58 The Sensations of Vision [50. N. The reaction takes place differently in other animals. In reptiles and in diurnal birds (birds of prey, hens), on illumination, there is sometimes a latent period lasting from 1/40 to 1/15 second, succeeded by a strong negative variation of the current; or else what happens is, at first a short positive “discharge,” succeeded then by the negative variation lasting while the illumination continues. If the eye is not illuminated, the current may drop at once to the strength of the “current of rest,” or, before doing this, it will show another negative variation. H. Piper’s work1 has helped a great deal to clarify this subject. With respect to nocturnal birds of prey, he confirmed the observation of Himstedt and Nagel, namely, that the only result of illumination in this case is a strong positive variation followed by an equally strong negative variation when the light is shut off. In mammals also the reaction consists chiefly of a positive variation. Every injury of the eyeball changes its electromotive behaviour and tends to promote negative variations. For example, the retina taken from the eye of a frog responds to the light stimulus at first with negative variation succeeded afterwards by a positive one. The positive dark variation occurs also in injured preparations of this kind. In perfectly fresh eyes of various animals Garten found that a very brief negative variation preceded the positive variation of the current as a regular result of stimulation. The sensitivity of these photo-electric reactions is sometimes very considerable, particularly in such animals as have numerous rods in their retinas and much visual purple. For example, in the eye of a frog the threshold value of the energy that is just sufficient to produce variation of current is perhaps very nearly the same as that which will just elicit the sensation of light in the dark-adapted human eye. Under stimulation by X-rays the eyes of frogs and of several species of owls also give a distinct photo-electric reaction, but a hen’s eye does not. The light of a cigar, moonlight, phosphorescent paint, each produce distinct photo-electric variations. Ultra-violet light has the same effect, evidently due to production of fluorescence in the ocular media. By a careful study of the quantitative relation between the retinal current and the intensity of the stimulating light, de Haas2 showed that the reaction does not obey the Weber-Fechner law except for a certain range of rather strong stimuli. For weaker stimuli the relation between current and stimulus is more complicated. The surprisingly long duration of the current variation that is observed after a brief “instantaneous” stimulus of sufficient intensity is curious. The 1 Arch. f. (Anat. u.) Physiol. 1905. Supplement. 2 Lichtprikkels en retinastroomen in hun quantitatief verband. Inaug.-Diss. Leiden 1903.