2 COLOUR VISION 



by the media of the eye, particularly the lens and retina. This " fluor- 

 escence " causes them to produce a lavender-hued sensation, which does 

 not denote true visibility of the short wave-length rays. Beyond the 

 red end are waves of greater length (extending to 60,000 /^/j), which when 

 absorbed cause a rise in temperature ; beyond the violet end are waves 

 of smaller wave-length, which are capable of causing chemical action. 

 So striking is the physiological phenomenon of the visibility of the 

 intermediate series that the heat rays are commonly spoken of as 

 " infra-red," and the actinic or chemical rays as " ultra-violet." This 

 custom is unfortunate, since it tends to obscure the importance 

 of the physical uniformity of the series. For example, not every 

 normal individual is able to see all the rays from 723 fifx to 397 /x/x ; for 

 most people the range is less extensive, roughly from 700 /x/x to 400 /x/x. 

 Again, though the ultra-violet rays are particularly potent in inducing 

 chemical action, the visible rays are also, but in less degree, actinic, 

 and the same is true, in still less degree, of the infra-red rays. Further, 

 all rays when absorbed cause a rise in temperature. The most convenient 

 and striking method of demonstrating actinicity is by the photographic 

 film, so that we have come to regard a photograph of the spectrum as a 

 complete analysis of the light under observation, too often forgetting 

 that the photographic effect varies with the specific sensitiveness of the 

 film to particular groups of waves. Thus it is only by specially sensitized 

 films, invented by Sir William Abney, that it is possible to demonstrate 

 infra-red rays photographically. 



It is further essential that the methods employed for analysis of the 

 light be suitable for their purpose. For example, an ordinary spectro- 

 scope, with glass prisms and lenses, suffices to demonstrate the visible 

 spectrum, but is almost useless for showing the ultra-violet rays, since 

 these are absorbed by the glass. In order to demonstrate the full 

 extent of the spectrum it is necessary to use a train of lenses and prisms 

 made of quartz or Iceland spar, which allows a maximum of rays to pass 

 unimpeded. 



Probably more error has crept into the subject of colour vision from 

 inexact description of experimental conditions and the nature of the 

 stimuli employed than from any other cause. Two green lights may 

 appear identical in colour to the eye, yet their physical characters may 

 differ widely. Again, mixing a yellow and a blue pigment will produce 

 a green pigment, yet the more general statement that green results 

 from mixing yellow and blue is not accurate. 



The complete range of simple colours can be obtained in a pure state 



