COLOUR- VISION 431 



frosted globe of a burning lamp or a white cloud on a 

 sunny day, through a pinhole in a card. When the card 

 is moved rapidly from side to side, but so as to keep the 

 pinhole always within the limits of the uidth of the piqiil, 

 the retinal blood vessels are " seen " as a fine branched 

 network of blaok lines in the bright field of vision. 



13. Sensations of Colour and Colour-blindness. — 

 ^^ e have spoken of the eye so far simply as the instru- 

 ment by which luminous sensations arise when the retina 

 is stimulated ; as an instrument which enables us to 

 appreciate the position of a source of light, and differences 

 in the intensity of the light which it emits or reflects, and 

 hence to perceive objects in the world around us as 

 regards their position, shape and size. But the objects 

 we see are characterised by something more than mere 

 shape and size ; they differ also in respect of what we 

 call their colour. 



When we look at a rainbow we are conscious of seven 

 broadly distinct kinds of colour-sensations ; these are red, 

 orange, yellow, green, blue, indigo-blue and violet, and 

 when ordinary white light is passed through a prism and 

 then allowed to fall into the eye we experience the same 

 seven coloured sensations. The prism has, in fact, resolved 

 the light into its several coloured constituents, and these 

 are known as the "colours of the spectrum." Each 

 colour which we recognise as such is characterised, just 

 as in the case of sounds, by certain qualities ; these are 

 (i) Hue, or colour as we ordinarily use the word to 

 denote what we call reds, greens, blues and so on. This 

 quality is dependent on the wave-length of the ethereal 

 vibrations which are giving rise to the sensation, and 

 hence corresponds to the "pitch" of a sound, (ii) In- 

 tensity or brightness. This depends on the amount 

 of light which falls on the retina in a given time and 

 corresponds to the loudness of a sound, (iii) Saturation, 

 or the amount of admixture with white light. Thus we 

 speak of a colour as being "pale" if mixed with much 



