1. Exner believed that the movement of the distal pigment during light-adaptation improved the visual acuity of the superposition eye. This hypothesis was tested by measurement of the visual acuity of Leander serratus, Pandalus montagui and Praunus flexuosus under different conditions.

2. When a dark-adapted animal is placed in the light the proximal pigment migrates into the light-adapted position more rapidly than the distal pigment. The distal pigment, but not the proximal pigment, undergoes diurnal rhythm and tends to migrate into the dark-adapted position at night, even when the animal is illuminated. The visual acuity may therefore be tested when the distal pigment is still in the dark-adapted position and the proximal pigment in the light-adapted position.

3. No difference in visual acuity could be detected as a result of changes in the position of the distal pigment.

4. Visual acuity increases and light sensitivity decreases when the dark proximal pigment migrates over the reflecting (tapetal) layer.

5. Eupagurus bernhardus, which lacks the typical tapetum, shows no detectable change in visual acuity or sensitivity after being kept in the dark.

6. These experiments do not support Exner's view of the function of the distal pigment. They indicate that visual acuity is improved by the presence of the dark proximal pigment at the base of the proximal retinulae, probably because this pigment reduces stray reflexions from the back of the eye (halation).

7. It is suggested that since the crystalline cones and crystalline tracts form optically continuous strands with a higher refractive index than that of the surrounding medium they may act as wave guides. If so they would retain light entering the corneal surface from sources close to the axis of the ommatidium, and so concentrate it on the rhabdomes and adjacent retinulae.

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