290 PHYSIOLOGY. 



The Ciliary Muscle. Arising from the sclerotic coat, 

 just within the outer border of the iris, is the ciliary muscle. 

 It is inserted in the margin of the lens capsule by means 

 of fibrous strands that form an intimate part of the capsule. 



Experiment with Lens to show Inversion of Image. Take a 

 double-convex lens, two ot which are in the common " tripod lens, 1 ' or 



Fig. 87. The Formation of an Image on the Retina. 



any hand magnifier. Hold this up in front of a window and catch the 

 inverted image o/ the window on a piece ot paper held back of the lens. 

 This illustrates how the image ot an external object is formed by the 

 crystalline lens upon the retina ot the eye. If two lenses of different 

 thickness can be obtained, it will be seen that the thicker lens (if both 

 have the same diameter) will make an image closer to the lens than the 

 thinner one. 



Experiments to illustrate the Adjustment for Distance. (i ) 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. 



Adjustment of the Lens for Seeing at Different Dis- 

 tances. If we look up from a book we are reading, we 

 do not realize that any change is necessary in the eye for 

 us to see a distant object. But the above experiments 

 prove that we cannot, at the same time, see distinctly a 

 near and a distant object. When the photographer places 

 his camera, he moves the ground-glass plate back and forth 

 till the image is distinctly formed on the plate. We cannot 



