THE IRIS 527 



and violet rays. Now this point appears differently to the observer according 

 to the distance for which the eye is adjusted. If it is adjusted for the red rays, 

 there appears a red spot with a violet halo; if it is adjusted for the violet rays, 

 a violet point with a red halo. This will be evident from Fig. 218 if one 

 imagines the retina in the first case to be located at r and in the second at v. 

 This experiment is particularly beautiful if an electric incandescent lamp be used. 

 The colored circles of subjective origin (H. Meyer's ring's) which are per- 

 ceptible around a source of light under certain abnormal circumstances, as in 

 conjunctivitis, or after the effect of osmium vapor, are to be explained by the 

 diffraction of the light about dead epithelial cells, mucous corpuscles, etc., on the 

 surface of the cornea. A similar color phenomenon (Bonder's rings) is pro- 

 duced by diffraction at the edges of the lens, but this occurs normally only when 

 the pupil is greatly dilated (Salomonsohn). 



F. SUMMARY 



Summarizing the optical defects of the eye, we may say that while it 

 exhibits various defects which could not be permitted in a good optical instru- 

 ment, yet its capacity as an organ of vision is surprisingly little interfered 

 with. The oblique incidence of the line of vision, the difference in the re- 

 fraction between the different meridians of the cornea, the imperfect correc- 

 tion of spherical and chromatic aberration none of these nor all of these 

 together diminish the capacity of the eye to such an extent as to produce any 

 perceptible disturbances in vision. But this is true only of the normal eye. 

 It happens not infrequently that these defects exceed the normal limits and 

 then the eye must be described as a rather poor optical instrument. Often- 

 times in such cases, the optical properties of the eye can be very considerably 

 improved by practical treatment. 



5. THE IRIS 



In order that a proper image formed in a camera may not be interfered 

 with by light reflected from the inner walls, the latter are always covered 

 with a dull black color. The retinal pigment and the strongly pigmented 

 choroid coat serve the same purpose in the eye. 



The iris which is but the anterior prolongation of the choroid coat like- 

 wise has an important function. It has been shown that the laws of refraction 

 in an optical system hold good in case only such rays enter as form a very small 

 angle with the optical axis, and the peripheral rays are shut out. This exclu- 

 sion of the peripheral rays, so important for the clearness of the images, is 

 provided for by the iris. The pupil can be constricted or dilated by con- 

 traction of circular or radial fibers respectively in the iris. Such alterations 

 in the size of the pupil serve the optical requirements of the eye in two ways : 

 in near vision, if this is accompanied by convergence of the optical axes, the 

 pupil constricts and thereby contributes to the sharpness of the image; again, 

 when the asymmetry of the cornea is great, the resulting astigmatism is 

 counteracted to a certain extent by constriction of the pupil. The pupil has 

 in addition the important function of protecting the retina from too intense 

 a light; it constricts in strong light and dilates in weak light. 



