In order to remedy the evils arising from the weak reflecting power of speculum metal, and from its tarnishing by exposure to the air, Sir Isaac proposed to substitute for the small oval speculum a triangular prism of glass or crystal ABC. Its side AB _ba_ he supposes to perform the office of that metal, by reflecting towards the eye-glass the light which comes from the concave speculum DF, _fig. 13_, whose light he supposes to enter into this prism at its side CB _bc_, and lest any colours should be produced by the refraction of these planes, it is requisite that the angles of the prism at A_a_ and B_b_ be precisely equal. This may be done most conveniently, by making them half right angles, and consequently the third angle at C_c_ a right one. The plane AB _ba_ will reflect all the light incident upon it; but in order to exclude unnecessary light, it will be proper to cover it all over with some black substance excepting two circular spaces of the planes A_c_ and B_c_, through which the useful light may pass. The length of the prism should be such that its sides A_c_ and B_c_ may be square, and so much of the angles B and _b_ as are superfluous ought to be ground off, to give passage for as much light as is possible from the object to the speculum. [Illustration: _Fig. 16._] One great advantage of this prism, which cannot be obtained from the oval metal, is, that without using two glasses the object may be erected, and the magnifying power of the telescope varied at pleasure, by merely varying the distances of the speculum, the prism, and the eye-glass. This will be understood from _fig. 16_, where AI represents the great concave speculum, EF the eye-glass, and BCD the prism of glass, whose sides BC and CD are not flat, but spherically convex. The rays which come from G, the focus of the great speculum AI, will, by the refraction of the first side BC, be reduced to parallelism, and after reflection from the base CD, will be made by the refraction of the next side BD to converge to the focus H of the eye-glass EF. If we now bring the prism BCD nearer the image at G, the point H will recede from BD, and the image formed there will be greater than that at G, and if we remove the prism BCD from G, the point H will approach to BD, and the image at H will be less than that at G. The prism BCD performs the same part as a convex lens, G and H being its conjugate foci, and the relative size of the images formed at these points being proportional to their distance from the lens. This construction would be a good one for varying optically the angular distance of a pair of wires placed in the focus of the eye-glass EF; and by bisecting the lenticular prism BCD, and giving the halves a slight inclination, we should be able to separate and to close the two images or disks which would thus be produced, and thus form a double image micrometer.