If we now converge the optic axes to a point _b_, and wish to ascertain the value of C_c_, which will give different _depths_, _d_, of the _hollow_ solids corresponding to different values of C_b_, we shall have E A_b_:—— = _d_:C_c′_, 2 _d_E and C_c′_ = ————— AB, 2 which, making AC = 8 inches, as before, will give the following results:— Depth. C_b_ = _d_ A_b_ = AC + _d_ C_c′_ Inches. 1 9 0.139 2 10 0.25 3 11 0.34 4 12 0.4166 5 13 0.48 6 14 0.535 7 15 0.58 8 16 0.625 9 17 0.663 10 18 0.696 11 19 0.723 12 20 0.75 The values of _h_ and _d_ when C_c_, C_c′_ are known, will be found from the formulæ 2AB · C_c_ _h_ = ————————————, E 2AB · C_c′_ and _d_ = ————————————. E. As C_c_ is always equal to C_c′_ in each pair of figures or dissimilar pictures, the depth of the _hollow_ cone will always appear much greater than the height of the raised one. When C_c_ = C_c′_ = 0.75, _h_:_d_ = 3:12. When C_c_ = C_c′_ = 0.4166, _h_:_d_ = 2:4, and when C_c_ = C_c′_ = 0.139, _h_:_d_ = 0.8:1.0. When the solids of which we wish to have binocular pictures are symmetrical, the one picture is the reflected image of the other, or its reverse, so that when we have drawn the solid as seen by one eye, we may obtain the other by copying its reflected image, or by simply taking a copy of it as seen through the paper. When the geometrical solids are not symmetrical, their dissimilar pictures must be taken photographically from models, in the same manner as the dissimilar pictures of other solids. _Portraits of Living Persons or Animals._ Although it is possible for a clever artist to take two portraits, the one as seen by his right, and the other as seen by his left eye, yet, owing to the impossibility of fixing the sitter, it would be a very difficult task. A bust or statue would be more easily taken by fixing two apertures 2½ inches distant, as the two points of sight, but even in this case the result would be imperfect. The photographic camera is the only means by which living persons and statues can be represented by means of two plane pictures to be combined by the stereoscope; and but for the art of photography, this instrument would have had a very limited application.