viz EETINAL EXCITATION 351 



differences of intensity, that is, by different degrees of brightness ; 

 the latter present both quantitative and qualitative differences, 

 represented by the different colours. 



There are a great number of coloured sensations, each of which 

 may not only present different degrees of brightness, but may also 

 differ from colourless sensation according to their degree of satura- 

 tion. The greater this difference, the more the colour is said to 

 be saturated ; the less it is, the paler the colour. Thus we may 

 have bright-saturated, or dark-saturated, bright-pale, or dark-pale 

 sensations of colour. 



The physiology of the visual sensations consists fundamentally 

 in determining the relations between the varying intensity and 

 quality of the sensations and the quantitative and qualitative 

 differences in the specific stimuli by which they are produced. 



We know from physics that the white light given out by the 

 sun consists of a complex of ether vibrations of different wave- 

 lengths, which by means of a prism can be resolved into the so- 

 called solar spectrum, owing to the different refrangibility of the 

 rays of which it is composed. These rays do not appear to us 

 white, like sunlight, but coloured. The colours of the spectrum are 

 im the following order red, orange, yellow, green, blue, indigo, and 

 violet. Eed is due to the least refrangible rays ; violet to the most 

 refrangible. The intermediate colours are due to rays of which the 

 refrangibility increases gradually in the series from red to violet. 



The degree of refrangibility of the different rays depends on 

 their different rate of velocity through solid or fluid media. They 

 further differ in the number of their vibrations and in their wave- 

 lengths. Measured in ^ ( = '0001 //,), this varies as follows for 

 the different simple colours : red = 760-647 ; orange = 647-586 ; 

 yellow = 586-535; green- 535-492; blue = 492-456 ; indigo = 

 456-424; violet = 424-397 (Fig. 172). 



There is a continuous series of gradations between the colours 

 of the spectrum, for the expression of which no simple and uni- 

 versal terms exist in language. But the number of colours and 

 hues represented in our spectrum does not include the whole of 

 the colour impressions our eyes are capable of perceiving : there is 

 no shade of purple, which is produced by the mixture of red and 

 violet, that is, of the two end-colours of the spectrum. 



The solar spectrum is not confined to the portion that our eyes 

 can appreciate. Beyond the red there are rays of greater wave- 

 length than 760 ^ (ultra-red rays), and beyond the violet, rays 

 of lesser wave-length than 397 /*/* (ultra-violet rays}. The 

 former are heat rays, the presence of which can be detected by 

 means of a thermo-electric couple ; the latter are chemical or 

 actinic rays, the existence of which is revealed by the chemical 

 effects which they produce upon certain salts of silver. 



On the other hand, the thermal, photic, and chemical action of 



