NEURO-MUSCULAE MECHANISM 131 



2. The Power of Localising- the Source or Direction of 

 Illumination. This may be determined in the same way as 

 in studying the sense of touch by finding how near two 

 points may be stimulated and still give rise to a double 

 sensation. 



3. Colour Sensation Physics of Light Vibration. Physi- 

 cally the various colours are essentially different rates of 

 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 illumination, 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 light fades. Again, a pure red when diluted 

 with all the spectrum i.e. with white light becomes pink 

 as it becomes less and less saturated. (Practical Physiology.} 



Physiology of Colour Sensation. 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 (fig. 62). 

 {Practical Physiology.) 



2. While the various sensations which we call colour are 

 generally produced by vibrations of different lengths falling 



