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They are shorter, cæteris paribus, for the focal length of a concave speculum is only half its radius, whereas that of a glass lens with equal surfaces is the whole radius.
They give brighter images, for there is less light lost in their reflexions, than in the refraction through an object glass.
Notwithstanding this, they are not so much used, because they are less manageable from their weight, more expensive, more apt to get out of order, and more troublesome to use with any nicety, as the least shaking of the instrument or its stand causes great confusion in the image, which is not the case in refracting telescopes.
Sir Isaac Newton's Telescope. Fig. 191.
173. This differs from Dr. Herschel's only in having a plane mirror placed at an angle of 45° to the axis, which throws the image to the side of the instrument, where the eye glass is placed. Newton sometimes used a rectangular prism of glass for a plane reflector.
The magnifying power is of course the same as in Herschel's telescope, as is likewise the field of view, provided the plane reflector be large enough to convey all the rays to the eye glass.
The Gregorian Telescope. Fig. 192.
174. In this the image formed, as in the last two instruments, at the focus q of a concave speculum A, is reflected by a second small concave mirror B having its focus f a little beyond q, so that there is a second image at q′, which is erect, and is viewed through an eye-piece fixed in an aperture in the center of the principal speculum.
This appears at first sight to be a very disadvantageous construction, as the central rays are all stopped by the smaller mirror, and the best part of the great speculum is lost. It is, however, to be observed, that the small reflector is usually of very confined dimensions, and when the object speculum is well ground, it is found that the lateral rays converge quite sufficiently well to make a distinct image.
To find the magnifying power, we must compare the angle subtended by the second image at the eye glass with that of the
