The little telescopes, MN, OP, may be made one and a half or even one inch long, and fitted up, either at a fixed or with a variable inclination, in a pyramidal box, like the lenticular stereoscope, and made equally portable. One of these instruments was made for me some years ago by Messrs. Horne and Thornthwaite, and I have described it in the _North British Review_[42] as having the properties of a _Binocular Cameoscope_, and of what has been absurdly called a _Pseudoscope_, seeing that every inverting eye-piece and every stereoscope is entitled to the very same name. The little telescope may be made of one piece of glass, _convex_ at each end, or _concave_ at the eye-end if a small field is not objectionable,—the length of the piece of glass, in the _first_ case, being equal to the _sum_, and, in the _second_ case, to the _difference_ of the focal lengths of the virtual lenses at each end.[43] [42] For 1852, vol. xvii. p. 200. [43] These solid telescopes may be made achromatic by cementing concave lenses of flint glass upon each end, or of crown glass if they are made of flint glass. 7. _The Eye-Glass Stereoscope._ As it is impossible to obtain, by the ocular stereoscope, pictures in relief from the beautiful binocular slides which are made in every part of the world for the lenticular stereoscope, it is very desirable to have a portable stereoscope which can be carried safely in our purse, for the purpose of examining stereoscopically all such binocular pictures. If placed together with their plane sides in contact, a plano-convex lens, AB, and a plano-concave one, CD, of the same glass and the same focal length, will resemble a thick watch-glass, and on looking through them, we shall see objects of their natural size and in their proper place; but if we slip the concave lens, CD, to a side, as shewn in Fig. 41, we merely displace the image of the object which we view, and the displacement increases till the centre of the concave lens comes to the margin of the convex one. We thus obtain a variable prism, by means of which we can, with the left eye, displace one of the binocular pictures, and lay it upon the other, as seen by the right eye. We may use semi-lenses or quarters of lenses, and we may make them achromatic or nearly so if we desire it. Double convex and double concave lenses may also be used, and the motion of the concave one regulated by a screw. In one which I constantly use, the concave lens slides in a groove over a convex quarter-lens. [Illustration: FIG. 41.] By employing two of these variable prisms, we have an _Universal Stereoscope_ for uniting pictures of various sizes and at various distances from each other, and the prisms may be placed in a pyramidal box, like the lenticular stereoscope. 8. _The Reading-Glass Stereoscope._