PART II 



VISION 

 BY H. HARTRIDGE. 



PAOM 



SECTION 1 .Properties of light, colour, and the spectrum . 4s<i 



2. Orbital cavity and its contents . .493 



3. Eyeball, its histology. Pupil reflex . . . . .500 



,, 4. Nutrition and protection of the eyeball 514 



tt 5. Optical media of eye, and accommodation . 519 



6. Optical properties and defects of the eye ... .529 



,, 7. Retina, its histology and physiology . 540 



,, 8. Response to light and colour 



tt . 9. Subjective phenomena of vision . ri!i 



ff 10. Defects of vision, and their detection . 



] l t Duplex theory and hypotheses of colour vision 



M 12. Binocular and stereoscopic vision . . 588 



SECTION I 

 PHYSICAL PROPERTIES OF LIGHT 



LIGHT is a form of energy, and consists of electro -magnetic waves which travel with 

 great velocity through the ether. Since we receive light from the stars, we conclude 

 that the ether permeates the whole of space. We know Also from electrical experi- 

 ments that the ether also permeates matter. We might expect therefore that light 

 would freely pass through matter, or in other words that all matter would be trans- 

 parent. This is not the case however, because most forms of matter have the property 

 of absorbing light energy; and therefore the property of transparency is relatively 

 rare. 



LIGHT IS A FORM OF WAVE MOTION. Because of this, one of its char- 

 acteristic properties is amplitude. But since this depends on the amount of light 

 energy present, it is equivalent to what is known as intensity. The other characteristic 

 property of wave motion is wavelength. In the case of ordinary light it is found by 

 experiment that a whole gamut of waves varying greatly in length is present. Those 

 falling between certain limits are able to stimulate the eye, and are therefore called 

 visual rays. These limits a.re slightly less than 8000 Angstrom units* for the longer 

 limit and 4000 A.U. for the shorter. Kays whose wavelength falls outside these limits 

 are invisible to the eye, and are called infra-red rays when they are too long, and 

 ultra-violet when they are too short. Since the infra-red rays are able to stimulate 

 the sensory end organs of the skin, which respond to heat, they are also called heat 

 while the ultra-violet rays from their ability to perform certain chemical. re- 

 actions, and notably those used in photography, are called actinic rays. There is 

 however no sharp line of demarcation between the three groups, which the use of 

 the.sr terms might be thought to imply. 



THE SPECTRUM. It is possible by suitable apparatus to cause the constituent 

 rays in a beam of light to arrange themselves neeunling to their wavelength. When 

 thus arranged they are said to form a spectrum. The apparatus is therefore called 

 a spectroscope. The visible lays thus arranged are seen as a coloured band which has 



* An Angstrom unit=one ten-millionth of a millimeter. 

 486 



