Printed in Great Britain OPTOKINETIC RESPONSES OF THE CRAB, CARCINUS TO A SINGLE MOVING LIGHT

In all measurements of the optokinetic response, with a wide significance in the analysis of the visual system, especially in arthropods, the stimulus of preference has been the striped drum. The reason is that the responses are then large and obvious and the striped drum is easy to set up experimentally, at least in its simplest form. However, striped drums have disadvantages: they cannot be switched instantaneously from one position to another; they move in only one plane; they are difficult to manufacture without subharmonics which arise from unequal illumination or faults in the stripes; they stimulate all exposed parts of the eye simultaneously and many eyes have different properties in different areas; they have upper and lower edges; centres which cannot be aligned with both eyes, and cannot easily be reproduced to exact specification. These and other difficulties are overcome if a small point source of light is used as a stimulus, but no one, apart from Rademaker & ter Braak (1948) on the rabbit, seems to have attempted to use this for optokinetic responses. To do so, the eye movements must be recorded in a light proof room when the only light is the minute stimulu