(2) The effect produced by a chemical reagent depends to some extent on the previous condition of the wire.

(3) A certain time is required for the full development of the effect.

With some reagents the full effect takes place almost instantaneously, while with others the effect takes place slowly. Again the effect may with time reach a maximum, after which there may be a slight decline.

(4) The after-effects of the reagents may be transitory or persistent; that is to say, in some cases the removal of the reagent causes the responses to revert to the normal, while in others the effect persists even after the removal of all traces of the reagent.

#Opposite effects of large and small doses.#--There remains a very curious phenomenon, known not only to students of physiological response but also known in medical practice, namely that of the opposite effects produced by the same reagent when given in large or in small doses.

Here, too, we have the same phenomena reproduced in an extraordinary manner in inorganic response. The same reagent which becomes a 'poison'

in large quant.i.ties may act as a stimulant when applied in small doses.

This is seen in record fig. 94, in which (_a_) gives the normal responses in water; KHO solution was now added so as to make the strength three parts in 1,000, and (_b_) shows the consequent enhancement of response. A further quant.i.ty of KHO was added so as to increase the strength to three parts in 100. This caused a complete abolition (_c_) of response.

[Ill.u.s.tration: FIG. 94.--OPPOSITE EFFECTS OF SMALL AND LARGE DOSES (TIN) (_a_) is the normal response; (_b_) is the stimulating action of small dose of potash (3 parts in 1,000); (_c_) is the abolition of response with a stronger dose (3 parts in 100).]

It will thus be seen that as in the case of animal tissues and of plants, so also in metals, the electrical responses are exalted by the action of stimulants, lowered by depressants, and completely abolished by certain other reagents. The parallelism will thus be found complete in every detail between the phenomena of response in the organic and the inorganic.

CHAPTER XVII

ON THE STIMULUS OF LIGHT AND RETINAL CURRENTS

Visual impulse: (1) chemical theory; (2) electrical theory--Retinal currents--Normal response positive--Inorganic response under stimulus of light--Typical experiment on the electrical effect induced by light.

The effect of the stimulus of light on the retina is perceived in the brain as a visual sensation. The process by which the ether-wave disturbance causes this visual impulse is still very obscure. Two theories may be advanced in explanation.

#(1) Chemical theory.#--According to the first, or chemical, theory, it is supposed that certain visual substances in the retina are affected by light, and that vision originates from the metabolic changes produced in these visual substances. It is also supposed that the metabolic changes consist of two phases, the upward, constructive, or anabolic phase, and the downward, destructive, or katabolic phase. Various visual substances by their anabolic or katabolic changes are supposed to produce the variations of sensation of light and colour. This theory, as will be seen, is very complex, and there are certain obstacles in the way of its acceptance. It is, for instance, difficult to see how this very quick visual process could be due to a comparatively slow chemical action, consisting of the destructive breaking-down of the tissue, followed by its renovation. Some support was at first given to this chemical theory by the bleaching action of light on the visual purple present in the retina, but it has been found that the presence or absence of visual purple could not be essential to vision, and that its function, when present, is of only secondary importance. For it is well known that in the most sensitive portion of the human retina, the _fovea centralis_, the visual purple is wanting; it is also found to be completely absent from the retinae of many animals possessing keen sight.

#(2) Electrical theory.#--The second, or electrical, theory supposes that the visual impulse is the concomitant of an electrical impulse; that an electrical current is generated in the retina under the incidence of light, and that this is transmitted to the brain by the optic nerve.

There is much to be said in favour of this view, for it is an undoubted fact, that light gives rise to retinal currents, and that, conversely, an electrical current suitably applied causes the sensation of light.

#Retinal currents.#--Holmgren, Dewar, McKendrick, Kuhne, Steiner, and others have shown that illumination produces electric variation in a freshly excised eye. About this general fact of the electrical response there is a widespread agreement, but there is some difference of opinion as regards the sign of this response immediately on the application, cessation, and during the continuance of light. These slight discrepancies may be partly due to the unsatisfactory nomenclature--as regards use of terms _positive_ and _negative_--hitherto in vogue and partly also to the differing states of the excised eyes observed.

Waller, in his excellent and detailed work on the retinal currents of the frog, has shown how the sign of response is reversed in the moribund condition of the eye.

As to the confusion arising from our present terminology, we must remember that the term _positive_ or _negative_ is used with regard to a current of reference--the so-called current of injury.

[Ill.u.s.tration: FIG. 95. RETINAL RESPONSE TO LIGHT The current of response is from the nerve to the retina.]

When the two galvanometric contacts are made, one with the cut end of the nerve, and the other on the uninjured cornea, a current of injury is found which _in the eye_ is from the nerve to the retina. In the normal freshly excised eye, the current of response due to the action of light on the retina is always from the nerve, which is not directly stimulated by light, to the retina, that is, from the less excited to the more excited (fig. 95). This current of response flows, then, in the same direction as the existing current of reference--the current of injury--and may therefore be called _positive_. Unfortunately the current of injury is very often apt to change its sign; it then flows through the eye from the cornea to the nerve. And now, though the current of response due to light may remain unchanged in direction, still, owing to the reversal of the current of reference, it will appear as _negative_. That is to say, though its absolute direction is the same as before, its relative direction is altered.

I have already advocated the use of the term _positive_ for currents which flow towards the stimulated, and _negative_ for those whose flow is away from the stimulated. If such a convention be adopted, no confusion can arise, even when, as in the given cases, the currents of injury undergo a change of direction.

#Normal response positive.#--The normal effect of light on the retina, as noticed by all the observers already mentioned, is a positive variation, during exposure to light of not too long duration. Cessation of light is followed by recovery. On these points there is general agreement amongst investigators. Deviations are regarded as due to abnormal conditions of the eye, owing to rough usage, or to the rapid approach of death. For just as in the dying plant we found occasional reversals from negative to positive response, so in the dying retina the response may undergo changes from the normal positive to negative.

The sign of response, as we have already seen in numerous cases, depends very much on the molecular condition of the sensitive substance, and if this condition be in any way changed, it is not surprising that the character of the response should also undergo alteration.

Unlike muscle in this, successive retinal responses exhibit little change, for, generally speaking, fatigue is very slight, the retina recovering quickly even under strong light if the exposure be not too long. In exceptional cases, however, fatigue, or its converse, the staircase effect, may be observed.

#Inorganic response under the stimulus of light.#--It may now be asked whether such a complex vital phenomenon as retinal response could have its counterpart in non-living response. Taking a rod of silver, we may beat out one end into the form of a hollow cup, sensitising the inside by exposing it for a short time to vapour of bromine. The cup may now be filled with water, and connection made with a galvanometer by non-polarisable electrodes. There will now be a current due to difference between the inner surface and the rod. This may be balanced, however, by a compensating E.M.F.

[Ill.u.s.tration: FIG. 96.--RECORD OF RESPONSES TO LIGHT GIVEN BY THE SENSITIVE CELL Thick lines represent the effect during illumination, dotted lines the recovery in darkness. Note the preliminary negative twitch, which is sometimes also observed in responses of frog's retina.]

We have thus an arrangement somewhat resembling the eye, with a sensitive layer corresponding to the retina, and the less sensitive rod corresponding to the conducting nerve-stump (fig. 96, _a_).

The apparatus is next placed inside a black box, with an aperture at the top. By means of an inclined mirror, light may be thrown down upon the sensitive surface through the opening.

On exposing the sensitive surface to light, the balance is at once disturbed, and a responsive current of positive character produced. The current, that is to say, is from the less to the more stimulated sensitive layer. On the cessation of light, there is fairly quick recovery (fig. 96, _b_).

The character and the intensity of E.M. variation of the sensitive cell depend to some extent on the process of preparation. The particular cell with which most of the following experiments were carried out usually gave rise to a positive variation of about 008 volt when acted on for one minute by the light of an incandescent gas-burner which was placed at a distance of 50 cm.

#Typical experiment on the electrical effect induced by light.#--This subject of the production of an electrical current by the stimulus of light would appear at first sight very complex. But we shall be able to advance naturally to a clear understanding of its most complicated phenomena if we go through a preliminary consideration of an ideally simple case. We have seen, in our experiments on the mechanical stimulation of, for example, tin, that a difference of electric potential was induced between the more stimulated and less stimulated parts of the same rod, and that an action current could thus be obtained, on making suitable electrolytic connections. Whether the more excited was zincoid or cuproid depended on the substance and its molecular condition.

Let us now imagine the metal rod flattened into a plate, and one face stimulated by light, while the other is protected. Would there be a difference of potential induced between the two faces of this same sheet of metal?

Let two blocks of paraffin be taken and a large hole drilled through both. Next, place a sheet of metal between the blocks, and pour melted paraffin round the edge to seal up the junction, the two open ends being also closed by panes of gla.s.s. We shall have then two compartments separated by the sheet of metal, and these compartments may be filled with water through the small apertures at the top (fig. 97, _a_).

[Ill.u.s.tration: FIG. 97 (_a_) A, B are the two faces of a brominated sheet of silver. One face, say A, is acted on by light. The current of response is from B to A, across the plate.]

[Ill.u.s.tration: FIG. 97 (_b_).--RECORD OF RESPONSES OBTAINED FROM THE ABOVE CELL Ten seconds' exposure to light followed by fifty seconds' recovery in the dark. Thick lines represent action in light, dotted lines represent recovery.]

The two liquid ma.s.ses in the separated chambers thus make perfect electrolytic contacts with the two faces A and B of the sheet of metal.

These two faces may be put in connection with a galvanometer by means of two non-polarisable electrodes, whose ends dip into the two chambers. If the sheet of metal have been properly annealed, there will now be no difference of potential between the two faces, and no current in the galvanometer. If the two faces are not molecularly similar, however, there will be a current, and the electrical effects to be subsequently described will act additively, in an algebraical sense. Let one face now be exposed to the stimulus of light. A responsive current will be found to flow, from the less to the more stimulated face, in some cases, and in others in an opposite direction.

It appears at first very curious that this difference of electric potential should be maintained between opposite faces of a very thin and highly conducting sheet of metal, the intervening distance between the opposed surfaces being so extremely small, and the electrical resistance quite infinitesimal. A h.o.m.ogeneous sheet of metal has become by the unequal action of light, molecularly speaking, heterogeneous. The two opposed surfaces are thrown into opposite kinds of electric condition, the result of which is as if a certain thickness of the sheet, electrically speaking, were made zinc-like, and the rest copper-like.

From such unfamiliar conceptions, we shall now pa.s.s easily to others to which we are more accustomed. Instead of two opposed surfaces, we may obtain a similar response by unequally lighting different portions of the same surface. Taking a sheet of metal, we may expose one half, say A, to light, the other half, B, being screened. Electrolytic contacts are made by plunging the two limbs in two vessels which are in connection with the two non-polarisable electrodes E and E' (fig. 98, _a_). On illumination of A and B alternately, we shall now obtain currents flowing alternately in opposite directions.

[Ill.u.s.tration: FIG. 98.--MODIFICATION OF THE SENSITIVE CELL]

Just as in the strain cells the galvanometer contact was transferred from the electrolytic part to the metallic part of the circuit, so we may next, in an exactly similar manner, cut this plate into two, and connect these directly to the galvanometer, electrolytic connection being made by partially plunging them into a cell containing water. The posterior surfaces of the two half-plates may be covered with a non-conducting coating. And we arrive at a typical photo-electric cell (fig. 98, _b_). These considerations will show that the eye is practically a photo-electric cell.

[Ill.u.s.tration: FIG. 99.--RESPONSES TO LIGHT IN FROG'S RETINA Illumination L for one minute, recovery in dark for two minutes during obscurity D. (Waller.)]

We shall now give detailed experimental results obtained with the sensitive silver-bromide cell, and compare its response-curve with those of the retina. A series of uniform light stimuli gives rise to uniform responses, which show very little sign of fatigue. How similar these response-curves are to those of the retina will be seen from a pair of records given below, where fig. 99 shows responses of frog's retina, and fig. 100 gives the responses obtained with the sensitive silver cell (fig. 100).

It was said that the responses of the retina are uniform. This is only approximately true. In addition to numerous cases of uniform responses, Waller finds instances of 'staircase' increase, and its opposite, slight fatigue. In the record here given of the silver cell, the staircase effect is seen at the beginning, and followed by slight fatigue. I have other records where for a very long time the responses are perfectly uniform, there being no sign of fatigue.

[Ill.u.s.tration: FIG. 100.--RESPONSES IN SENSITIVE SILVER CELL Illumination for one minute and obscurity for one minute. Thick line represents record during illumination, dotted line recovery during obscurity.]

Another curious phenomenon sometimes observed in the response of retina is an occasional slight increase of response immediately on the cessation of light, after which there is the final recovery. An indication of this is seen in the second and fourth curves in fig. 99.

Curiously enough, this abnormality is also occasionally met with in the responses of the silver cell, as seen in the first two curves of fig. 100. Other instances will be given later.

CHAPTER XVIII