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.

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