If in place of two right or two left eye pictures, as shewn in Figs. 29, 31, and 32, we use one right eye and one left eye picture, and combine the reflected image of the one with the reflected image of the other, we shall have a _raised_ cone with the stereoscope, shewn in Fig. 31, and a _hollow_ cone with the one in Fig. 32. The double reflecting stereoscope, in both its forms, is a general instrument for portraits and landscapes, and thus possesses properties peculiar to itself. The reflectors may be glass or metallic specula, or total reflexion prisms. 4. _The Total Reflexion-Stereoscope._ This form of the stereoscope is a very interesting one, and possesses valuable properties. It requires only a small prism and _one_ diagram, or picture of the solid, as seen by one eye; the other diagram, or picture which is to be combined with it, being created by total reflexion from the base of the prism. This instrument is shewn in Fig. 33, where D is the picture of a cone as seen by the left eye L, and ABC a prism, whose base BC is so large, that when the eye is placed close to it, it may see, by reflexion, the whole of the diagram D. The angles ABC, ACB must be equal, but may be of any magnitude. Great accuracy in the equality of the angles is not necessary; and a prism constructed, by a lapidary, out of a fragment of thick plate-glass, the face BC being one of the surfaces of the plate, will answer the purpose. When the prism is placed at _a_, Fig. 34, at one end of a conical tube LD, and the diagram D at the other end, in a cap, which can be turned round so as to have the line _mn_, Fig. 33, which passes through the centre of the base and summit of the cone parallel to the line joining the two eyes, the instrument is ready for use. The observer places his left eye at L, and views with it the picture D, as seen by total reflexion from the base BC of the prism, Figs. 33 and 35, while with his right eye R, Fig. 33, he views the real picture directly. The first of these pictures being the reverse of the second D, like all pictures formed by one reflexion, we thus combine two dissimilar pictures into a _raised_ cone, as in the figure, or into a _hollow_ one, if the picture at D is turned round 180°. If we place the images of two diagrams, one like one of those at A, Fig. 31, and the other like the one at B, vertically above one another, we shall then see, at the same time, the _raised_ and the _hollow_ cone, as produced in the lenticular stereoscope by the three diagrams, two like those in Fig. 31, and a third like the one at A. When the prism is good, the dissimilar image, produced by the two refractions at B and C, and the one reflexion at E, is, of course, more accurate than if it had been drawn by the most skilful artist; and therefore this form of the stereoscope has in this respect an advantage over every other in which two dissimilar figures, executed by art, are necessary. In consequence of the length of the