Part 13 (1/2)

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.

We shall next proceed to study the effect, on the response of the sensitive cell, of all those conditions which influence the normal response of the retina. We shall then briefly inquire whether even the abnormalities sometimes met with in retinal responses have not their parallel in the responses given by the inorganic.

[Ill.u.s.tration: FIG. 101.--INFLUENCE OF TEMPERATURE ON RESPONSE Illumination 20”, obscurity 40”.

In (_a_) is shown a series of responses at 20 C.--the record exhibits slight fatigue. (_b_) is the slight irregular response at 50 C.

(_c_) is the record on re-cooling; it exhibits 'staircase'

increase.]

#Effect of temperature.#--It has been found that when the temperature is raised above a certain point, retinal response shows rapid diminution.

On cooling, however, response reappears, with its original intensity. In the response given by the sensitive cell, the same peculiarity is noticed. I give below (fig. 101, _a_) a set of response-curves for 20 C. These responses, after showing slight fatigue, became fairly constant. On raising the temperature to 50 C. response practically disappeared (101, _b_). But on cooling to the first temperature again, it reappeared, with its original if not slightly greater intensity (fig. 101, _c_). A curious point is that while in record (_a_), before warming, slight fatigue is observed, in (_c_), after cooling, the reverse, or staircase effect, appears.

[Ill.u.s.tration: 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) ill.u.s.trating 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 diminis.h.i.+ng from 7 to 1, and immediately afterwards increasing from 1 to 7, in the second.

[Ill.u.s.tration: FIG. 103.--RESPONSES OF SENSITIVE CELL TO VARIOUS INTENSITIES OF LIGHT On the left the responses are for diminis.h.i.+ng 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

diminis.h.i.+ng)

increasing)

+-----------+--------------+-------------+--------+--------------------+

7

43

39

41

630 10^{-} volt

5

31

29

30

461 ”