No. 1072, Vol. 42J 



NATURE 



55 



COLOUR-VISION AND COLOUR-BLINDNESS} 



IT is a matter of familiar knowledge that the sense of 

 vision is called into activity by the formation, on the 

 retina or internal nervous expansion of the eye, of an 

 inverted optical image of external objects— an image 

 precisely analogous to that of the photographic camera. 

 The retina lines the interior of the eyeball over somewhat 

 more than its posterior hemisphere. It is a very delicate 

 transparent membrane, about one-fifth of a millimetre in 

 thickness at its thickest part, near the entrance of the 

 optic nerve, and it gradually diminishes to less than half 

 that thickness at its periphery. It is resolvable by the 

 microscope into ten layers, which are united together by 

 a web of connective tissue, which also carries blood- 

 vessels to minister to the maintenance of the structure. 

 I need only refer to two of these layers : the anterior or 

 fibre-layer, mainly composed of the fibres of the optic 

 nerve, which spread out radially from their point of 

 entrance in every direction, except where they curve 

 around the central portion of the membrane; and the 

 perceptive layer, which, as viewed from the interior of the 

 eyeball, may be likened to an extremely fine mosaic, each 

 individual piece of which is in communication with a 

 nerve fibre, by which the impressions made upon it are 

 conducted to the brain. The terminals of the perceptive 

 layer are of two kinds, called respectively rods and cones ; 

 the former, as the name implies, being cylindrical in 

 shape, and the latter conical. The bases of the cones 

 are directed towards the interior of the eye, so as to 

 receive the light ; and it is probable that each cone may 

 be regarded as a collecting apparatus, calculated to gather 

 together the light which it receives, and to concentrate 

 this light upon its deeper and more slender portion, or 

 posterior limb, which is believed to be the portion of the 

 whole structure which is really sensitive to luminous 

 impressions. The distribution of the two elements differs 

 greatly in different animals ; and the differences point to 

 corresponding differences in function. The cones are 

 more sensitive than the rods, and minister to a higher 

 acuteness of vision. In the human eye, there is a small 

 central region in which the perceptive layer consists of 

 cones only, a region which the fibres avoid by curving 

 round it, and in which the other layers of the retina are 

 much thinner than elsewhere, so as to leave a depression, 

 and are stained of a lemon-yellow colour. In a zone 

 immediately around this yellow spot each cone is sur- 

 rounded by a single circle of rods ; and, as we proceed 

 outwards towards the periphery of the retina, the circle of 

 rods around each cone becomes successively double, 

 triple, quadruple, or even more numerous. The yellow 

 spot receives the image of the object to which the eye is 

 actually directed, while the images of surrounding objects 

 fall upon zones which surround the yellow spot ; and the 

 result of this arrangement is that, generally speaking, the 

 distinctness of vision diminishes in proportion to the 

 distance of the image of the object from the retinal 

 centre. The consequent effect has been well described 

 by saying that what we see resembles a picture, the 

 central part of which is exquisitely finished, while the 

 parts around the centre are only roughly sketched in. 

 We are conscious that these outer parts are there ; but, if 

 we desire to see them accurately, they must be made the 

 objects of direct vision in their turn. 



The indistinctness with which we see lateral objects is so 

 completely neutralized by the quick mobility of the eyes, 

 and by the manner in which they range almost uncon- 

 sciously over the whole field of vision, that it seldom 

 or never forces itself upon the attention. It may be 

 conveniently displayed by means of an instrument called 

 a perimeter, which enables the observer to look steadily 

 at a central spot, while a second spot, or other object, is 



' Lecture delivered at the Royal Institution, on Friday, May 9, 1890, by 

 Mr. K. Brudenell Carter. 



moved along an arc, in any meridian, from the circum- 

 ference of the field of view towards the centre, or vice 

 versa. Slight differences will be found between in- 

 dividuals ; but, speaking generally, a capital letter 

 one third of an inch high, which is legible by direct 

 vision at a distance of sixteen feet, and is recognizable 

 as a dark object at 40^ or 50° from the fixing point, will 

 not become legible, at a distance of one foot, until it 

 arrives within about 10', 



The image formed upon the retina is rendered visible 

 by two different conditions— that is to say, by differences 

 in the amount of light which enters into the formation of 

 its different parts, and by differences in the quality of 

 this light, that is, in its colour. The former conditions 

 are fulfilled by an engraving, the latter by a painting. It 

 is with the latter conditions only, and with the power of 

 perceiving them, that we are concerned this evening. 



Before such an audience as that which I have the 

 honour to address, it is unnecessary to say more about 

 colour than that it depends upon the power, possessed by 

 the objects which we describe as coloured, to absorb and 

 retain certain portions of white or other mixed light, and 

 to reflect or transmit other portions. The resulting effect 

 of colour is the impression produced upon the eye or 

 upon the brain by the waves of light which are left, after 

 the process of selective absorption has been accomplished. 

 Some substances absorb two of the three fundamental 

 colours of the solar spectrum, others absorb one only, 

 others absorb portions of one or more. Whatever 

 remains is transmitted through the media of the eye ; 

 and, in the great majority of the human race, suffices to 

 excite the retina to a characteristic kind of activity. 

 Few things are more curious than the multitude of 

 different colour sensations which may be produced by 

 the varying combinations of the three simple elements, 

 red, green, and violet ; but this is a part of the subject 

 into which it would be impossible for me now to enter, 

 and with which most of those who hear me must already 

 be perfectly familiar. 



Apart from the effect of colour as one of the chief 

 sources of beauty in the world, it is m'anifest that the 

 power of distinguishing it adds greatly to the acuteness 

 of vision. Objects which differ from their surroundings 

 by differences of colour are far more conspicuous than 

 those which differ only by differences of light and shade. 

 Flowers are much indebted to their brilliant colouring for 

 the visits of the insects by which they are fertilized ; and 

 creatures which are the prey of others find their best 

 protection in a resemblance to the colours of their 

 environment. It is probably a universal truth that the 

 organs of colour-perception are more highly specialized, 

 and that the sense of colour is more developed, in all 

 animals, in precise proportion to the general acuteness of 

 vision of each. 



From a variety of considerations, into which time 

 will not allow me to enter, it has been concluded that 

 the sense of colour is an endowment of the retinal 

 cones, and that the rods are sensitive only to differences 

 in the quantity of the incident light, without regard 

 to its quality. Nocturnal mammals, such as mice, 

 bats, and hedgehogs, have no cones ; and cones are 

 less developed in nocturnal birds than in diurnal ones. 

 Certain limitations of the human colour sense may 

 almost be inferred from the anatomy of the retina. It 

 is found, as that anatomy would lead us to suppose, 

 that complete colour sense exists only in the retinal 

 centre, or in and immediately around the yellow spot 

 region, and that it diminishes as we pass away from 

 this centre towards the periphery. The precise facts 

 are more difficult to ascertain than might be supposed ; 

 for, although it is easy to bring coloured objects from 

 the circumference to the centre of the field of vision 

 on the perimeter, it is by no means easy to be quite sure 

 of the point at which the true colour of the advancing 



