56 



NATURE 



[May 15, 1890 



object can first be said to be distinctly seen. Much 

 depends, moreover, on the size of this advancing object ; 

 because, the larger it is, the sooner will its image fall 

 upon some of the more sparsely distributed cones of the 

 peripheral portion of the retina. Testing the matter 

 upon myself with coloured cards of the size of a man's visit- 

 ing card, I find that I am conscious of red or blue at about 

 40° from the fixing point, but not of green until it comes 

 within about 30'" ; while, if I take three spots, respectively 

 of bright red, bright green, and bright blue, each half a 

 centimetre in diameter, and separated from its neighbour 

 on either side by an interval of half a centimetre, spots 

 which would be visible as distinct and separate objects at 

 eight metres, I cannot fairly and distinctly see all three 

 colours until they come within 10" of the centre. Beyond 

 40°, albeit with slight differences between individuals, and 

 on different meridians for the same individual, colours are 

 only seen by the degree of their luminosity— that is, they 

 appear as light spots if upon a dark ground, and as dark 

 spots if upon a light ground. Speaking generally, there- 

 fore, it may be said that human vision is only tri- 

 chromatic, or complete for the three fundamental colours 

 of the solar spectrum, over a small central area, which 

 certainly does not cover more than 30"^ of the field ; that 

 it is bi-chromatic, or limited to red and violet, over an 

 annulus outside this central area ; and that it is limited 

 to light and shade from thence to the outermost limits of 

 the field. 



The nature and limitations of the colour-sense in man 

 long ago suggested to Thomas Young that the retina 

 might contain three sets of fibres, each set capable of 

 responding to only one of the fundamental colours ; or, 

 in other words, that there are special nerve fibres for 

 red, special nerve fibres for green, and special 

 nerve fibres for violet. It has also been assumed 

 that the differences between these fibres might essen- 

 tially consist in the ability of each set to respond 

 only to light-vibrations of a certain wave-length, much 

 as a tuned string will only respond to a note with 

 which it is in unison. In the human subject, so far as has 

 yet been ascertained, no optical differences between the 

 cones are discoverable ; but the analogy of the ear, and 

 the facts which have been supplied by comparative 

 anatomy, combine to render Young's hypothesis exceed- 

 ingly probable, and it is generally accepted, at least pro- 

 visionally, as the only one which furnishes an explanation 

 of the facts. It implies that elements of all three 

 varieties are present in the central portion of ths retina ; 

 that elements sensitive to green are absent from an 

 annulus around the centre ; and that the peripheral por- 

 tions are destitute of any elements by which colour-sense 

 can be called into activity. 



According to the observation already made, that the 

 highest degree of acuteness of vision is necessarily at- 

 tended by a corresponding acuteness of colour-sense, we 

 should naturally expect to find such a highly-developed 

 colour-sense in birds, many of which appear, as regards 

 visual power, to surpass all other creatures. I need not 

 dwell upon the often-described acuteness of vision of 

 vultures, or upon the vision of fishing birds ; but may 

 pass on to reir-ark that the acuteness of their vision 

 appears not only to be unquestionable, but also to be 

 much more widely diffused over the retina than is the 

 case with man. If we watch domestic poultry, or 

 pigeons, feeding, we shall frequently see a bird, when 

 busily picking up food immediately in front of its beak, 

 suddenly make a lateral dart to some grain lying sideways 

 to its line of sight, which would have been practically in- 

 visible to a human eye looking in the same direction as 

 that of the fowl. When we examine the retina the ex- 

 planation both of the acuteness of vision and of its dis- 

 tribution becomes at once apparent. In birds, in some 

 reptiles, and in fishes, not only are cones distributed over 

 the retina much more abundantly and more evenly than 



in man, but the cones are provided with coloured globules,, 

 droplets of coloured oil, at their apices, through which the 

 light entering them must pass before it can excite sensation, 

 and which are practically impervious to any colour but 

 their own. Each globule is so placed as to intervene 

 between what is regarded as the collecting portion of the 

 cone and what is regarded as its perceptive portion in 

 such a way that the latter can only receive colour which 

 is capable of passing through the globule. The retinas of 

 many birds, especially of the finch, the pigeon, and the 

 domestic fowl, have been carefully examined by Dr. 

 Waelchli, who finds that near the centre green is the pre- 

 dominant colour of the cones, while among the green cones 

 red and orange ones are somewhat sparingly interspersed, 

 and are nearly always arranged alternately, a red cone 

 between two orange ones, and vice versa. In a surround- 

 ing portion, called by Dr. Waelchli the red zone, the red 

 and orange cones are arranged in chains, and are larger 

 and more numerous than near the yellow spot ; the green 

 ones are of smaller size, and fill up the interspaces. Near 

 the periphery the cones are scattered, the three colours 

 about equally numerous and of equal size, while a few 

 colourless cones are also seen. Dr. Waelchli examined 

 the optical properties of the coloured cones by means of 

 the micro-spectroscope, and found, as the colours would 

 lead us to suppose, that they transmitted only the corre- 

 sponding portions of the spectrum ; and it would almost 

 seem, excepting for the few colourless cones at the peri- 

 pheral part of the retina, that the birds examined must 

 have been unable to see blue, the whole of which would 

 be absorbed by their colour globules. It would be neces- 

 sary to be thoroughly acquainted with their food in order 

 to understand any advantage which the birds in question 

 may derive from the predominance of green, red, and 

 orange globules over others ; but it is impossible to con- 

 sider the structure thus described without coming to the 

 conclusion that the birds in which it exists must have a 

 very acute sense of the colours corresponding to the 

 globules with which they are so abundantly provided, and 

 that this colour-sense, instead of being localized in the 

 centre, as in the human eye, must be diffused over a very 

 large portion of the retina. Dr. Waelchli points out that 

 the coloration of the yellow spot in man must, to a 

 certain extent, exclude blue from the central and most 

 sensitive portion of his retina. 



It is hardly necessary to mention how completely the 

 high differentiation of the cones in the creatures referred 

 to tends to support the hypothesis of Young, that a similar 

 differentiation, although not equally manifest, exists also 

 in man. If this be so, we must conclude that the region 

 of the yellow spot contains cones, some of which are 

 capable of being called into activity by red, others by 

 green, and others by violet ; that a surrounding annulus 

 contains no cones sensitive to green, but such as are 

 sensitive to red or to violet only ; and that, beyond and 

 around this latter region, such cones as may exist are 

 not sensitive to any colour, but, like the rods, only to 

 differences in the amount of light. When cones of only 

 one kind are called into activity, the sensation produced 

 is named red, green, or violet ; and, when all three 

 varieties are stimulated in about an equal degree, the 

 sensation produced is called white. In the same way, 

 the innumerable intermediate colour-sensations of which 

 the normal eye is susceptible, must be ascribed to stimu- 

 lation of the three varieties of cones in unequal degrees. 



The conditions of colour-sense which, in the human 

 race, or at least in civilized man, exist normally in outer 

 zones of the retina, are found, in a few individuals, to exist 

 also in the centre. There are persons in whom the 

 region of the yellow spot is absolutely insensitive to 

 colour, and recognizes only differences in the amount or 

 quantity of light. To such persons, the term " colour- 

 blind" ought perhaps in strictness to be limited ; but the 

 individuals in question are so rare that they are hardly 



