NERVE 155 



at and the eyes closed ; a persisting image of the window- 

 may be seen. 



2. The Power of Localising the Source or Direction of 



Illumination. 



This may be determined by finding how close together 

 two separate stimuli may fall and still give rise to a double 

 sensation. Over the central spot two points of illumination 

 mav be very near, and still two sensations be experienced 

 (fig. 74). 



On passing to the more peripheral part of the retina, 

 where the cones are more scattered, the power of localising 

 decreases. 



3. Colour Sensation. 



1. Physics of Light Vibration. — Physically the various 

 colours are essentially ditferent rates of vibration of the 

 ether, and only a comparatively small range of these vibra- 

 tions stimulates the retina. The slowest acting vibrations are 

 at the rate of about 435 biUions per second, while the fastest 

 are not more than 764 billions — the relationship 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 

 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 moditied 

 by the brightness of illumination, and by admixture with 

 other parts of the spectrum (saturation). 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 pinker as it becomes less and less 

 saturated (Practical Fhysiology). 



2. Physiology of Colour Sensation. — (1) The peripheral part 

 of the retina in man is colour blind — is incapable of acting^ so 

 as to produce colour sensations. Yellow and blue can be 

 distinguished further out upon the retina than can red and 

 green. There is a zone of retina which is blind to red and 



