1. A method is described for measuring the principal characteristics of the light reaction of the ammocoete, and data are presented on the relation between the intensity and duration of the stimulus, dark adaptation, intensity discrimination and spectral sensitivity.

2. The intensity/time relation approximates to the Bunsen-Roscoe Law at low intensities of illumination. Use is made of this fact to define a criterion for a constant response to a given stimulus, for measuring the other physiological properties of the system.

3. Dark adaptation and intensity discrimination at different levels of adapting illumination both yield simple curves, which suggest that the system contains a single photosensitive pigment. The range of dark adaptation following a period of light adaptation is from 10 to 30 times. The threshold of excitation of a dark adapted ammocoete is from 0.25 to 0.95 millilambert.

4. The spectral sensitivity possesses a sharply peaked maximum about 530 mµ. Reasons are presented for suggesting porphyropsin (visual violet) as the photosensitive pigment concerned.

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