_Rules for taking Binocular Photographs._--In viewing very distant objects with the eyes, the images formed on the retinæ are not sufficiently dissimilar to produce a very Stereoscopic effect; hence it is often required, in taking binocular pictures, to separate the Cameras more widely than the two eyes are separated, in order to give a sufficient appearance of relief. Mr. Wheatstone's original directions were, to allow about one foot of separation for each twenty-five feet of distance, but considerable latitude may be permitted. If the Cameras be not placed far enough apart, the dimensions of the stereoscopic image from before backwards will be too small,--statues looking like bas-reliefs, and the circular trunks of trees appearing oval, with the long diameter transverse. On the other hand, when the separation is too wide, the reverse obtains,--objects for instance which are square, assuming an oblong shape pointing towards the observer. To understand the cause of this, the following law in optics should be studied:--"The distance of objects is estimated by the extent to which the axes of the eyes must be converged to view them." If we have to turn our eyes strongly inwards, we judge the object to be near; but if the eyes remain nearly parallel, we suppose it to be distant. [Illustration] The above figures represent six-sided truncated pyramids, each with its apex towards the observer, the centres of the two smaller interior hexagons being more widely separated than those of the larger exterior ones. By converging the eyes upon them so as to unite the central images in the manner represented in page 68 a greater amount of convergence will be required to bring together the two summits than the bases, and hence the summits will appear the nearest to the eye; that is to say, the resultant central figure will acquire the additional dimension of _height_, and appear as a solid cone, standing perpendicularly upon its base: further, the more widely the summits are separated in relation to the bases, the taller will the cone be, although a greater effort will be required to coalesce the figures. Binocular Photographs taken with too much separation of the Cameras, are distorted from a similar cause,--so strong a convergence being required to unite them that certain parts of the picture appear to approach near to the eye; and the depth of the solid image is increased. This effect is most observable when the picture embraces a variety of objects, situated in different planes. In the case of views which are quite distant, no near objects being admitted, the Cameras may be placed with especial reference to them, even as far as twelve feet apart, without producing distortion.