Suppose the focal distance of the great mirror was 9 inches, and the focal distance of the small mirror 1-1/2 inch--were we to remove the eye piece of this telescope, and look through the hole of the great mirror, we should see the image of the object depicted upon the face of the small speculum, and magnified, in the proportion of 9 to 1-1/2, or, 6 times, on the same principle as a common convex object glass 9 inches focal length, with an eye glass whose focus is 1-1/2 inch magnifies 6 times. This may be regarded as the first part of the magnifying power. If now, we suppose the small speculum placed a little more than 1-1/2 inch from the image formed by the great speculum, a second image is formed about _f_, as much exceeding the first in its dimensions as it exceeds it in distance from the small speculum, on the principle on which the object glass of a compound microscope forms a large image near the eye glass. Suppose this distance to be 9 times greater, then the whole magnifying power will be compounded of 6 multiplied by 9, or 54 times. As a telescope it magnifies 6 times, and in the microscope part 9 times.--Such is a _general_ idea of the Gregorian telescope, the minute particulars and structure of which can only be clearly perceived by a direct inspection of the instrument. _The Newtonian Reflector._--This instrument is somewhat different both in its form and in its mode of operation from that of Gregory. It is represented in fig. 63, where BAEF is the tube, and BE, the object concave mirror, which reflects the parallel rays _ab_ to a _plane_ speculum G, placed 45°, or half a right angle to the axis of the concave speculum. This small plane reflector must be of an oval form, the length of the oval should be to the breadth as 7 to 5, on account of the obliquity of its position. It is supported on an arm fixed to the side of the tube; an eye-glass is placed in a small tube, moveable in the larger tube, so as to be perpendicular to the axis of the large reflector, the perpendicular line passing through the centre of the small mirror. The small mirror is situated between the large mirror and its focus, that its distance from this focal point may be equal to the distance from the centre of the mirror to the focus of the eye-glass. When the rays _ab_ from a distant object fall upon the large speculum at _cd_, they are reflected towards a focus at _h_; but being intercepted by the plane mirror G, they are reflected perpendicularly to the eye-glass at I, in the side of the tube, and the image formed near that position at _e_ is viewed through a small plano-convex lens. The magnifying power of this telescope is in the proportion of the focal distance of the speculum to that of the eye-glass. Thus, if the focal distance of the speculum be 36 inches, and that of the eye-glass 1/3 of an inch, the magnifying power will be 108 times. It was this form of the reflecting telescope, that Newton invented, which Sir. W.