608 A TEXTBOOK OF PHYSIOLOGY 



Their dendrites pass outwards to arborize in the inner molecular 

 layer; their axons pass in towards the optic nerve, forming 



9. The layer of non-medullated nerve-fibres (Fig. 340). 



10. The internal limiting membrane is formed by the expanded 

 bases of the supporting fibres of Miiller. 



SECTION II 



LAWS OF DIOPTRICS 



The generally accepted hypothesis of the phenomena of light is 

 that first enunciated by Thomas Young namely, that light is a mode 

 of motion of the ether which pervades space. By the molecular 

 movements of luminous bodies the ether is caused to vibrate in a 

 series of waves, forming a ray of light. The component particles of the 

 ray move at right angles to the direction in which the ray is travelling, 

 just as do the particles of water in the waves caused by disturbing 

 the smooth surface of a pool. 



Waves of big amplitude give rise to a sensation of bright light, 

 small waves to one of dim light. All the waves are of very rapid rate 

 of vibration 435,000,000 to 764,000,000 per second. Outside these 

 limits the eye is not stimulated, although there are present both 

 infra-red and ultra-violet rays. Beyond the ultra-violet rays come 

 the Rontgen rays, and below the infra-red the electrical waves in 

 unbroken sequence. The light rays, with the low rate of vibration, 

 give rise to the sensation of red, those with the high rate to that of 

 violet. In between we have the colours orange, green, blue, indigo, 

 gradually merging into one another. Ordinary sunlight, as Isaac 

 Newton showed, is composed of this series of colours blended together. 



Light travels through the ether at about 190,000 miles per second. 

 A ray, falling upon a polished surface, is reflected, the angle with the 

 perpendicular of the reflected ray being equal to that which the 

 incident ray makes with the same perpendicular. 



When a ray passing through one transparent medium, such as air, 

 meets perpendicularly another medium, such as water, part of it 

 passes into the new medium, and part is reflected upon itself. If, 

 however, the ray meets the new medium obliquely, the part which 

 passes into the medium is bent out of its course, or refracted. This 

 refracted ray is in the same plane as before, and in the case of a ray 

 passing from a rarer to a denser medium the deflection is toward the 

 perpendicular. The law for single refraction is that, whatever the 

 obliquity of the incident ray, the ratio of the sine of the incident 

 angle to the sine of the angle of refraction is constant for the same 

 two media, but varies with different media. This ratio of the angles 

 is known as the index of refraction, and, knowing this index for any 

 two media, the direction which will be taken by a ray of light can 



