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--Résumé.
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.
[Illustration: 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.
[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 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 × 10^{-} volt |
| 5 | 31 | 29 | 30 | 46·1 × " |
| 3 | 18·5 | 17·5 | 18 | 27·7 × " |
| 1 | 10 | 9 | 9·5 | 14·6 × " |
+-----------+--------------+-------------+--------+--------------------+
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
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
analogues to this phenomenon will be noticed later.
[Illustration: 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 circumstances various
complicated modifications are observed in the response.
[Illustration: 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.
[Illustration: 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_).
[Illustration: 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 passage of the
current, but on breaking the current there is seen a second responsive
twitch.
#Résumé.#--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.
[Illustration: 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.