1890.] on Colour- Vision and Colour-Blindness. 117 



zone immediately around this yellow si^ot each cone is surrounded by a 

 single circle of rods [s]iow7i] ; 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 [shown]. 

 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 arrange- 

 ment 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 exqui- 

 sitely finished, while the parts around the centre are only roughly 

 sketched in. We are conscious tbat these outer j)arts 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 com- 

 pletely neutralised by the quick mobility of the eyes, and by the 

 manner in which they range almost unconsciously 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 [sAoii;w], which enables the observer to look steadily at a 

 central spot, while a second spot or other object is moved along an arc, 

 in any meridian, from the circumference of the field of view towards 

 the centre, or vice versa. Slight differences will be found between 

 individuals ; but, speakirg generally, a capital letter one-third of an 

 inch high, which is legible by direct vision at a distance of sixteen 

 feet, and is recognisable 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 differ- 

 ences 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 thiit which I have the honour to 

 address, it is unnecessary to say more about colour than that it de- 

 pends 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 majority of 

 the human race, suffices to excite the retina to a characteristic kind 



