A highly efficient hand telescope for astronomical purposes might be constructed along this line, the great shortening of the instrument making it possible to use somewhat higher powers than the ordinary without too much loss of steadiness. There is also constructed a binocular for strictly astronomical use consisting of a pair of small hand comet-seekers. One of these little instruments is shown in Fig. 112. It has a clear diameter of objectives of 1⅜ inch, magnification of 5, and a brilliant and even field of 7½° aperture. The objectives are triplets like Fig. 57, already referred to, the oculars achromatic doublets of the type of Fig. 104_a_. With the exception of these specialized astronomical field glasses the most useful and generally available hand instrument is the prism glass now in very general use. It is based on reversal of the image by internal total reflection in two prisms having their reflecting surfaces perpendicular each to the other. The rudiments of the process lie in the simple reversion prism shown in diagram in Fig. 113. [Illustration: FIG. 112.—Astronomical Binocular.] This is nothing more nor less than a right angled glass prism set with its hypothenuse face parallel and with its sides at 45° to the optical axis of the instrument. Rays falling upon one of its refracting faces at an angle of 45° are refracted upon the hypothenuse face, are there totally reflected and emerge from the second face of the prism parallel to their original course. Inspection of Fig. 113 shows that an element like A B perpendicular to the plane of the hypothenuse face is inverted by the total reflection so that it takes the position A′ B′. It is equally clear that an element exactly perpendicular to A B will be reflected from the hypothenuse face flatwise as it were, and will emerge without its ends being reversed so that the net effect of this single reflection is to invert the image without reversing it laterally at the same time. On the other hand if a second prism be placed behind the first, flat upon its side, with its hypothenuse face occupying a plane exactly perpendicular to that of the first prism, the line A′B′ will be refracted, totally reflected and refracted again out of the prism without a second inversion, while a line perpendicular to A′B′ will be refracted endwise on the hypothenuse face of the second prism and will be inverted as was the line A B at the start. [Illustration: FIG. 113.—Reversion Prism.]