Colour vision : being the Tyndall lectures delivered in 1894 at the Royal Institution

Preface

JHE writer had for some years past, in conjunc¬ tion with General Festing, and recently as Secretary and iMember of the Colour Vision Com¬ mittee of the Royal Society, carried out a series of investigations on colour vision, and selected that subject when he was invited, in 1894, to deliver the Tyndall Lectures at the Royal Institution,

The brief time allotted for these lectures — an hour on three successive Saturday afternoons — restricted

the discussion of some aspects of the question, and confined its treatment in the main to those features

most readily explicable by the physicist, and to bringiner into notice the latest results which had been obtained from physical experiments. Howj far the viii Preface.

writer has succeeded in the task which he then out-, lined it is for the reader to determine.

There was no intention in the first instance to publish these lectures. .After their delivery, many persons expressed a desire that the information they contained should be rendered accessible to such as were interested in the theory of colour vision, and in deference to that desire the lecture-notes have been re-cast in liook form. For the reader’s con¬ venience the matter is now divided into chapters instead of into lectures, and a few additions have been made in the text to explain some of the ex¬ perimental work to those who have not facilities for its repetition.

The writer has to acknowledcre several debts of

O

gratitude. First, to Mr. E. Nettleship, for his kindness in looking over the proofs, and making valuable sug¬ gestions whilst the work was passing through the press ; and also, as will be seen throughout its pages, for many of the interesting cases of defective colour perception which have been examined by the some-

Preface. ix

what novel methods described. Next, the writers gratitude is due to Professor M. Foster for the per¬ mission he has given to use his admirable description of the Hering theory ; and, lastly, to the Royal Society for the permission it accorded to use various diagrams which have served as illustrations to papers which have appeared in its “ Philosophical Transactions ” and “Proceedings.”

I

COLOUR VISION.

Chapter I

J ML^ST commence this course by saying that 1 feel the honour that has been done me in askins; me to undertake it, connected as it is with the name of Tyndall, whose recent removal from our midst has been deplored by all lovers of science, and Ijy none more than by those who have had the privilege of listening to him at this Institution. It is my duty to speak on some subject of physics, and the subject I have chosen is Colour Vision. I hope it will not be considered inappropriate, since it was Thomas Young, the physicist, whose connection with this Institution is well known, who first propounded a really philosophical theory of the subject. Interesting as it may be to trace how old theories have failed and

B o

Colon)' Vision.

new ones have started, I feel that for those wlio, like' myself, have hnt little time at command in which to address you, the historical side of this (jnestion must of necessity he treated iiicom2)letely.

Colour vision is a subject which enters into the domains both of physics and physiology, and it is thus diffienlt for any one individual to treat of it exhaustively unless he be a Helmholtz, who was as distinguished in the one branch of science as he was in the other. I am not a physiologist, and at the most can only pretend to an elementary knowledge of the physiology of the eye, l)ut I trust it is sufficient to prevent myself from falling into any grievous error. I shall try and show you, however, that the subject is capable of being made subordinate to physical methods of examination. I must neces¬ sarily commence by a very brief description of those parts of the eye in which it is supposed the seat of vision lies, l)ut in terms which are not too technical. As to the mere optical properties of the eye I shall say but little, for they are not necessary for my purjDose, although more particularly adapted to mathematical treatment than the other properties I have to describe.

The eye may be diagrammatically represented as in the tigure which is supposed to be a horizontal

The Eye.

section of it, the figure being reproduced from Professor Michael Foster’s Physiology.

As fiir as the perception of colour is concerned, the principal part of the eye which is not distinctly optical — i.e. for the produc¬ tion of imao;es ■ — is the retina, and this it will 1)6 seen is in reality an outcrop of the brain, the connec¬ tion between the two being the optic nerve.

Owino; to this connection, it is not easy to determine where the seat of colour percep¬ tion is located ; but for the purpose of physical investigation this is not of firstrate importance, nor does it affect the discussion of rival theories except in a minor degree. There are other subsidiary adjuncts in the eye to which, however, I must call attention, as they have a distinct bearing on the apparent intensity

B 2

Fig. 1.

Sd is tbe sclerotic coat. Ch the choroid coat, ■with CP the ciliary process. I is the body of the Iris. B is the retina or inner 'wall. PE the pigment epithelium or outer wall. L the lens held by the suspensory ligament sp.l. VH is the vitreous humour. ON the optic nerve, ox is the optic axis, in this case made to pass through the fovea centralis, /.c.

Colour Visiofi.

of some colours and of tlie hue that mixtures of others are perceived. The first is what is called tlie “ macula lutea,” or yellow spot, a spot which it may be assumed exists in every eye. It is horizontally oval in form, and is situated in the ver}' centre of the retina, em1)racing some G° to 8° in angular mea¬ sure. It has a 1)rownish or yellowish tint, and the retina at this part is slightly depressed, being bounded by a slightly raised rim. In the centre of this area the retina becomes very thin, having a depression about of an inch or '3 millimetres in diameter, which is named the “ fovea centralis,” where it is said that vision is the most acute. This statement can be well credited when we come to consider where the seat of the stimulation of sensation lies. The colour which tints the yellow spot is strongest at the crater-like rim, and fades away centrally and periphe¬ rally, and is said to be wholly absent in the fovea centralis.

As the colour of this spot is yellow or brown in the living eye (and that it is probably brown the absorption indicates), it follows that white light passing through it must be deprived of some of its compo¬ nents, though in differing degrees. If the seat of sensation is at the outer layer of the retina, as we

The Yellow Spot. 5

shall shortly see must he the case, it will further be seen that when light of any colour which the brown pigment will absorl) more or less completely tails on cliflferent parts of the oval area, the absorption must vary at each part, and the intensity of the perceived light will be least at the rim and’increase centrally and peripherally. As the centre of the yellow spot or fovea is coincident approximately with the point where the axis of the eye cuts the retina, the image of an evenly illuminated object, when looked at directly, must fall on the yellow spot. If, therefore, a patch of such light, the image of which more than covers the spot, be observed, it ought to exhibit a varying brightness of colour corresponding to the strength of the colouring matter which exists at the different parts. This it but rarely does, for habit and constant interpretation of what should be seen prevents the mind from distinguishing these variations ; Imt if the colour brightness, as perceived l)y the different parts, be submitted to measurement by proper means, the varia¬ tions in brightness of the image can be readily recognised. A very common method of exhibiting the presence of the pigment is to look at a bright white cloud through a layer of chrome alum. Chrome alum transmits red and blue - green rays. Now as

Colotir Vision.

the sjjectrum-ljlue rays are those which the pigment will ahsorl), it follows that the colour of the solution should appear ruddy to the central part of the eye, but on the rest of the retina it should appear of its ordinary j^urplish colour. At a first glance, and before the eye has become fatigued, this is the case, but the pheiionieiioii soon disappears. Another way of forming an idea as to what the yellow spot absorbs is to throw a feeble spectrum on a white surface and cause the eye to travel along it. If the sj^ectruni be viewed so that it does not occuj^y more than about 40° of the retina, the movement of the eye will show a dark band travelling along the green, blue, and violet regions as the image of these parts of the sj^ectrum fall on the yellow s2)ot, and their a2)parent brightness will increase as they fall outside the absorbing area. This proves that an absorption takes j^lace in this area.

The retina consists essentially of an inner and outer wall, enclosing matter wdiich is similar to the grey matter of the brain. On the inner wall are the vessels which are connected with the optic nerve. The outer wall is epithelium coloured with a pig¬ ment, and it is here that the visual impulses begin, although the rays of light giving rise to them have to j-^ss through the thickness of the retina

Purkiiije s Figures. 7

before so doing. It has already been stated that the light has to pass through the thickness of the yellow spot before a visual sensation is felt in the centre of the field, and the experiments just given offer a fair proof of the truth of the assertion, but there is still another which is perhaps more conclusive. Suppose we have a hollow reflecting ball, as shown in Fig. 2, and through an orifice A we project a beam of light to B, which meets an obstruction, S, in its path, then A B would be reflected from B to C on a screen C F, and the ob¬ struction S would f)e marked at C.

If another beam from D was di¬ rected so as to meet the same obstruction, its presence would be marked at F. Knowing the distance of the centre 0 of the hollow sphere from F C and its diameter, and measuring the distance between F and C and their respective distances from the axis of the sphere, the distances S B and S E can be calculated. This method is applied in the formation of what are known as Purkinje’s figures. The simplest case is where a beam of lig-ht is directed through the sclerotic and

O O

transmitted through the lens. Images of the retinal

r) o

Fm. 2.

C F

Colour Vision.

vessels are distinouislied as at S, and it is found tliat they cast shadows, which are seen as dark lines in the glare of the field of vision. The sensation of light must therefore come from behind these vessels, and calculation shows that the seat of the sensation is close to the pigmented inner wall of the retina.

Lying here is a layer of what are known as rods and cones, which have a connection, either actual or functioiial, with the optic fibres which largely compose the inner wall of the retina, and are connected with the optic nerve. In the yellow spot the cones arc much more numerous than the rods, but in the peri¬ pheral part the reverse is the case. In the fovea the rods a|jpear to be altogether al)sent. The total number of cones in the eve has been calculated to be about 3,000,000, of which about 7,000 are in the small fovea. The number of cones will mve an idea of their dimen-

O

sions. This detail has lieen entered into as it has been supposed that these rods and cones are all-im23ortant in translating light-waves into visual impulses. The inner wall of the retina of most human eyes, as has been mentioned, is stained with a black j^igment, fuscin, though in albinos it is absent. What its par¬ ticular use may be is still unknown, for its change by

Visual Purple. 9

light is so slow that it can scarcely l)e the cause of

vision. In the outer parts of the rods is, however,

diftused a substance highly sensitive to light, called

the “ visual purple,” from its colour, and a theory founded on chemical action, produced l)y a change in this substance, has been promulgated. Fascinating, however, as such a theory must l)e, it lacks confirma¬ tion. The fact that the cones do not contain it, and that in the fovea are cones alone, renders it difficult to reconcile the theory with the fact that this part of the retina possesses, we are told, the srreatest acuteness of sensation as regards lio;ht and

colour.

The eyes of most vertebrate animals, it may l»e remarked, have this visual purple, but in those of the bat, owl, hen, and some others the colouringmatter seems to be absent. A^isual purple is an in¬ teresting substance, however, and as it is found in the eye it probably exercises some useful function, though what that function may be is at present unknown. That images of objects can be formed on the retina, owing to the ])leaching of this substance, has been proved by experiment. The purple is first changed to a yellow colour, and then passes into white. These “ optograms,” as they are called, can lO

CoUnir l^isio7i.

1)6 fixed ill an excised eye if the retina be detached,

and then l)e treated with a weak solution of alum.

i\lany persons are not aware of the extent of the field of view which the eye embraces. Ver¬ tically it takes in about 100°, whilst horizontally it will take in some 145°, more or less. The field is smaller on the nasal than on the temporal side.

AVhen both eyes are used, the combined field of view is larger horizontally, being about 180°.

The field of view which is common to both eyes is roughly a circle of about 90°. There

Distinctness of Vision. i r

is, however, a marked difference in the distinctness with which objects are perceived in the different parts of field of view. On the fovea centralis twt) dots placed so as to subtend an angle of 60'' Avill be perceived as double. That is to say, if a piece of paper, on which are two dots gig- of an inch apart, be placed 10 feet away from the oljserver, these dots will be seen as separated, whilst dots (in this case they should be black and of good dimensions) placed half - an - inch apart would still appear as one if viewed at the same distance near the periphery of the retina. In the yellow spot the distance apart of the cones is such that they subtend about the same angle as the dots when they are seen separate, viz., about 60"; that is, they are about of an inch apart, and hence

may have something to say to the limit of separation. The field for the perception of colour is different to that for light.

The diagrams (Fig. 3) will show the fields in a satisfactory manner. The concentric circles are sup¬ posed to be circles lying on the retina corresponding to parallels of latitude on a globe, and are not, there¬ fore, equi-distant when seen in projection. To make these circles it must be imao'ined that we have a bowl, in the middle of which is a thin rod standing upright

Colour Vision.

I 2

and passing tlirongli the centre, and another rod attached to it at the centre of the sphere of exactly the length of the radius. If this last arm be opened to make an angle of 5° with the fixed rod, and be twisted round like the leg of a compass against the IkjwI, it will make a circle, the projection of which will give the innermost circle of the diagram ; if opened to 10° it will give the next circle, and so on for every sul)se(]uent 10°. The lines j^fissing through the centre are 30° from one another, the line stretching from 360° to 180° being a line supposed to l)e vertical. By means of an instrument called the perimeter, the field of vision for each eye can be measured. With its aid any small object can be made to fall on any part of the retina by directing the axis of the eye to a fixed j^oint and moving the ol)ject along one of the diameters. SupjDose we wish to ascertain the field for a white object, a small white disc is moved, say, along the horizontal line, and the angles at which the retina just no longer sees it are noted. This gives two points ill the field, and they are plotted on the chart — in Fig. 3 one touches the outside circle, and the other is at an angle of about 65°. The field of vision is next tested along another line, say 300° to 120°, and other points noted and marked on the chart. When

Colour Fields.

the whole circle has been examined, the various points are joined together, and we have the boundary of vision for a white object. The boundaries of the colour perception for (say) small red and green discs are found in the same way. The former is depicted in the left-hand chart and gives the field for the right eye, and the latter with that for white in the right-hand chart for the same eye. It will be noticed that two boun¬ daries are given, one taken at mid-day and the other at 6 p.ni. The brighter the colour, the larger is the boundary in both cases, showing that the field of colour vision varies according to the illumination. Now it is difficult from this method of experimenting to determine whether the fields for different colours are the same or differ in extent, as we have no information as to whether the colours themselves which were used were physiologically equal. The only way l)y which this can be satisfactorily determined is Ijy using spectrum colours each of known brightness and area. (Some preliminary experiments made by myself regarding the colour fields will be found in the appendix, and will be referred to later.) It must not be thought that the various colour boundaries mark the limit at which light is perceived, but only the limit at which colour is seen ; outside the boundaries the objects appear of a nondescript colour,

Colotir Vision.

to which we shall by-and-by call attention. The yellow spot lies within the circle of 5°, and the Idind spot on which no sensation of light is stimulated is shown by the black dot about 15° away from the centre.

I have only attempted to sketch, in unphysiological language, the primary apparatus with which our experiments in colour have perforce to be made.

{ ‘5 )

Chapter II

It will Ije seen, then, that in measuring colour or light several circumstances have to be taken into account. These are not simple, and rerpiire differen¬ tiating one from another before the results of colour measures can be finally laid down as correct, or as being held to be applicable to all cases.

We must naturally ask, what is colour ? The answer I should like to pass over entirely. It can only lie <lescribed as a sensation, just as we should describe touch as a sensation. It has, however, one advantage over most sensations, in that it is a sensation which can be submitted to empyric measurement. The f|uestion whether certain phenomena, such as the colours produced by simultaneous contrast, are sub¬ jective or real, does not require answering for the purpose that we have in view, ljut the results recorded may probably help to throw light on it. ('olour is an impression caused by the stimulation in the eye

Colon 7' Vis 1071.

i6

of some apparatus, that lies near the outer wall of the retina, the effect of the stimulation heiim conveyed by the optic nerve to the brain. If this apparatus be complicated by being made up of distinct parts, each of which transmits its own kind of impression to the brain, it is not only cpite possible, but more than probable, that when one part is absent or injured the particular impression for which it is responsible will be lacking, and that the sum of the imjDressions due to the remainder will be unlike that perceived when they are all working together.

In every investigation, whether it be in j^hysical or in any other In-anch of science, it is better to work up from the sim2:)le to the more complicated ; and acting on this j^lan, it is l)etter to commence experi¬ menting with simple rather than with complex colours, though they may apparently produce precisely the same sensations. I shall, with this in view, devote most of the remaining part of this chapter to some necessary experiments with simple colours. The simple colours are those of the spectrum, and are the result of motion in the ether, which pervades all space. The motion is in the form of undulations or waves, and each colour is due to a series of these waves, which have a definite length. Thus, 65G2 ten-millionths of

Wave-Lengths of Light. ly

a millimetre produces to most of us a red colour in the spectrum (see Plate L), occupying the position indicated by a black line known as the C line in the solar spectrum.

A table of wave-lengths of certain lines in the solar spectrum is given below : —

TABLE OF WAVE-LENGTHS IN TEN-MILLIONTHS OF A MILLIMETRE.

Lithium, cherry red

F, bluish green

C, red

Lithium, blue .

D, orange

G, violet.

E, green .

H, extreme violet

The rays in

the

different parts of the spectrum

being due to these

simple

vibratory motions.

cannot

be decomposed

further.

And it makes no

matter

whether we see them as different colours or not, they will always issue at the same angle from the same prism (if the prism be used to form the spectrum), when it is turned to the same angle to the incident light. Milestones are useful along a road to tell us where we are in reference to some central place, and’ these black lines in the spectrum serve the same end. But they have the advantage over the milestone, for whilst the last will tell us how far we are from, say, York or London, the former tell us our distance from c

Colour Vision.

a zero point. We thus have a scale of light of different wave-lengths laid down for us, which we can apply to the study of the sensations stimulated in the eye, and so have the means of instituting a comparison l)etween the colour vision of different eyes. A mixed or composite colour is in a different category, however, to the simple colour, as you will see directly. It is one which may he formed hy any number of rays of different wave-lengths falling on the eye. What these rays are we can only tell by analysing the light and referring them to the spectrum.

The instrument before you is one which I have used before in this theatW ; but as the major part of my experiments have l)een carried out with it, in case those who are present may not be acquainted with it, it will be necessary to describe it very briefly. The general arrangement of the apparatus is given in the accompanying diagram, Fig. 4.

R R are rays coming from the source of light, be it sun lio;ht or the electric light, and an image of the one or the other is formed by a lens Li on the slit Si of the collimator C. The j^arallel rays produced by the lens La are partially refracted and partially reflected. The former pass through the prisms Pj, Pa, and are focussed to form a spectrum at D by a lens L3. D is a

Colour Patch Apparatus. 19 movable screen in which is an aperture So, the width of which can be varied as desired. The rays are again collected by a lens L4, and form a white image of the surface of the last prism on the screen E. If the light passing through S2 is alone used, the image at E is c 2

Colour Vision.

formed of practically mono-chromatic light. Part of the rays falling on Pi are, as just said, reflected, hut as it and the refracted part are portions of the light passing through the slit Si, they both must vary pro¬ portionally. If then we use the reflected portion as a comparison light to the spectrum colours, the rela¬ tive intensities of the two, though they may vary intrinsically, will remain the same. The rays reflected from Pi fall on G, a silver or glass mirror, and, by means of another lens L3, also can be caused to form a white patch on the screen E, alongside the patch of colour. At M, or anywhere in the path of the beams, an electro-motor dri\'ing a sector with aper¬ tures which can be opened or closed whilst rotating, is placed, and the illumination of either beam can be altered at will. To obtain a large spectrum on the screen E, all that’, is necessary is to interpose a lens of fairly short focus in front of L4, when a spectrum of great purity and brightness can be formed.

If it be required to measure the width of the slits S2 (which we shall see further on is often necessary), a small lens of short focal length placed behind L4 and near the slit will cast a magnified image on E, and by means of a scale placed there, the widths of each slit, if there are more than one, can be read off on the

Coloiir Patch Appar scale by bringing them successively into the same colour.

Originally the com¬ parison light was a candle, and it answered its purpose fairly well, and for obtainiiiQ; absolute measures is con¬ venient at the present time. Fig. 5 will show its arrangement, but as both the candle and the electric light may vary independently of each other, it will be seen that for merely the comparison of the dif¬ ferent spectrum colours, the previous arrange¬ ment is the better. In both cases the two beams — the direct and the comparison — may be made to cast shadows

Colour Vi si 0)1.

])y placing a rod in their path, the shadow cast hy one light is then illuminated Ity the other light. By moving the rod towards or from the sci'een the shadows can l)e brought side by side.

With this i,nstrument it is easy to demonstrate that a mixed colour may be mistaken for a simple colour of the spectrum. In a glass cell with parallel sides is a solution of potassium l;)ichromate, which, to mj'self and probal)ly most of you, has a l)eautiful orange colour. The spectrum of white light is now on the screen, and if this orange liquid is placed in the path of the white light before it reaches the prisms, all the violet, l)lue, and most of the green is cut off, leavino; some m-eenyellow, orange and red only on the screen. That these form the orange colour of the bichromate is readily shown by removing the auxiliary lens. The spectrum, which has its focus at D, is now recombined into a patch of light, which is at once seen to be the colour of the solution.

The colour of the bichromate is therefore a complex or mixed colour according to our definition, for it is made up of a large numl )er of simple colours. What I desire to show, however, is that this complex colour can be mistaken by the eye for a simple colour. First, let us interpose the cell with the bichromate in the path

Simple and Complex Colours.

of the reflected beam, and throw the patch of light formed bv it on a white surface A (Fig. 6), alongside the patch of light B formed by the spectrum. Next let us pass a single aperture (Fig. 7), which can be opened and closed by a screw arrangement, through the spectrum. By careful movement we at length come to an orange ray, which is spread out l)y the apparatus to form a patch on B, that to the majority (and the word majority is used with intention) of j^eople exactly matches the coloui’ of the bichromate. Thus we have a proof that, as far as the eye is concerned, the simple and the complex colours are identical. This illustration of the want of power of the eye to analyse colour might be repeated as often as we like. We may pass coloured wools, for in¬ stance, through the length of the spectrum and show that they have the property of appearing bright in, and therefore of reflecting, some colours and of almost disappearing in others — a sure indication that these colours are mixed colours as they are made up of the rays which are reflected. Yet when viewed in white light they can in many cases be matched wdth simple