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THE SENSITIVENESS OF THE EYE TO

colour, wbich as a rule attract very little attention, as LIGHT AND COLOUR.1

they are common ones, are the subjects of my discourse

to-night. THERE may be some here who have had the pleasure | Experiments which can be shown to a large audience

-or the pain--of rising very much betimes in a on this subject are naturally rather few in number, but I Swiss centre of mountaineering in order to gain some will try and show you one or two. mountain peak before the sun has had power enough to We are often told that the different stages of heat ta render the intervening snow-fields soft, or perhaps dan which a body can be raised are black, red, yellow, and gerous. Those who have will recollect what were the white heat, but I wish to show you that there is an intersensations they experienced as they sallied out of the mediate stage between black and red heat, viz. a grey comfortable hotel, after endeavouring to swallow down heat. An incandescent lamp surrounded by a tissue breakfast at 2 a.m., into the darkness outside. Perhaps paper shade, has a current flowing through it, and in this the night may have been moonless, or the sky slightly absolutely dark room nothing is seen, for it is black hot. overcast, and the sole light which greeted them have been An increase of the current, however, shows the shade of the nervous glimmer of the guides' lanterns. By this a dim grey, whilst a further increase shows, it as illamfeeble light they may have picked their way over the inated by a red, and then a yellow light. 'A bunch of stony path, and between the frequent stumbles over some flowers placed in the beam of the electric light shows every half hidden piece of rock lying in the short grass they ! colour in perfection; the light is gradually dimmed down, may have had time to look around and above them, and and the reds disappear, whilst the blue colours remain and notice that the darkness of the night was alone broken the green leaves become dark. These two experiments by stars which gave a twinkle through a gap in the show that there is a colour, if grey may be called a colour clouds, or if the sky were cloudless, every star would be with which we have to reckon. seen to lie on a very slightly illuminated sky of trans- Now the question arises whether we can by any means parent blackness. Although giant mountains may have ascertain at what stage a colour becomes of this grey hue, been immediately in front of them, their outlines would and at what stage of illumination the impression of mere be almost if not quite invisible. As time went on the sky light also disappears, and wbether in any case the two would become a little brighter, and what is termed the petit disappear simultaneously. 20ur would be known to be approaching. The outlines As all colours in nature are mixed colours, it is at the of the mountains beyond would become fairly visible, the outset useless to experiment with them in order to arrive tufts of grass and the flowers along the path would still at any definite conclusion, hence we are forced—and the be indistinguishable, and most things would be of a cold forcing in this direction to the experimentalist is a very grey, absolutely without colour. The guide's red woollen i agreeable process-we are forced to come to the spectrus scarf which he bound round his neck and mouth would ! for information. be black as coal. But a little more light, and then some! The apparatus on this table is one which I have before flowers amongst the grass would appear as a brighter described in this theatre, and it is needless for me to grey, though the grass itself would still appear dark ; but describe it again. I can only say that it has in all that red scarf would still be as black as a funeral gar- !

colour investigations been of such service that any ment. The mountains would have no colour. The sky

attempt on my part to do without it would have been would look leaden, and were it not for the stars above it

most disadvantageous. The apparatus enables a patch might be a matter of guesswork whether it were not

of what is practically pure monochromatic light of any covered over with cloud.

spectrum colour to be placed upon the screen at once, More light still, and the sky would begin to blush in

and an equally large patch of white light alongside it, the part where the sun was going to rise, and the rest by means of the beam reflected from the first surface of would appear as a blue-grey ; the blue flowers will now

the first prism. be blue, and the white ones white ; the violet or lavender

It should be pointed out that this beam of white light coloured ones will still appear of no particular colour, and

reflected from the first prism of the apparatus, having the grass will look a green grey, whilst the guide's neck first passed through the collimator, must of necessity gear will appear a dull brown.

diminish with the intensity of the spectrum, when the The sun will be near rising, the white peaks beyond

collimator slit is closed. will appear tipped with rose ; every colour will now be

Having got these patches, the next step is to so en. distinguished, though they would still be dull; and,

feeble the light that their colour and then their visible finally, the daylight will come of its usual character, and

illumination disappear. the cold grey will give place to warmth of hue.

An experiment which well demonstrates loss of colour But there may be others who have never experienced

er experienced is made by throwing a feeble white light on one part of this early rising, and prefer the comfort of an ordinary the screen, and then in succession patches of red, green, English tramp to that just described ; but even then they and violet alongside it. The luminosity of the coloured may have felt something of the kind. In the soft autumn light gradually diminishes till all the colour disappears, evening, when the sun has set, they may have wandered

red , the white patch being a comparison for the loss of into the garden and noticed that flowers which in the day- Colour time appear of gorgeous colourings-perhaps a mixture

If red, green, and vioiet patches be placed alongside of red and blue-in the gloaning will be very different in

each other, and they are bedimmed in brightness together, aspect. The red flowers will appear dull and black; a

it will be noticed that the red disappears first, then the red geranium, for instance, in very dull light, being a

green, and then the violet ; or I may take a red and sable black, whilst the blue flowers will appear wbitish

green patch overlapping, which when mixed form orange, grey, and the brightest pale yellow flowers of the same

and extinguish the colour: the slit allowing red light to tint; the grass will be grey, and the green of the trees the

fall on the screen may be absolutely closed, and no alterasame nondescript colour. A similar kind of colouring

tion in the appearance of the patch is found to occur. will also be visible in moonlight when daylight has

This shows, I think, that when all colour is gone from a entirely disappeared, though the sky will have a trans- once brilliant colour, a sort of steel-grey remains behind, parent dark blue look about it, approaching to green.

and that red fails to show any luminosity when the green These sensations, or rather lack of sensations of light and

still retains its colour. TAL:cture delivere la the Roval Inititution of Great Britain by Cap ain

The measurement of the extinction of colour from the W. de W. Abney, C. B., R.E., DC.L, F.RS.

different parts of the spectrum was made on these prin. to

350

ples. A box, similar to Fig. 2, was prepared, but having out for the ordinates; each curve is therefore made ro apertures, one at each side. Through one the on a scale ten times that of its neighbour, counting from Noured ray was reflected, and through the other a white | the centre.

In the diagram the sodium light of the spectrum before extinction was made of the luminosity of the amylacetate lamp (hereafter called AL),which is about 8 of a standard candle, at i foot distance from the source. Before it ceased to cause an impression on the eye, the illumination had to be reduced

· AL.
10,000,000
E light to

- 65

10,000,000 F light

150 or ,

15 10,000,000 1,000,000 G light ,

3000 or 3

10,000,000 10,000 light 11,000 or II

10,000,000 10,000

70,000 0 10

or 7 18 26 34 42 50

B light 58 - 66

,

10,000,000 1000

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Of its spectrum luminosity.

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There was one objection which might have been offered to this method, and that was to the use of the rotating

sectors, and perhaps to the ground FIG, I.--Extinction of Spectrum Colours.

glass. This objection was met by

first of all reducing the light by beam of light to a white screen. Both beams were means of a double reflection of the beam forming the diminished, and when the white and coloured patches patch from one or two plain glass mirrors, and also by appeared the same hue, the amount of illuinination was using a plain glass mirror in the box instead of a silvered calculated. Fig. I shows graphically the reduction of glass. By this plan the light falling on the first plain illumination, when the D light of the spectrum is the glass mirror was reduced, before it reached the end of same intensity as one amyl-acetate lamp at one foot the box, 1000 times; and again, by narrowing the slit of from the screen. To measure the extinction of light, a the collimator, and also the slit placed in the spectrum, box was made as in the diagram, closed at each end, but having two apertures as shown, Fig. 2:

-E is a tube through which the eye looks at s, which is a black screen with a white spot upon it, and which can be illuminated by light coming through the diaphragm Dfirst falling on a ground glass which closes the aperture, and reflected on to it by M a mirror.

The patch of light of any colour being thrown on D rotating sectors, the apertures of which could be opened and closed at pleasure, were placed in the path of the beam, thus enabling the intensity of the patch to be diminished. D could be made of any desired aperture, and thus the illumination of the ground glass would be diminished at pleasure. After keeping the eye in darkness for some time, the eye was placed at E, when the white spot illumi. nated by the colour thrown on D was visible, and the sectors closed till the last scintilla of light was extinguished. This was repeated

Fig. 2.-Extinction Box. for rays at different parts of the spectrum, and the results are shown in Fig. 3 by the con- ! another similar reduction would be effected. All rays tinuou's curved lines. The diagram would have been thus enfeebled were within the range of extinction. It too large had the same scale been adopted through- was found that neither ground glass nor rotating sectors

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had any prejudicial effect, and therefore this extinction a large patch, the image will fall on a part of curve may be taken as correct.

the retina of considerably larger area than the yellow spot, and it will appear too green for those at a distance; but it is correct for myself. If I place a mirror at a distance, and make a match again by the reflected image, the match is complete for us all, as we all see it through the yellow absorbing medium. If I look at it direct from where I stand the match is much too pink. It may be asked why the comparison patches and the mixed colours do not always match since both images are received on the same part of the retina. The reason is that the green I have selected for mixture is in the part of the spectrum where great absorption takes place, whilst the comparison white contains the green of the whole spectrum, some parts of which are much less absorbed than others. I may remark that just outside the yellow spot the eye is less sensitive to the red than is the centre, and this is one additional cause of

the difference. See Fig. 5. 150

More on this subject I have not time to say on this occasion, but it will be seen that the extinction of light for the centre and the outside of the eye differs on account

of this. 100

I must take you to a theory of colour vision which, though it may not be explanatory of everything, at all events explains most phenomena-that is, the Young. Helmholtz theory. The idea embodied in it is that we have three sensations stimulated in the eye, and that these three sensations give an impression of a red, a green, and a violet. These three colours I have said can be mixed to match any other colour, or, in other words, the three sensations are excited in different degrees, in order to produce the sensation of the intermediate spectrum colours, and those of nature as well.

The diagram Fig. 4 shows the three sensations as derived from colour equations made by Koenig. It will

be seen that there are three complete colour sensations, Fig. 3.-Extinction of the Spectrum.

all of which are present in the normal eye. I would ask

you to note that at each end of the spectrum only one In the curves there are two branches at the violet side, sensation is present, viz. at the red end of the spectrum, and this requires explanation. One shows the extinction the red sensation, and at the violet end the violet. when viewed by the most sensitive part of the eye, wherever that may be, and the other when the central portion of the eye was employed. The explanation of this difference in perception is chiefly as follows :

In the eye we have a defect-at least we are apt to call it a defect, Ithough no doubt Providence has made it for a purpose—in that there is a yellow spot which occupies some 6° to go of the very centre of the retina, and as it is on this central part that we receive any small image, it has a very important bearing on all colour experiments. The yellow spot absorbs the blue-green, blue, and violet rays, and exercises its strongest absorption towards the centre, though probably absent in the very centre, that is, in the “fovea centralis," and is less at

н с F DE

с ва the outer edges. That absorption of colour by the yellow spot takes place can be shown you in this way. Any colour in nature can be imitated by mixing a red, a green; and violet together, and with these I will make a match with white and then with brown, two very representative colours,

Fig. 4.-Colour Sensations. if we may call them colours. Now if I, standing at this lecture table, match a white This is a matter of some importance, as we shall now

mixing these three colours together, using | see.

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It will be recollected that in making the extinctions, the This being so, I think it will be pretty apparent that, at D'light of the spectrum was made equal to one amyl-ace all events from the extreme violet to the Fraunhofer line tate lamp, and the other rays had the relative luminosity D of the spectrum, the extinction is really the extinction to it, which they had in the spectrum before they were of the violet sensation, a varying amount of which is exextinguished. The luminosity curve of the spectrum is cited by the different colours. If then we take the recipshown in Fig. 5.

rocals of the numbers which give extinction of the Suppose we make all the luminosities of the different spectrum, we ought to get the curve of the violet sensation rays equal to one A L., we should not get the same ex on the Young-Helmholtz theory. For if one violet sentinction value, as shown in the continuous lines in Fig. 3. sation has to be reduced to a certain degree before it is The violet would have to be much more reduced, but unperceived, and another has to be reduced to half that by multiplying the extinction by the luminosity we should amount, it is evident that the violet sensation must be get the curve of reduction for equal luminosities, and we double in one case to what it is in the other ; that is, the get the dotted curves in Fig. 3.

degrees of stimulation are expressed by the reciprocal of It will be seen that it is the violet under such circumstan the reduction. ces that would be the last to be extinguished, and that all. Such a curve is shown in Fig. 5 (in which also are the rays at the violet end of the spectrum would be extin drawn the curves of luminosity of the spectrum when guished simultaneously, as would also those at the extreme viewed with the centre of the retina and outside the yellow red. This looks like a confirmation of the Young-Helm- | spot). And it will be noticed that it is a mountain which

reaches its maximum about E. Remember that the height of the curve signifies the amount of stimulation given to the violet sensatory apparatus by the particular ray indicated in the scale beneath.

Turning once more to Fig. 3, it will be noticed that if any one or two of the three sensations are absent, the persons so affected are, what is called, colour-blind. Thus if the red sensation is absent, they are red-blind ; if the green, then green-blind ; if the violet, then violet-blind ; if both red and green sensations are absent, then the person would see every colour, including white, as violet. The results of the measurement of the luminosity of the spectrum by persons who have this last kind of monochromatic vision

should be that they give a curve exactly 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64

or at all events very approximately, of VIOLET

BLUE
GREEN ORANGE RED

the same form as the curve given by
the reciprocals of the extinction curve
obtained by the normal eye, as the
violet sensation is that which is last
stimulated.

It has been my good fortune to Fig. 5.

examine two such persons, and I find

that this reasoning is correct, the two holtz theory which I have briefly explained, for we can coinciding when the curves for the centre of the retina are not imagine that it can be anything but a single sensation employed. which fails to be excited.

Further, I examined a case of violet blindness, and The violet is extinguished when it is - 15 A L, measured the luminosity of the spectrum as apparent to

10,000,000

him. Now if the Young-Helmholtz theory be correct, that is, a screen placed 817 feet away and illuminated by then in his case the violet sensation ought to be absent, an A L violet lamp would be invisible. The blue-green and the difference between his luminosity and that of the (F) light when it is

normal eye ought to give the same curve as that of the 10 millionths or 770 feet away.

violet sensation. This was found to be the case. The green (E) light — 35

Again, the reciprocal of the extinction curves of the ,

red-blind and green-blind ought to be the same as those

of the normal eye, for the violet sensation must be present orange (D) light is extinguished as before at 35.0

10 millionths

with them also. This was found to be so. We have or 180 feet away, whilst the red (c) light has only to be re

still one more proof that the last sensation to disappear is

the violet. duced to 2200

or an A L lamp radiating c light If we reduce the intensity of the spectrum till the green 10 millionths of

and red disappear to a normal eye, and measure the would have to be placed only 67 feet away, whilst the

luminosity of the spectrum in this condition, we shall find radiation for an AL of the colour of the B light of the that it also coincides with the persistency curve. On the spectrum would have to be diminished to but

2600

' screen we have a brilliant spectrum, but by closing the

10 millionths | slit admitting the light and placing the rotating sectors or the screen would have to be placed 60 feet away.

in the spectrum and nearly closing the apertures, we can It is therefore apparent that with equal luminosities reduce it in intensity to any degree we like. The whole the violet requires about 175 times more reduction to ex spectrum is now of one colour and indistinguishable in tinguish it than does the red, and probably about 25 times hue from a faint white patch thrown above it. If the more than the green.

luminosity of this colourless spectrum be measured we

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10 millionths or 550 feet away. The

shall get the result stated. The curve obtained in this clusions here arrived at, and wbich concern the act of way is in reality identical with the other curves. By these deglutition in the lower classes of mammals, lead to a four methods then we arrive at the conclusion that the consideration of other organs of the oral cavity, and to last colour to be extinguished is the sensation which when an attempt at establishing a connection between these strong gives the sensation of violet, but which when feeble and the apparatus consisting of the epiglottis and soft gives a blue-grey sensation.

palate. This in turn induces a minute investigation of One final experiment I may show you. It has been the structure of the epiglottis, and of its relationship to remarked that moonlight passing through painted glass the framework of the larynx and the general structure of windows is colourless on the grey stone floor of a cathe the respiratory organs in the lowest forms of animal life. dral or church.

In the last chapter the author summarizes the results obWe can imitate the painted glass and moonlight. Here tained by his comparative studies and throws out such is a diaper pattern of different coloured glasses, and by suggestions concerning the origin, development, and means of the electric light lantern we throw its coloured function of the epiglottis as would seem justified by his pattern on the screen. The strength of moonligbt being researches. known, we can reduce the intensity of the light of the Brief as this survey of the course of Gegenbaur's essay lamp till it is of the same value. When this is done it necessarily has been, it will be sufficient to show that it is will be seen that the pattern remains, but is now colour quite impossible to give in the space of a short review a less, showing that the recorded observations are correct, detailed analysis of its contents. Conclusions derived and I think you are now in a position to account for the from the synthetic conception of an enormous number of disappearance of the colour.

single observations, which extend over a large part of the I have now carried you through a series of experiments entire animal kingdom, can only be properly appreciated which are difficult to carry out perfectly before an by a study of the original, and this may be warmly recomaudience, but at any rate I think you will have seen mended. enough to show you that the first sensation of light is The final and most important conclusion arrived at by what answers to the violet sensation when it is strong the author may be briefly summarized as follows :enough to give the sensation of colour. The other sen Whilst as high up in the scale as in the sauropsidæ, sations seem to be engrafted on this one sensation, but parts of two branchial arches only contribute towards in what manner it is somewhat difficult to imagine. forming the primary hyoid, three more arches are added Whether the primitive sensation of light was this and the in the mammals. Two of these growing together form others evolved, of course we cannot know. It appears the transition into the thyroid, which becomes intimately probable that even in insect life this violet sensation is connected with the larynx. predominant, or at all events existent. Insects whose The mammalian larynx, however, has received a food is to be found in flowers seek it in the gloaming, further addition, viz. the epiglottis, the cartilage of which when they are comparatively safe from attack. Prof. can only be looked upon as the further development of Huxley states that the greater number of wild flowers are the fourth branchial arch, which in fishes still serves its certainly not red, but more or less of a blue colour. This primitive function, and in the amphibia appears in a rudimeans that the insect eye has to distinguish these flowers mentary form. The exact manner in which this rudiment at dusk from the surrounding leaves, which are then of a passes over into the supporting organ of the epiglottis in dismal grey ; a blue flower would be visible to us whilst mammals is, on the whole, still obscure. So much, how a red flower would be as black as night. That the insects ever, is certain, that the cartilage of the epiglottis is not a single out these flowers seems to show that they partici product of mucous membrane, but a genuine part of the pate in the same order of visual sensations. I venture to skeleton, and that it communicates its supporting think, without adopting it in its entirety, that these results function to the whole of the epiglottis, which serves as at all events give an additional probability as to the gene well the purpose of keeping the air-passages open as of ral correctness of the Young-Helmholtz theory of colour protecting the vestibule of the larynx. vision. Where the seat of colour sensation may be is not From this final conclusion it will be seen that, accordthe point, it is only the question as to what the colour ing to Gegenbaur, the role of the epiglottis in its highest sensations make us feel which the physicist has to deal development is purely a respiratory and protective one. with. The simpler the theory, the more likely is it to be Pathological observation in man does not admit of the true one, and certainly the Young-Helmholtz theory these functions of the part being looked upon in any way has the advantage over others of simplicity.

as indispensable for the existence of the individual. Total loss of the epiglottis has often been observed in various

diseases, without the patients either suffering from dyspTHE EPIGLOTTIS.1

nea or from increased liability to the entrance of foreign

bodies into the lower air-passages, the constrictor testFROM an anthropotomical point of view the epiglottis buli laryngis (Luschka) in such cases vicariously taking I had for a long time been generally looked upon as its function. The supposed phonatory rôle of the epia kind of sentinel for the protection of the upper air glottis, upon which much stress is laid by some eminent passages, when Rückert's comparative anatomical obser singing masters (e.g. Stockhausen), inasmuch as they vations showed that the human epiglottis greatly differed maintain that it influences, according to its more erect or from that of mammals, in so far as its relations to the soft more horizontal position, the "timbre” of the singing palate were entirely altered, and that its physiological voice, is not even mentioned in Gegenbaur's essay. Thus conditions pari passu had undergone important modifica many points connected with this subject still demand tions. The new points of view thus obtained induced elucidation. Still it is impossible to withhold the expres Gegenbaur to study the comparative anatomy of the sion of admiration and of gratitude to the author of the epiglottis and its relations to the larynx, and the present present work for his patient and extensive researches in a volume is the outcome of his investigations.

very obscure field of comparative anatomy. The inquiry being undertaken from a morphological point of view the author begins with a study of the different forms of the epiglottis or epiglottoid structures in low classes of animal life. He next discusses the mammalian

NOTES. epiglottis and its relations to the soft palate. The con On Saturday the British Eclipse Expedition to West Afria 1 "Die Epiglottis," Vergleichend anatomische Studie, by Carl Gegenbaur,

arrived safely at Bathurst. The Alecto was there, ready to with two plates, &c. (Leipzig: Wilh. Engelmann, 1892.)

convey the party up the Salum River to the selected site.

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