CHAPTEE IY. 



REFRACTION IN THE EYE. 



The eye as an optical instrument Laws of refraction, dispersion, etc., bearing 

 upon the physiology of vision Angle alpha Theories of light Theories 

 of colors Color-blindness Refraction by lenses Myopia and hyperme- 

 tropia Spherical aberration Chromatic aberration Formation of images 

 in the eye Area of distinct vision Punctum caecum Mechanism of re- 

 fraction in the eye Refraction by the cornea Refraction by the crystal- 

 line lens Simple, schematic eye Astigmatism. 



IT is simply impossible to obtain a clear idea of the physi- 

 ology of vision without having carefully studied the physio- 

 logical anatomy of the visual organs ; and, for this reason, we 

 have been as exact as possible and somewhat minute in our 

 description of the structure of the eye. If the student will 

 carefully study the anatomy of the parts, a very succinct 

 statement of some of the well-established laws of refraction 

 will render the physiology so simple that it will follow al- 

 most without explanation. 



In applying the laws of the refraction of light to the 

 transparent media of the eye, it is necessary to bear in mind 

 certain general facts with regard to vision, that have as yet 

 been referred to either very briefly or not at all. 



The eye is not by any means a perfect optical instrument, 

 looking at it from a purely physical point of view. This 

 statement, however, should not be understood as implying 

 that the arrangement of the organs of vision is not such as to 

 adapt them perfectly to the functions which they have to per- 

 form in connection with the proper appreciation of visual im- 

 pressions. By physical tests, it can be demonstrated that the 

 eye is not entirely achromatic ; but, in ordinary vision, the 



