ELEMENTARY EXPERIMENTAL PHYSIOLOGY 17 



conclusions as to the relative curvatures of the different surfaces. 



The bright clear erect image is formed from the anterior surface 

 of the cornea. The small sharply-defined inverted image is formed 

 from the back of the crystalline lens. The large, dim erect image 

 which seems to lie behind the first is formed from the anterior 

 surface of the crystalline lens. These, then, are the three refracting 

 surfaces of the eye at which rays of light are bent so that they may 

 be focussed on the retina. 



Why is no image formed from the posterior surface of the cornea ? 



The refractive indices of the media are : 



Cornea . .1-33 Lens . .144 



Aqueous . . 1'33 Vitreous . . 1-33 



From the results of these observations, make a diagram of the 

 physiological lens of the eye. 



3. Can Near and Far Objects be seen at the same Time ? 



METHODS. (i) Close one eye and fix the other on the far corner 

 of the room, and then hold up a pencil at about a foot from the eye 

 and see if at the same time both objects can be distinctly seen. 

 Another student should note any change in the pupil when the eye 

 is directed to the pencil. 



(ii) Schemer's Experiment. Make two holes in a horizontal line 

 in a sheet of paper so near that they both fall within the diameter 

 of the pupil. Now stand at about two or three yards from a wall 

 on which a small vertical line is drawn and look at it through the 

 holes. While keeping the eye fixed on the line, bring a needle 

 vertically in front of the holes at about 8 inches from the eye, 



FIG. 6. To show the formation of images in Schemer's Experiment. 

 (From Nogl Patera's Essentials.) 



and note the appearance of the needle when the distant line is 

 looked at, and of the line when the needle is looked at. 



Make a diagram of the experiment and formulate the conclusions 

 to be drawn. 



4. Is the Power of Focussing Limited or Unlimited ? 



METHOD. Bring the point of a pencil held vertically nearer and 

 nearer to the eye ; a point is reached within which it cannot be 

 distinctly seen the near point. Measure the distance of this from 

 the eye and record it. 



5. What Change takes place in the Eye in Near Vision ? 

 METHOD. Repeat the experiment on the refracting surfaces 



of the eye (p. 16, 1.2), when the observed eye is looking at a distant 

 and at a near object. 



C 



