In fig. 77, AB represents the plano-convex lens next the object, which is about 2 inches in focal length, and 3/4 inch in diameter; CD, a plain metallic speculum of an oval form, well polished, and placed at half a right angle to the axis of the tube; and EF another plano-convex lens, about 1-1/2 inch focal distance. The centre of the speculum is about 1-1/4 inch from the lens AB, and about 1/2 or 1/3 inch from EF; so that this eye-piece is a _positive_ one, on the principle proposed by Ramsden. The rays proceeding from the lens AB, and falling upon the speculum, are reflected in a perpendicular direction to the lens EF, where they enter the eye at G, which looks down upon the object through the side of the tube. The real size of this eye-piece is much about the same as that represented in the figure. When applied to an achromatic telescope of 44-1/2 inches focal distance it produces a magnifying power of 36 times, and exhibits a very beautiful view of the whole of the full moon. It likewise presents a very pleasing prospect of terrestrial objects, which appear as if situated immediately below us. [Illustration: _figure 78._] Another plan of the diagonal eye-piece is represented in fig. 78, where the speculum is fixed _within_ the sliding tube which receives the eye-piece, or immediately below it. The part of the tube at AB slides into the tube of the telescope, CD is the speculum placed at half a right angle to the axis of the tube, and EF, the tube containing the lenses, which stands at right angles to the position of the telescope, and slides into an exterior tube, and the eye is applied at G. This construction of the diagonal eye-piece may be used with any eye-piece whatever, whether the Huygenian or that of Ramsden. It will admit of any magnifying power, and if several different eye-pieces be fitted to the sliding tube, they may be changed at pleasure. This form of the diagonal eye-piece, I therefore consider as the best and the most convenient construction, although it is not commonly adopted by opticians.