CHAPTER XXVII 

 REFRACTION IN THE EYE ACCOMMODATION 



Refraction in the eye Certain laws of refraction, dispersion etc., bearing on the physiology of 

 vision Refraction by lenses Spherical monochromatic aberration Chromatic aberra- 

 tion Formation of images in the eye Visual purple and visual yellow and accommoda- 

 tion of the eye for different degrees of illumination Mechanism of refraction in the eye 

 Astigmatism Movements of the iris Direct action of light on the iris Accommodation 

 of the eye for vision at different distances Changes in the crystalline lens in accommoda- 

 tion Changes in the iris in accommodation Erect impressions produced by images in- 

 verted upon the retina Field of indirect vision Binocular vision Corresponding points 

 The horopter Duration of luminous impressions (after-images) Irradiation. 



IN applying some of the laws of refraction of light to the action of 

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

 general facts in regard to vision, that have as yet been considered very 

 briefly or have been omitted. The eye is not 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 arrange- 

 ment of the parts is not such as to adapt them perfectly to their uses in 

 connection with the appreciation of visual impressions. By physical 

 tests it can be demonstrated that the eye is not entirely achromatic ; but 

 in ordinary vision the dispersion of colors is not appreciated. There 

 is but a single point in the retina, the fovea centralis, where vision is 

 absolutely distinct ; and it is upon this point that images are made to 

 fall when the eye is directed toward any particular object. 



The refracting apparatus is* not exactly centred, a condition essen- 

 tial to the satisfactory performance of perfect optical instruments. For 

 example, in a compound microscope or a telescope, the centres of the 

 different lenses entering into the construction of the instrument are 

 situated in a straight line. Were the eye a perfect optical instrument, 

 the line of vision would coincide exactly with the optical axis ; but this 

 is not the case. The visual line a line drawn from an object to its 

 image on the fovea centralis deviates from the optical axis, in normal 

 eyes, to the nasal side. The visual line, therefore, forms an angle with 

 the optical axis, which is known as the angle alpha. This deviation of 

 the visual line is observed both in the horizontal and in the vertical 

 planes. The horizontal deviation varies by two to eight degrees, and the 

 vertical, by one to three degrees. Of course this want of exact centri* 



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