warming, slight fatigue is observed, in (_c_), after cooling, the
reverse, or staircase effect, appears.

[Illustration: FIG. 102.--RESPONSE-CURVES FOR INCREASING DURATION OF
ILLUMINATION FROM 1" TO 10"
In (_a_) the source of light was at a distance of 50 cm.; in (_b_) it
was at a distance of 25 cm. Note the after-oscillation.]

#Effect of increasing length of exposure.#--If the intensity of light be
kept constant, the magnitude of response of the sensitive cell increases
with length of exposure. But this soon reaches a limit, after which
increase of duration does not increase magnitude of effect. Too long an
exposure may however, owing to fatigue, produce an actual decline.

I give here two sets of curves (fig. 102) illustrating the effect of
lengthening exposure. The intensities of light in the two cases are as 1
to 4. The incandescent burner was in the two cases at distances 50 and
25 cm. respectively. It will be observed that beyond eight seconds'
exposure the responses are approximately uniform. Another noticeable
fact is that with long exposure there is an after-oscillation. This
growing effect with lengthening exposure and attainment of limit is
exactly paralleled by responses of retina under similar conditions.

#Relation between intensity of light and magnitude of response.#--In the
responses of retina, it is found that increasing intensity of light
produces an increasing effect. But the rate of increase is not uniform:
increase of effect does not keep pace with increase of stimulus. Thus a
curve giving the relation between stimulus and response is concave to
the axis which represents the stimulus.

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