The same is true of the sensation of light. That is to say, within wide limits, intensity of sensation does not increase so rapidly as stimulus. This particular relation between stimulus and effect is also exhibited in a remarkable manner by the sensitive cell. For a constant source of light I used an incandescent burner, and graduated the intensity of the incident light by varying its distance from the sensitive cell. The intensity of light incident on the cell, when the incandescent burner is at a distance of 150 cm., has been taken as the arbitrary unit. In order to make allowance for the possible effects of fatigue I took two successive series of responses (fig. 103). In the first, records were taken with intensities diminishing from 7 to 1, and immediately afterwards increasing from 1 to 7, in the second. [Illustration: FIG. 103.--RESPONSES OF SENSITIVE CELL TO VARIOUS INTENSITIES OF LIGHT On the left the responses are for diminishing intensities in the ratios of 7, 5, 3, and 1. On the right they are for the increasing intensities 1, 3, 5, and 7. The thick lines are records during exposures of one minute; the dotted lines represent recoveries for one minute.] TABLE GIVING RESPONSE TO VARYING INTENSITIES OF LIGHT (The intensity of an incandescent gas-burner at a distance of 150 cm. is taken as unit.) +-----------+--------------+-------------+--------+--------------------+ | | Response | Response | | | | Intensity | (Light | (Light | Mean | Value in volt | | of Light | diminishing) | increasing) | | | +-----------+--------------+-------------+--------+--------------------+ | 7 | 43 | 39 | 41 | 63.0 x 10^{-} volt | | 5 | 31 | 29 | 30 | 46.1 x " | | 3 | 18.5 | 17.5 | 18 | 27.7 x " | | 1 | 10 | 9 | 9.5 | 14.6 x " | +-----------+--------------+-------------+--------+--------------------+ As the zero point was slightly shifted during the course of the experiment, the deflection in each curve was measured from a line joining the beginning of the response to the end of its recovery. A mean deflection, corresponding to each intensity, was obtained by taking the average of the descending and ascending readings. The two sets of readings did not, however, vary to any marked extent. The deflections corresponding to the intensities 1, 3, 5, 7, are, then, as 9.5 to 18, to 30, to 41. If the deflections had been strictly proportionate to the intensities of light stimulus they would have been as 9.5 to 28.5, to 47.5, to 66.5. [Illustration: FIG. 104.--CURVES GIVING THE RELATION BETWEEN INTENSITY OF LIGHT AND MAGNITUDE OF RESPONSE In (_a_) sensitive cell, (_b_) in frog's retina.]