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 diminishing 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 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

630 10^{-} volt

5

31

29

30

461 "

3

185

175

18

277 "

1

10

9

95

146 "

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

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 95 to 18, to 30, to 41. If the deflections had been strictly proportionate to the intensities of light stimulus they would have been as 95 to 285, to 475, to 665.

[Ill.u.s.tration: 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 stimulus.

#After-oscillation.#--When the sensitive surface is subjected to the continued action of light, the E.M. effect attains a maximum at which it remains constant for some time. If the exposure be maintained after this for a longer period, there will be a decline, as we found to be the case in other instances of continued stimulation. The appearance of this decline, and its rapidity, depends on the particular condition of the substance.

When the sensitive element is considerably strained by the action of light, and if that light be now cut off, there is a rebound towards recovery and a subsequent after-oscillation. That is to say, the curve of recovery falls below the zero point, and then slowly oscillates back to the position of equilibrium. We have already seen an instance of this in fig. 102. Above is given a series of records showing the appearance of decline, from too long-continued exposure and recovery, followed by after-oscillation on the cessation of light (fig. 105). Certain visual a.n.a.logues to this phenomenon will be noticed later.

[Ill.u.s.tration: FIG. 105.--AFTER-OSCILLATION Exposure of one minute followed by obscurity of one minute. Note the decline during illumination, and after-oscillation in darkness.]

#Abnormal effects.#--We have already treated of all the normal effects of the stimulus of light on the retina, and their counterparts in the sensitive cell. But the retina undergoes molecular changes when injured, stale, or in a dying condition, and under these circ.u.mstances various complicated modifications are observed in the response.

[Ill.u.s.tration: FIG. 106.--TRANSIENT POSITIVE AUGMENTATION GIVEN BY THE FROG'S RETINA ON THE CESSATION OF LIGHT L (WALLER)]

#1. Preliminary negative twitch.#--When the light is incident on the frog's retina, there is sometimes a transitory negative variation, followed by the normal positive response. This is frequently observed in the sensitive cell (see fig. 96, _b_).

#2. Reversal of response.#--Again, in a stale retina, owing to molecular modification the response is apt to undergo reversal (Waller). That is to say, it now becomes negative. In working with the same sensitive cell on different days I have found it occasionally exhibiting this reversed response.

#3. Transient rise of current on cessation of light.#--Another very curious fact observed in the retina by Kuhne and Steiner is that immediately on the stoppage of light there is sometimes a sudden increase in the retinal current, before the usual recovery takes place.

This is very well shown in the series of records taken by Waller (fig. 106). It will be noticed that on illumination the response-curve rises, that continued illumination produces a decline, and that on the cessation of light there is a transient rise of current. I give here a series of records which will show the remarkable similarity between the responses of the cell and retina, in respect even of abnormalities so marked as those described (fig. 107). I may mention here that some of these curious effects, that is to say, the preliminary negative twitch and sudden augmentation of the current on the cessation of light, have also been noticed by Minchin in photo-electric cells.

[Ill.u.s.tration: FIG. 107.--RESPONSES IN SILVER CELL The thick line represents response during light (half a minute's exposure), and dotted line the recovery during darkness. Note the terminal positive twitch.]

#4. Decline and reversal.#--We have seen that under the continuous action of light, response begins to decline. Sometimes this process is very rapid, and in any case, under continued light, the deflection falls.

(1) The decline may nearly reach zero. If now the light be cut off there is a rebound towards recovery _downwards_, which carries it below zero, followed by an after-oscillation (fig. 108, _a_).

[Ill.u.s.tration: FIG. 108--DECLINE UNDER THE CONTINUED ACTION OF LIGHT (_a_) Decline short of zero; on stoppage of light, rebound downwards to zero; after-oscillation.

(_b_) Decline below zero; on stoppage of light, rebound towards zero, with preliminary negative twitch.

(_c_) The same, decline further down; negative twitch almost disappearing.]

(2) If the light be continued for a longer time, the decline goes on even below zero; that is to say, the response now becomes apparently negative. If, now, the light be stopped, there is a rebound upwards to recovery, with, generally speaking, a slight preliminary twitch downwards (fig. 108, _b_, _c_). This rebound carries it back, not only to the zero position, but sometimes beyond that position. We have here a parallel to the following observation of Dewar and McKendrick: 'When diffuse light is allowed to impinge on the eye of the frog, after it has arrived at a tolerably stable condition, the natural E.M.F. is in the first place increased, then diminished; during the continuance of light it is still slowly diminished to a point where it remains tolerably constant, and on the removal of light there is a sudden increase of the E.M. power nearly up to its original position.'[18]

(3) I have sometimes obtained the following curious result. On the incidence of light there is a response, say, upward. On the continuation of light the response declines to zero and remains at the zero position, there being no further action during the continuation of stimulus. But on the cessation or 'break' of light stimulus, there is a response downwards, followed by the usual recovery. This reminds us of a somewhat similar responsive action produced by constant electric current on the muscle. At the moment of 'make' there is a responsive twitch, but afterwards the muscle remains quiescent during the pa.s.sage of the current, but on breaking the current there is seen a second responsive twitch.

#Resume.#--So we see that the response of the sensitive inorganic cell, to the stimulus of light, is in every way similar to that of the retina. In both we have, under normal conditions, a positive variation; in both the intensity of response up to a certain limit increases with the duration of illumination; it is affected, in both alike, by temperature; in both there is comparatively little fatigue; the increase of response with intensity of stimulus is similar in both; and finally, even in abnormalities--such as reversal of response, preliminary negative twitch on commencement, and terminal positive twitch on cessation of illumination, and decline and reversal under continued action of light--parallel effects are noticed.

[Ill.u.s.tration: FIG. 109.--CERTAIN AFTER-EFFECTS OF LIGHT]

We may notice here certain curious relations even in these abnormal responses (fig. 109). If the equilibrium position remain always constant, then it is easy to understand how, when the rising curve has attained its maximum, on the cessation of light, recovery should proceed _downwards_, towards the equilibrium position (fig. 109, _a_). One can also understand how, after reversal by the continued action of light, there should be a recovery _upwards_ towards the old equilibrium position (fig. 109, _b_). What is curious is that in certain cases we get, on the stoppage of light, a preliminary twitch away from the zero or equilibrium position, upwards as in (_c_) (compare also fig. 107) and downwards as in (_d_) (compare also fig. 108 _b_).

In making a general retrospect, finally, of the effects produced by stimulus of light, we find that there is not a single phenomenon in the responses, normal or abnormal, exhibited by the retina which has not its counterpart in the sensitive cell constructed of inorganic material.

FOOTNOTES:

[18] _Proc. Roy. Soc. Edin._, 1873 p. 153.

CHAPTER XIX

VISUAL a.n.a.lOGUES

Effect of light of short duration--After-oscillation--Positive and negative after-images--Binocular alternation of vision--Period of alternation modified by physical condition--After-images and their revival--Unconscious visual impression.