SECTION II 



THE SPECTRUM AS SEEN BY THE LIGHT-ADAPTED 



(PHOTOPIC) EYE 



CHAPTER I 



THE SPECTRUM : HUE, LUMINOSITY, SATURATION 



If a pure spectrum, e.g. that of the arc light, of moderate intensity 

 is observed a band of colours is seen. Of these, four are clearly defined 

 as separate and distinct from each other, viz. red, yellow, green and 

 blue, the red region consisting of the least refracted rays, the blue of the 

 most refracted. Between the red and yellow we distinguish a region 

 which is called orange. The gradation from red to yellow is gradual 

 and it will be generally admitted that orange partakes of the natures 

 of both red and yellow psychologically, the red element diminishing as 

 we pass from red to yellow and the yellow element correspondingly 

 increasing. Between yellow and green a somewhat similar gradation 

 occurs, the yellow gradually becoming more and more tinged with green 

 until we fail to recognise any yellow at all and the colour gives the 

 impression of pure green. Passing further towards the blue an inter- 

 mediate green-blue region is met with, showing the same gradual 

 transition until the blue no longer gives any impression of green. Pass- 

 ing beyond the blue we gradually come to a region in which the pre- 

 dominant sensation is still of the order " blue," but it is not pure blue. 

 It is called violet. Now violet is a colour which occurs rarely in nature. 

 There is, however, a colour in nature which is often called violet, but 

 which is really purple. True purple does not occur in the spectrum, 

 but it can be obtained by mixing pure red light with pure blue light, 

 and we can pass from blue to red through violet and the mixtures of 

 blue and red which are called purple and carmine. 



We have thus travelled in a circle and returned to the original 



