Part 13 (2/2)
3
185
175
18
277 ”
1
10
9
95
146 ”
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As the zero point was slightly s.h.i.+fted 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.
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