ABSTRACT
Many experiments have shown that when a frog is disturbed it escapes towards the light, and that blue light is more effective than any other colour in guiding this response (see Pearse, 1910, for a review). A previously reported experiment (Muntz, 1962 b) has shown that the greater effectiveness of blue light is not simply due to the animals’ possessing a greater sensitivity to light in this part of the spectrum, but involves colour vision; the animals will respond preferentially to blue irrespective of the intensity or saturation of the stimulating lights. They will, for example, choose a blue illuminated window in preference to a window illuminated by an additive mixture of the same blue light together with a green light, in spite of the fact that the latter window contains as much blue as the former, as well as the green.
Wald (1945-6) has shown that Rana caterbiana, the bull-frog, has visual pigments based on vitamin A, before metamorphosis, and only gets the typical frog pigments, based on vitamin A, when metamorphosis is complete. This would make the curve obtained in test I incompatible with cone function, for it does not agree with the maximum of photopic sensitivity expected for a system based on vitamin At, which would be at about 610 mp. However, Wald’s finding does not apply to all anurans, and Collins, Love & Morton (1953) have shown that in R. temporaria and R. eiculenta rhodopsin with λmax. 502 mp is the only visual pigment in both tadpoles and adults. In these species, therefore, the photopic sensitivity of the tadpole would be expected to be maximal at about 560 mμ.
