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, half-way 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.