When the stereoscope is perfectly adapted to the eyes of the observer, and the _general_ union of the pictures effected, the remote parts of the picture, that is, the objects seen in the distance, may be under refracted, while those in the foreground are over refracted, so that while eyes which have the power of convergence beyond the picture, unite the more distant objects which are under refracted, they experience much difficulty in uniting those in the foreground which are over refracted. In like manner, eyes which have the power of near convergence will readily unite objects in the foreground which are over refracted, while they experience much difficulty in uniting objects in the distance which are under refracted. If the requisite power over the optical axes is not acquired by experience and perseverance, when the stereoscope is suited to the eyes of the observer, the only suggestion which we can make is to open the eyes wide, and expand the eyebrows, which we do in staring at an object, or in looking at a distant one, when we wish to converge the axes, as in Fig. 22, to a point _beyond_ the pictures, and to contract the eyes and the eyebrows, which we do in too much light, in looking at a _near_ object, when we wish to converge the optic axes, as in Fig. 21, to a point _between_ the pictures and the eye. When the binocular pictures are taken at too great an angle, so as to produce a startling amount of relief, the distance between similar points in each picture, both in the distance and in the foreground, is much greater than it ought to be, and hence the difficulty of uniting the pictures is greatly increased, so that persons who would have experienced no difficulty in uniting them, had they been taken at the proper angle, will fail altogether in bringing them into stereoscopic relief. In these observations, it is understood that the observer obtains distinct vision of the pictures in the stereoscope, either by the adjustment of the moveable eye-tubes, if they are moveable, as they ought to be, or by the aid of convex or concave glasses for both eyes, either in the form of spectacles, or separate lenses placed immediately above, or immediately below the semi-lenses in the eye-tubes. If the eyes have different focal lengths, which is not unfrequently the case, lenses differing in convexity or concavity should be employed to equalize them. EDINBURGH: T. CONSTABLE, PRINTER TO HER MAJESTY.