120 



HUMAN PHYSIOLOGY 



vibration of the ether, and only a comparatively small range 

 of these vibrations stimulate the retina. The slowest acting 

 vibrations are at the rate of about 435 billions per second, 

 while the fastest are not more than 764 billions the relation- 

 ship of the slowest to the fastest is something like four to 

 seven. The apparent colour of objects is due to the fact 

 that they absorb certain parts of the spectrum, and either 



transmit onwards other parts, 

 or reflect other parts. The 

 vast variety of colours which 

 are perceived in nature is 

 due to the fact that the pure 

 spectral colours are modified 

 by the brightness of illumina- 

 tion, and by admixture with 

 other parts of the spectrum. 

 Thus a surface which in bright 

 sunlight appears of a brilliant 

 red, becomes maroon, and 

 finally, brown and black, as the 



4 



A g am > a P Ure 



of the retina (Colour Perimeter). re when diluted with all 

 A indicates the extent of retina fch spe ctrum i.e. With white 

 stimulated by white and black ; . r . 



J5, the part also capable of stimula- light becomes pink as it be- 

 tion by blue and yellow; and C, comes less and less saturated. 



of n>y"ii<*y of Colour Sensa - 



tion. 1. The peripheral part 

 of the retina is colour blind is incapable of acting so as to 

 produce colour sensations. This may be shown by means 

 of the perimeter and coloured chalks. Until the chalk is 

 brought well within the field of vision its colour cannot 

 be made out. As the image of the chalk travels in along 

 the retina it is found that yellow and blue can be dis- 

 tinguished before red and green that is, that there is a 

 zone of retina which is blind to red and green, but which 

 can distinguish blue and yellow. Only the central part 

 of the retina is capable of being stimulated by all colours. 

 These zones are not sharply defined, and vary in extent 

 with the size and brightness of the coloured image. (Ex- 

 periment.) (Fig. 58.) 



Fio. 58. Distribution of Colour Sensa- , -, . r , 

 tion in relationship to the surface h g ht fades - 



