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.]

In another set of records, with a different cell, I obtained the
deflections of 6, 10, 13, 15, corresponding to light intensities of 3,
5, 7, and 9.

The two curves in fig. 104, giving the relation between response and
stimulus, show that in the case of inorganic substances, as in the
retina (Waller), magnitude of response does not increase so rapidly as