The Special Senses 



251 



OBJECT 



EYE 



393. Inner Structure of the Eye. Let us imagine an 

 eyeball divided through the middle from above downwards. 

 Let us now start in front and observe its parts. We come 

 first to the cornea, 

 which has just 

 been described 

 (Fig. 129). The 



iris forms a sort 1Ncnvt \\\ CHAMBER 

 of partition, divid- 

 ing the cavity of 



the eyeball into FIG. 130. Diagram illustrating the Manner in which 



two chambers tne I ma e f an Object is brought to a Focus on 



,-r,, the Retina. 



1 he anterior 



chamber occupies the space between the cornea and the 

 iris. It is filled with a thin, watery fluid called the aqueous 

 humor. 



Experiment 112. The retina is not sensitive where the optic nerve 

 enters the eyeball. This is called the "blind spot." Put two ink 

 bottles, about two feet apart, on a table covered with white paper. 

 Close the left eye and fix the right steadily on the left-hand inkstand, 

 gradually varying the distance from the eye to the ink bottle. At a 

 certain distance the right-hand bottle will disappear, but nearer or 

 farther than that, it will be plainly seen. 



Experiment 113. Close one eye and look steadily at the small a in 

 the figure below. The other letters will also be visible at the same 

 time. If now the page be brought slowly nearer to the eye, while the 

 eye is kept steadily looking at the small a, the large A will disappear 

 at a certain point, reappearing when the book is brought still nearer. 



oAx 



On the reappearance of the A it will be noted that it comes into view 

 from the inner side, the x being seen before it. If now we move the 

 book back towards its original place, the A will again disappear, com- 

 ing again into view from the outer side when the o is seen before it. 



