thetic grouping derived from vitamin A or retinene would yield no materials not known to be present in the eye. CONCLUSIONS The modulator analogues which have been obtained fulfil nearly all the requirements of the colour receptors whose presence is implied by Granit's work, e.g. during fading processes, the maxima fall at different rates, suggesting the presence, not of one product with several bands, but of several products with one maximum to each. The conditions under which these modulator analogues have been produced are obviously unphysiological. If the problem posed by Granit's modulators is put in the form ofa query whether or not the vitamin A-retineneoxidation-reductionsystemcouldpossibly account for the phenomena observed, the answer is clearly in the affirmative. Which catalysts, old or new, must be invoked before the modulator analogues can be obtained under less obviously unphysiological conditions is a matter for further research. SUMMARY 1. The absorption spectra of vitamin A and of retinene, in concentrated sulphuric, phosphoric and hydrochloric acids have been studied. Sharp absorption bands characteristic of unstable ionized molecules were obtained. 2. The results show that vitamin A and retinene1 can give rise in vitro to materials simulating the photopic modulators of Granit. 3. The hypothesis that the system vitamin Aretinene, is important in photopic as well as scotopic vision is strengthened.

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