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TEXT-BOOK OF PHYSIOLOGY. 



eye is accommodated for one set of rays, it is not for another, and the result is a 

 fringe of colors around the image. This defect in the normal eye is so slight 

 that the mind fails to take cognizance of it. That the eye is incapable simul- 

 taneously focalizing rays of widely different refrangiblity, as those which 

 give rise to the blue and red colors, is shown by the following experiment: 

 The eye being directed to a luminous point, a plate of cobalt-glass is 

 placed between the light and the observer close to the eye. This substance 

 has the property of intercepting all rays but the red and the blue and hence 

 these alone will be seen. The center of the image produced will be 

 red and clearly defined, the periphery blue and ill-defined. The reason 

 for this is clear. The eye more readily accommodates itself for the 

 red rays, and hence their focal point is distinct. The blue rays, having a 

 higher degree of refrangibility, come to a focus, cross and diverge, and give 

 rise to diffusion-circles. If a biconcave glass be placed before the cobalt, 

 the blue rays can be focalized on the retina, while the red will fall on the retina 

 without focalization. The image will now be blue and distinct in the center, 

 the periphery red and ill-defined. With the removal of the minus glass the 

 reverse condition again obtains. 



Imperfect Centering. From a purely physical point of view, the eye 

 is not a perfect optic instrument. In addition to the defects noticed in the 



FIG. 327. DIAGRAM SHOWING THE CORNEAL Axis D D, THE OPTIC Axis O A, THE VISUAL 



AXIS V L, AND THE LINE OF FIXATION V C\ ALSO THE THREE ANGLES, , 0, V. 



foregoing paragraphs, there is yet another, viz.: an imperfect centering of 

 the refracting surfaces. In first-class optic instruments the lenses are 

 centered that is, their optic centers are situated on the same axis. In 

 viewing an object through such a system the visual line corresponds with the 

 axis of the lens system. This is not the case with the refracting system of the 

 eye. A line passing through the center of the cornea and the center of the 

 eye, the optic axis (O A in Fig. 327), does not pass exactly through the 

 center of the lens and does not fall into the point for most distinct vision, the 

 fovea. This has lead to the recognition of other lines the relations of which 

 must be kept in mind in all optic discussions, viz. : 



i. The visual axis or visual line (V L), the line connecting the point viewed, 

 the nodal point, and the fovea centralis. 



