The lenticular form of the stereoscope is admirably fitted for its application to small and microscopic objects. The first instruments of this kind were constructed by myself with quarter-inch lenses, and were 3 inches long and only 1 and 1½ deep.[44] They may be carried in the pocket, and exhibit all the properties of the instrument to the greatest advantage. The mode of constructing and using the instrument is precisely the same as in the common stereoscope; but in taking the dissimilar pictures, we must use either a small binocular camera, which will give considerably magnified representations of the objects, or we must procure them from the compound microscope. The pictures may be obtained with a small single camera, by first taking one picture, and then shifting the object in the focus of the lens, through a space corresponding with the binocular angle. To find this space, which we may call _x_, make _d_ the distance of the object from the lens, _n_ the number of times it is to be magnified, or the distance of the image behind the lens, and D the distance of the eyes; then we shall have D _nd_ : _d_ = D : _x_, and _x_ = ———. _n_ that is, the space is equal to the distance between the eyes divided by the magnifying power. [44] _Phil. Mag._, Jan. 1852, vol. iii. p. 19. With the binocular microscope of Professor Riddell,[45] and the same instrument as improved by M. Nachet, binocular pictures are obtained directly by having them drawn, as Professor Riddell suggests, by the camera lucida, but it would be preferable to take them photographically. Portraits for lockets or rings might be put into a very small stereoscope, by folding the one lens back upon the other. [45] _American Journal of Science_, 1852, vol. xv. p. 68. CHAPTER VIII. METHOD OF TAKING PICTURES FOR THE STEREOSCOPE. However perfect be the stereoscope which we employ, the effect which it produces depends upon the accuracy with which the binocular pictures are prepared. The pictures required for the stereoscope may be arranged in four classes:— 1. The representations of geometrical solids as seen with two eyes. 2. Portraits, or groups of portraits, taken from living persons or animals. 3. Landscapes, buildings, and machines or instruments. 4. Solids of all kinds, the productions of nature or of art. */ _Geometrical Solids._