The Sense of Sight. 255 



room. The iris has circular muscle fibers that make the pupil smaller 

 when there is too much light for the eye, and when the light is feeble 

 the pupil opens wider. 



The Aqueous Humor. There is a small space between 

 the cornea and the iris. In this space is the clear, watery 

 aqueous humor. 



The Vitreous Hmour. All but the front part of the 

 space within the eye is fille with a clear, jelly-like sub- 

 stance, the vitreous humor. 



Fig. 97. The Formation of an Image on the Retina. 



The Crystalline Lens. Just back of the iris is a double- 

 convex lens, clear as crystal, and of about the consistency 

 of a gumdrop. It is less convex on the front surface. 



Experiment with Lens to show Inversion of Image. Take a 

 double-convex lens or any hand magnifier. Hold this up at the rear 

 of the room and catch the inverted image of the window on a piece of 

 paper held back of the lens. This illustrates how the image of an 

 external object is formed by the crystalline lens upon the retina of 

 the eye. 



Experiments to illustrate the Adjustment for Distance. (0 Stick 

 a pin at each' end of a book cover. Hold the book at about the usual 

 distance for reading, so that the two pins are in a line with the eye. 

 Look closely at the nearer pin, and the second pin will appear indistinct. 

 Now look closely at the head of the farther pin. The nearer one may 

 be seen, but not sharply. (2) Hold the tip of a pencil in a line with 

 any object, say a picture, on a wall opposite. In looking at the tip of 

 the pencil the picture is dim. Now look by the pencil at the picture, 

 and the point of the pencil will be blurred. 



Accommodation. We cannot, at the same time, see 

 distinctly a near and a distant object. When we look at 



