Variations of the latent period of vision

The intensity variation of the latent period of vision was measured in two observers with a binocular method which involved the use of an episcotister. The test stimulus of 3 ms duration subtended at the eye an angle of only $2.7^{\prime}$. This made it possible to separate the foveal from the extra-foveal mechanism. In the fovea, the latent period decreases with a rise in intensity near the threshold, reaches a minimum, and then slightly rises again. In the extra-fovea the decrease in latent period is very much more marked and uniform. The two latent period functions converge towards a common value at about 6 logarithmic threshold units. A prediction made on the basis of these results as regards the mechanism underlying the Pulfrich effect was confirmed.