1890.] on Colour-Vision and Colour-Blindness, 119 



seen by the degree of tlieir luminosity — that is, they appear as light 

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

 Speaking generally, therefore, 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 funda- 

 mental 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 difi'erences between these 

 fibres might essentially 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 differ- 

 ences 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 exceedingly probable, and it 

 is generally accej)ted, at least provisionally, 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 the retina ; that 

 elements sensitive to green are absent from an annulus around the 

 centre ; and that the peripheral portions 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 attended by a correspond- 

 ing 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 remark 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 

 invisible to a human eye looking in the same direction as that of 

 the fowl. When we examine the retina, the explanation both of 

 the acuteness of vision and of its distribution 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 



