SUGGESTIONS FOR LABORATORY WORK 605 



a candle flame, and a screen in a darkened room. .The visual de- 

 fects of myopia and hypermetropia (p. 262) may be illustrated by 

 shifting the screen in such a manner as to throw the image on it 

 out of focus. When the screen is too far away from the lens for 

 the image to be clear the situation is as in myopia. A double con- 

 cave lens placed in the path of the rays illustrates the correction 

 for this defect. Throwing the image out of focus by bringing the 

 screen too near the lens gives the situation seen in hypermetropia. 

 This may be corrected with a double convex lens. 



SOME PHENOMENA OF VISION 



Visual Reference. The eye learns by experience to refer visual 

 stimuli outward through a point called the "nodal point" (p. 267). 

 This point is within the crystalline lens, three-quarters of the dis- 

 tance from retina to cornea. 



Schemer's Experiment. The fact that images on the right side 

 of the retina are interpreted as coming from objects to the left of 

 the visual axis, and vice versa, was demonstrated by Scheiner in 

 1619. 



Pierce a card with two pin holes about one-tenth inch apart. 

 Look through the pin holes at a distant object. Place in the line 

 of vision about a foot from the eye a pin mounted in a block. Do 

 not accommodate for the pin. Two images of the pin are seen. 

 Slide a card over the right hand pin hole. The left hand image 

 disappears. 



Place the pin four feet from the eye. Look through the pin 

 holes at a second pin in line with the first one, but only four to six 

 inches distant from the eye. Two images of the far pin are seen. 

 Slide a card over the right hand pin hole. Now the right hand image 

 disappears. For the explanation see diagram, Fig. 158. 



The Blind Spot. Make a small black spot near the left margin 

 of a sheet of note paper. Place the paper on the desk. Let one 

 student of a pair look fixedly at the spot with the right eye, holding 

 the head stationary, about twelve inches, over the spot. The other 

 member of the pair should move a black-headed hatpin from a 

 point in the right margin of the paper directly opposite the black' 

 spot toward the spot itself. The subject should report the instant 

 the head of the pin disappears, and the place should be marked 



