CHAPTEK VI 



MKMI. \\ISM OK THE KVK 



CONTKNTS. --1. Criioral anatomy of eyeball. '_'. Formation of retina] images; 

 underlying optical prinoiples. '!. Optic constants of the eye. i. statir nTrartiun 



of the eye (?mni<-/i-o/n'<i and .i//i.//-ry/<\ 5. Refraction of the t-yr : nirrhaniMii and 

 inin Tvation of accommodation, *>. Far point ami near point of clear vision ; ran<, r e 

 and speed of accommodation. 7. V>imal inipciTrrtion~ in tin- dioptric- apparatus 

 of the rye. 8. Dioptric importan'-r nf tin- in*. '.'. Mechanism and inncrvalioii 

 of jmpil in a ...... innioilatioii -, theory of pupil - reflexes. 10. Absorption and rdlrc- 



ot' li;, r lit in (lie rye : ophthalmoscope and skiavnpy. Bibliography. 



HEAUINI; ami Vision arc tlio two higlir-t senses tin- best, developed 

 and best, differentiated both from tin- biological and from the 

 psycho-physieal standpoint. TH<-}I only makes us aware of the 

 existence of surh bodies in the external world as come into direct 

 contact with our skin ; Jn-uri n<i only enables us to perceive sonor- 

 ous vibrations at a variable distance : ri*i<>n informs us of objects 

 at vast, immeasurable distances, provided they -ive out, or reflect, 

 light. While the adequate stimulus of auditory sensations 

 consists in the vibrations of elastic bodies between relatively 

 narrow limits of frequency and intensity, the adequate stimulus 

 of visual sensations is represented by ///////, that is (according to 

 physicists) by the vibrations of an imponderable medium, at a 

 frequency of 480-760 trillions per second, while its wave-length 

 is comprised between 700 and 430 /*// (millionth^ of a milli- 

 metre). These very rapid ethereal vibrations penetrate the 

 transparent media of the eye and stimulate the sensitive terminal 

 elements of the retina, and we are then, owing to the marvellous 

 structure of the visual organ, able not only to recognise light 

 and colour but also to estimate the form, size, position, and 

 structure of the surrounding bodies. 



I. Each eyeball is an elastic, almost spherical body. As a 

 whole it constitutes the peripheral sense-organ of the optic nerve. 



The earlier anatomists described the human eye as an organ 

 composed of three concentric coats and three fluids. These were 

 the humor aqueus (which still bears the same name), the humor 

 crystallina (now known as the crystalline lens), and humor vitreus 

 (vitreous body). 



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