306 STUDIES IN PHYSIOLOGY 



A study of Fig. 140 will make clear the explanation of 

 these facts we have been observing. In all three positions 

 of the book (A, B, C) the rays of light pass from the cross 

 in a straight line through the cornea, aqueous humor, lens, 

 and vitreous humor, and reach a point on the retina directly 

 behind. This is the region of keenest vision, for here the 

 retina is thinnest and the cones are most numerous. Since 

 the color of this portion of the retina is yellow, this is called 

 the yellow spot. 



In position A the rays of light from the black dot enter 

 the eye obliquely, and strike the retina outside the yellow 

 spot ; hence the image is not as clear as is that of the cross. 

 In position B the image of the dot falls upon the interior 

 end of the optic nerve ; here rods and cones are wanting, 

 no stimulus is sent to the brain, and we therefore call this 

 the blind spot. When the page is brought to position (7, the 

 light rays from the dot again fall upon the retina, and the 

 dot reappears to our vision. 



Defective Eyes. A normal, healthy eye has the power of 

 adjusting itself so that objects become visible which are 



w ithin five to ten inches, or those 

 which are as far away as a dis- 

 tant horizon. Many people, 

 however, find that they can see 

 objects near at hand much more 

 clearly than those at a distance ; 

 in other words, they are near- 

 sighted. Others, on the other 

 hand, are far-sighted. These 

 defects in vision are due to im- 

 perfect formation of the eyeball, 

 IG. ltt._Tert^ Astigma- and can be corrected Qnly by the 



use of eyeglasses or spectacles, 



which should be purchased only after a careful examination 

 of the eyes has been made by a specialist. 



Another very common defect of the eye is known as a-stig' 



