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A new type of stereoscope has recently been described (Asher and Law, 1952) in which the lateral separation of the cards and their distance from the lenses may both be varied separately, thus exercising accommodation and convergence independently of each other. It was expected that when the images of the cards were at infinity, the normal orthophoric observer would find fusion easiest with the visual axes parallel. However, it was at once apparent that this was not so, since the majority of observers found it easier to combine the pictures with the visual axes converged by as much as 100 or even more, and quite difficult to combine them with the visual axes parallel. Even patients with a pronounced Lamp exophoria would set the instrument so as to combine the images in an over-converged position.Enquiry showed that this effect is familiar to many who use instruments of this kind. Hofstetter Lens (1942), Morgan (1944), and Ittelson and Ames Fio. LY1-ergence and dspar-The importance of the proximal factor may be ity clues from the real img of the lamp give information shown by the following very simple experiment that the lamip is on the near (Fig. 1). side; nevertheless it appears at first to be on the far, side. A small lamp is placed at about 20 cm. from a + lOD of the lamp does not obstrucrt spherical lens, so that a real inverted image of the lamp the view of anything behind it. is formed at a distance of 20 cm. from the lens and on the *

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