THE PRINCIPLES OF VISION 187 



A plus lens is neutralized by placing a minus lens of 

 equal ''strength" before it. For example, place a +1.00 

 D. S. before a — i.oo D. S. and it has the effect of a glass 

 whose sides are parallel. 



When the rays of light enter the eye from an object 

 at infinity, that is, from a distance of 20 feet or more, 

 the normal eye should be at rest, and the object will be 

 ''focused" or formed sharply upon the macula. The 

 image on the retina is inverted. The rays cross at a 

 point which is, approximately, in man 15 mm. anterior 

 to the retina and 5 mm. posterior to the cornea. This 

 is according to a schematic eye devised by Bonders. 

 (These distances would be relative in animals' eyes, 

 according to the size of the eye.) An object i meter 

 long vertically, placed at 15 meters distance from the 

 eye, would produce a retinal image in vertical measure- 

 ments, I mm. The size of the retinal image is influenced 

 by the variations of the visual angle, and the latter varies 

 according to the size and distance of the object from the 

 eye or the optical center of the lens. 



The acuity of vision is the ability to see objects of a 

 certain size and at a certain distance distinctly. This 

 depends upon a normal visual apparatus and proper 

 light. Under normal conditions the visual acuity of 

 animals of a kind should be the same. A bird, however, 

 can see a grain or creeping thing at a much greater 

 distance than can a cow or horse. They, therefore, 

 have a greater visual acuity. Man can count the bricks 



