. 3. It is usual to fix a plate with a hole in it, at _a b_, the focus of the eye glass S, of such a diameter as will circumscribe the image, so as to exhibit only that part of it which appears distinct, and to exclude the superfluous rays. 4. There is an adjusting screw on the outside of the great tube, connected with the small speculum, by which that speculum may be pushed backwards or forwards to adjust the instrument to distinct vision. The hand is applied for this purpose at T. _Newtonian Telescopes._--These telescopes are now more frequently used for celestial observations than during the last century, when Gregorian reflectors were generally preferred. Sir W. Herschel was chiefly instrumental in introducing this form of the reflecting telescope to the more particular attention of astronomers, by the splendour and extent of the discoveries which it enabled him to make. In this telescope there is no hole required in the middle of the great speculum, as in the Gregorian construction, which circumstance secures the use of all the rays which flow from the central parts of the mirror.