CHAPTEK VI 



DIOPTRIC MECHANISM OF THE EYE 



CONTENTS. 1. General anatomy of eyeball. 2. Formation of retinal images ; 

 underlying optical principles. 3. Optic constants of the eye. 4. Static refraction 

 of the eye (emmetropia and ametropia). 5. Refraction of the eye ; mechanism and 

 innervation of accommodation. 6. Far point and near point of clear vision ; range 

 and speed of accommodation. 7. Normal imperfections in the dioptric apparatus 

 of the eye. 8. Dioptric importance of the iris. 9. Mechanism and innervation 

 of pupil in accommodation ; theory of pupil -reflexes. 10. Absorption and reflec- 

 tion of light in the eye ; ophthalmoscopy and sldascopy. Bibliography. 



HEARING and Vision are the two highest senses, the best developed 

 and best differentiated both from the biological and from the 

 psycho-physical standpoint. Touch only makes us aware of the 

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

 contact with our skin ; hearing only enables us to perceive sonor- 

 ous vibrations at a variable distance ; vision informs us of objects 

 at vast, immeasurable distances, provided they give 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 light, 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 ^ (million ths 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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