Modified major amblyoscope.

Angle between Fixation Axes (" Objective" Angle of Squint) The most commonly used methods of determining this angle depend on abolishing any ocular movement when fixation changes from one eye to the other. In using the major amblyoscope for this purpose, a fixation object is supplied alternately to each eye, the arms of the instrument being adjusted until no ocular movement takes place. There is thus complete dissociation between the eyes, and the conditions are far from being those obtaining in the normal use of the eyes; moreover, even though the images are optically at infinity, they often appear to be suspended in space at a short distance from the observer. In using Duane's parallax test for measuring the angle, alternate occlusion is again employed, with a prism in front of one or both eyes, so that again the conditions are unusual. It is not surprising that the angle measured by these methods often differs from that suggested by the patient's appearance, by the position of the corneal reflexes, and by -the behaviour on cover test. Methods depending on the corneal reflex may be carried out under more normal conditions of viewing, but are in general either inaccurate or clumsy, and require an independent measurement of the angle gamma. A method which removes some of these disadvantages has been described by Graham and Naylor (1957).