Just as in the strain cells the galvanometer contact was transferred from the electrolytic part to the metallic part of the circuit, so we may next, in an exactly similar manner, cut this plate into two, and connect these directly to the galvanometer, electrolytic connection being made by partially plunging them into a cell containing water. The posterior surfaces of the two half-plates may be covered with a non-conducting coating. And we arrive at a typical photo-electric cell (fig. 98, _b_). These considerations will show that the eye is practically a photo-electric cell. [Illustration: FIG. 99.--RESPONSES TO LIGHT IN FROG'S RETINA Illumination L for one minute, recovery in dark for two minutes during obscurity D. (Waller.)] We shall now give detailed experimental results obtained with the sensitive silver-bromide cell, and compare its response-curve with those of the retina. A series of uniform light stimuli gives rise to uniform responses, which show very little sign of fatigue. How similar these response-curves are to those of the retina will be seen from a pair of records given below, where fig. 99 shows responses of frog's retina, and fig. 100 gives the responses obtained with the sensitive silver cell (fig. 100). It was said that the responses of the retina are uniform. This is only approximately true. In addition to numerous cases of uniform responses, Waller finds instances of 'staircase' increase, and its opposite, slight fatigue. In the record here given of the silver cell, the staircase effect is seen at the beginning, and followed by slight fatigue. I have other records where for a very long time the responses are perfectly uniform, there being no sign of fatigue. [Illustration: FIG. 100.--RESPONSES IN SENSITIVE SILVER CELL Illumination for one minute and obscurity for one minute. Thick line represents record during illumination, dotted line recovery during obscurity.] Another curious phenomenon sometimes observed in the response of retina is an occasional slight increase of response immediately on the cessation of light, after which there is the final recovery. An indication of this is seen in the second and fourth curves in fig. 99. Curiously enough, this abnormality is also occasionally met with in the responses of the silver cell, as seen in the first two curves of fig. 100. Other instances will be given later. CHAPTER XVIII INORGANIC RESPONSE--INFLUENCE OF VARIOUS CONDITIONS ON THE RESPONSE TO STIMULUS OF LIGHT Effect of temperature--Effect of increasing length of exposure--Relation between intensity of light and magnitude of response--After-oscillation--Abnormal effects: (1) preliminary negative twitch; (2) reversal of response; (3) transient positive twitch on cessation of light; (4) decline and reversal--Resume.