ELEMENTARY EXPERIMENTAL PHYSIOLOGY 19 



A knowledge of the origin and insertion of the ciliary muscle 

 explains how this change is brought about. 



Imperfections in the Dioptric Mechanism of the Eye. Use some 

 modification of Kiihne's artificial eye with a candle about 3 feet 



FIG. 10. To show the way in which rays of light from the far point and 

 near point of vision are focussed in the normal (Emmetropic), in the 

 old sighted (Presbyopic), in the long sighted (Hypermetropic), and in 

 the near sighted (Myopic) eye. 



(From No61 Paton's Essentials.) 



in front of it so that nearly parallel rays enter the eye. Adjust 

 the screen so as to get a clear image of the candle. Now 

 by moving the screen study the effect of making the eye (1) too long 

 and (2) too short upon (a) the image 

 formed, (6) the distance to which the 

 candle must be moved to give a 

 clear image. Which condition gives 

 short-sightedness (Myopia) and which 

 long-sightedness (Hyp&rrmtropia) ? 



Using Clark's model with (1) a 

 cylindrical lens convex from above 

 downwards, (2) and one convex from 

 side to side, throw the image of a 

 cross upon the screen. Note the 

 character of the image. Now slide 

 (2) in front of (1) and note the 

 change in the image. 



This represents in an exaggerated 

 form the result of any of the 



FIG. 11. To show that in the 

 astigmatic eye rays from ver- 

 tical and horizontal lines are 

 not focussed simultaneously 

 upon the retina. 



(From Noel Paton's Essentials.) 



refracting surfaces of the eye having its curvature unequal in 

 different planes. This is the condition of astigmatism. 



Using the card provided, test your eye for astigmatism. If this 



