PREVENTION OF OPTICAL DEFECTS 283 



suiilighl these are tlie yellow rays (Fi<4'. 1), and other faint blurred 

 images are produced by the vioiet-blue-grceu which come to a 

 I'ocus in I'rout of the sensitive surface and by the red rays wliich 

 would focus behind the retina. 



Chromatic Differences of Magnification. Tlie fo\'ea, the area of 

 the retina most sensitive to light (q.v.), is eccentric and, therefore, 

 violet, blue and green images should appear smaller on it than 

 yellow and red ones. This manifest fault, however, acts as a 

 corrective to chromatic aberration. 



Comma. This is not shown by lens systems obeying the sine 

 law. Apparently the optical system of the eye fulfils this condition. 



Halation does not occur in the normal eye, as the reflecting 

 layer and the sensitive layer are practically identical. 



Flare. Table XXXVII. shows that the refractive indices of the 

 media bounding the lens differ only slightly from that of the lens 

 itself. Further, the R.I. of the peripheral parts of the lens is even 

 closer to the R.I. of the humours. Flare is therefore negligible. 



Irradiation. This does occur in the eye if the illumination is 

 very bright, and causes bad definition of objects. 



Scattered Light. In spite of the liberal supply of protective 

 pigment in the layer of the retina lying immediately under the 

 layer of rods and cones, some light is reflected back into the 

 viscus of the eye. It is this reflecte'd light that makes retinoscopy, 

 etc., possible. The reflected light, however, is in great part 

 absorbed by the pigment of the iris and by the insensitive anterior 

 portion of the retina. On the other hand, light entering the eye 

 at a great angle would first strike the anterior retina and be 

 reflected to the posterior sensitive part. 



The possibility of the entry of oblique rays is reduced by shelter- 

 ing the eye by the barriers of eyebrows, eyelids, eyelashes, cheeks 

 and nose, and by the smallness of the pupil when light is intense. 

 Under ordinary conditions one is not much troubled by scattered 

 light in the eye. 



Defects of the normal eye as an optical instrument. 



(i.) The eye is not perfectly centred, i.e. the axis of the lens 

 does not coincide with the axis of the cornea. 



(ii.) The optical axis passing through the centre of the cornea 

 and the centre of the lens does not coincide exactly with the 

 visual axis passing through the centre of the cornea and the 

 fovea centralis. 



Neither of these defects interferes appreciably with the accuracy 

 of vision 



(iii.) Slight degrees of astigmatism are almost always present. 



