eyes, or from some peculiarity in the power of converging the optical axes, which it is not easy to explain.

If the distance between the pupils of the two eyes, E, E', Fig. 52, which has been already explained on Fig. 18, is less than the distance between the semi-lenses L, L', then, instead of looking through the middle portions no, n'o', of the lenses, the observer will look through portions between o and L, and o' and L', which have a greater power of refracting or displacing the pictures than the portions no, n'o', and therefore the pictures will be too much displaced, and will have so far overpassed one another that the observer is not able to bring them back to their place of union, halfway between the two pictures in the slide.

If, on the other hand, the distance between the pupils of the observer's eyes is greater than the distance between the semi-lenses L, L', then, instead of looking through the portions no, n'o' of the lenses, the observer will look through portions between n and L, and n' and L', which have a less power of refracting or displacing the pictures than the portions no, n'o', and therefore the pictures will be so little displaced as not to reach their place of union, and will stand at such a distance that the observer is not able to bring them up to their proper place, half-way between the two pictures in the slide.

Now, in both these cases of over and under displacement, many persons have such a power over their optical axes, that by converging them to a point nearer than the picture, they would, in the first case, bring them back to their place of union, and by converging them to a point more remote than the picture, would, in the second case, bring them up to their place of union; but others are very defective in

this power of convergence, some having a facility of converging them beyond the pictures, and others between the pictures and the eye. This last, however, namely, that of near convergence, is by far the most common, especially among men; but it is of no avail, and the exercise of it is injurious when the under refracted pictures have not come up to their place of union. The power of remote convergence, which is very rare, and which would assist in bringing back the over refracted pictures to their place of union, is of no avail, and the exercise of it is injurious when the pictures have been too much displaced, and made to pass beyond their place of union.

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.

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