As every photographer possesses a camera with a lens between 2½ and 3 inches in diameter, it may be useful to him to know how he may convert it into a binocular instrument. In a cover for the lens take two points equidistant from each other, and make two apertures, _c_, _d_, Fig. 43, ²/₁₀ths of an inch in diameter, or of any larger size that may be thought proper, though ²/₁₀ is the proper size. Place the cover on the end of the tube, and bring the line joining the apertures into a horizontal position. Closing one aperture, take the picture of the sitter, or of the statue, through the other, and when the picture is shifted aside by the usual contrivances for this purpose, take the picture through the other aperture. These will be good binocular portraits, fitted for any stereoscope, but particularly for the Achromatic Reading Glass Stereoscope. If greater relief is wanted, it may be obtained in larger lenses by placing the two apertures at the greatest distance which the diameter of the lens will permit. [55] It is only in a horizontal direction that we can see 180° of the hemisphere. We would require a circle of eyes 2½ inches distant to see a complete hemisphere. _The Binocular Camera made the Stereoscope._ If the lenses of the binocular camera, when they are whole lenses, be made to separate a little, so that the distance between the centres of their inner halves may be equal to 2½ inches, they become a lenticular stereoscope, in which we may view the pictures which they themselves create. The binocular pictures are placed in the camera in the very place where their negatives were formed, and the observer, looking through the halves of his camera lenses, will see the pictures united and in relief. If the binocular camera is made of semi-lenses, we have only to place them with their thin edges facing each other to obtain the same result. It will appear, from the discussions in the following chapter, that such a stereoscope, independently of its being achromatic, if the camera is achromatic, will be the most perfect of stereoscopic instruments. The preceding methods are equally applicable to landscapes, machines, and instruments, and to solid constructions of every kind, whether they be the production of nature or of art.[56] [56] See Chapters X. and XI. CHAPTER IX. ON THE ADAPTATION OF THE PICTURES TO THE STEREOSCOPE. —THEIR SIZE, POSITION, AND ILLUMINATION. Having described the various forms of the stereoscope, and the method of taking the binocular portraits and pictures to which it is to be applied, we have now to consider the relation that ought to exist between the instrument and the pictures,—a subject which has not been noticed by preceding writers.