If D = 9·24 inches, C = 2·50, then _d_ = 2·14, _d′_ = 2·42, and MN = 0·283, the height of the cone. DC When C = _d_, MP = ———— 2C. As the summit plane _op_ rises above the base _mn_ by the successive convergency of the optic axes to different points in the line _o_N_p_, it may be asked how it happens that the conical frustum still appears a solid, and the plane _op_ where it is, when the optic axes are converged to points in the line _m_M_n_, so as to see the base distinctly? The reason of this is that the rays emanate from _op_ exactly in the same manner, and form exactly the same image of it, on the two retinas as if it were the summit CD, Fig. 19, of the real solid when seen with both eyes. The only effect of the advance of the point of convergence from N to M is to throw the image of N a little to the _right_ side of the optic axis of the left eye, and a little to the _left_ of the optic axis of the right eye. The summit plane _op_ will therefore retain its place, and will be seen slightly doubled and indistinct till the point of convergence again returns to it. It has been already stated that the two dissimilar pictures may be united by converging the optical axes to a point beyond them. In order to do this, the distance SS′ of the pictures, Fig. 21, must be greatly less than the distance of the eyes L, R, in order that the optic axes, in passing through similar points of the two plane pictures, may meet at a moderate distance beyond them. In order to explain how the relief is produced in this case, let AB, CD, _ab_, _cd_, Fig. 22, be the dissimilar pictures of the frustum of a cone whose summit is CD, as seen by the right eye, and _cd_ as seen by the left eye. From L and R, as before, draw lines through all the leading points of the pictures, and we shall have the points A, _a_ united at _m_, the points B, _b_ at _n_, the points C, _c_ at _o_, and the points D, _d_ at _p_, the points S, _s_ at M, and the points S′, _s′_ at N, forming the cone _mnop_, with its base _mn_ towards the observer, and its summit _op_ more remote. If the cone had been formed of lines drawn from the outline of the summit to the outline of the base, it would now appear _hollow_, the inside of it being seen in place of the outside as before. If the pictures AB, _ab_ are made to change places the combined picture would be in relief, while in the case shewn in Fig. 21 it would have been hollow. Hence the _right_-eye view of any solid must be placed on the _left_ hand, and the _left_-eye view of it on the _right_ hand, when we wish to obtain it in relief by converging the optic axes to a point between the pictures and the eye, and _vice versa_ when we wish to obtain it in relief by converging the optic axes to a point beyond the pictures. In every case when we wish the combined pictures to represent