1. Bees respond by a characteristic reflex to a movement in their visual field. By confining the field to a series of parallel stripes of different brightness it is possible to determine at any brightness of one of the two stripe systems the brightness of the second at which the bee will first respond to a displacement of the field. Thus intensity discrimination can be determined.

2. The discriminating power of the bee's eye varies with illumination in much the same way that it does for the human eye. The discrimination is poor at low illumination; as the intensity of illumination increases the discrimination increases and seems to reach a constant level at high illuminations.

3. The probable error of

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decreases with increasing I exactly in

the same way as does

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itself. The logarithm of the probable error

of ΔI is a rectilinear function of log I for all but the very lowest intensities. Such relationships show that the measurements exhibit an internal self-consistency which is beyond accident.

4. A comparison of the efficiency of the bee's eye with that of the human eye shows that the range over which the human eye can perceive and discriminate different brightnesses is very much greater than for the bee's eye. When the discrimination power of the human eye has reached almost a constant maximal level the bee's discrimination is still very poor, and at an illumination where as well the discrimination power of the human eye and the bee's eye are at their best, the intensity discrimination of the bee is twenty times worse than in the human eye.

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