SECTION V 

 SPECIAL SENSES. 

 CHAPTER XVII. 



VISION. 



In order to gain accurate information through the sense of sight 

 about objects in the external world, their size and shape, and 

 their positions relative to one another, two things are necessary. 

 There must in the first place be a group of cells sensitive to light 

 waves and so connected to the central nervous system that stimu- 

 lation of any part of the sensitive surface gives rise to a sensation 

 different from that caused by stimulation of any other part. 

 Secondly some system of lenses is needed so that each point in 

 the sensitive layer does not receive rays from all directions in the 

 external field, but has focussed on it only those which arise from a 

 single part of the field. In the eye the sensitive elements are con- 

 tained in the retina, and the cornea and lens together make up the 

 focussing apparatus. In the discussion and experiments which 

 follow we shall first consider the way in which light waves are 

 brought to a focus in the eye and later the response of the retina 

 to them. Before taking up the complex arrangement of refracting 

 surfaces which exist in the eye it is well to review some of the simple 

 cases of the formation of images both by lenses and by mirrors. 

 Although the focussing in the eye is entirely done by refracting 

 surfaces and reflection plays no part in it, it is by reflected rays 

 that one examines the condition of the eye and on this account 

 it is necessary to have clearly in mind the laws which underlie the 

 reflection, as well as those of refraction. 



Physiological Optics. 

 Reflection. 



When rays of light diverging from a point are reflected by 

 a mirror their course is so changed that they appear to an observer 



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