"There is yet another species of reflecting telescope to be noticed, the specula of which are made of unsilvered glass, employing only the portions of light reflected at their first surfaces. The object of this construction is to obviate the necessity of employing darkening glasses in viewing the sun with telescopes of great power, which break by the heat and endanger the eye, besides other inconveniences. It might be imagined that this end would be effected by simply contracting the aperture of any good telescope, and so shutting out all superfluous rays. But this does not succeed in practice. Perfect distinctness is not attainable in telescopes with very small apertures and high magnifying powers, when directed to the sun, moon, or planets. A certain ratio of the aperture to the focal length (not less than one-twelfth or one-fifteenth as the minimum) is requisite; so that, if we would obtain the full advantage of a large aperture and high power, some means of suppressing, or otherwise getting rid of, a large percentage of the sun's rays, becomes necessary. This is done by the following arrangement: The large speculum of a Newtonian telescope is a double concave crown or plate-glass lens, the radius of curvature of whose anterior or reflecting surface is twice the proposed focal length This surface must be worked to a true parabolic figure. The back requires only to be well polished, and worked (whether accurately or not is of no moment) to any moderately deep concavity. Neither is the quality of the glass, as to veins or streaks, of any consequence, provided it be colourless.