118 Mr. B. Brudenell Carter [May 9, 



of activity. Few things are more curious tLan 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 effects of colour as one of the chief sources of 

 beauty in the world, it is manifest 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 fertilised ; and creatures which are 

 the prey of others find their best protection in a resemblance to the 

 colours of their environment. It is probably an universal truth that the 

 organs of colour-perception are more highly specialised, 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 lias 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 diffi- 

 cult 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 j^erimeter, it is bv no means easy to be quite sure 

 of the point at which the true colour of the advancing 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 visiting 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 [ 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 



