190 ADVANCED LESSONS IN PRACTICAL PHYSIOLOGY 



cobalt glass permits only the red and violet rays to pass. In accom- 

 modating for the red rays a violet halo is obtained and on accommodat- 

 ing for the violet rays, a reddish halo. 



On a black background place a strip of red paper and one of blue 

 paper. The red appears nearer than the blue. Since the red rays are 

 less refrangible, a greater effort at accommodation is required to focal- 

 ize them. This gives rise to an erroneous judgment of distance. 



Place a heart colored red upon a bright blue sheet of paper. In a 

 room lighted only by a candle hold the heart below the level of the eyes 

 and move it gently from side to side. The red heart will then appear 

 to flutter over the blue background. 



3. Spheric Aberration. Construct an emmetropic eye. Note that 

 while the focal point upon the screen seems perfectly sharp, a close 

 observation shows that this "point" is really drawn out backward. 

 This distortion is due to the development of caustics, i. e., the rays of 

 light traversing the marginal zone of the lens are focalized somewhat 

 behind those traversing its central area. Correct this defect by cover- 

 ing the peripheral sphere of the lens with a circle cut out of paper. How 

 is spheric aberration prevented in our eye? 



4. Mechanical Stimulation of the Retina. Phosphenes. Shut one 

 eye and turn it inward. With the point of a pencil press gently upon 

 the outer surface of the upper eyelid. To what part of the field of vision 

 is the yellowish circular image referred. Explain. 



Place a light in front of the eyes. Close them and move them 

 quickly from side to side. When the eyes reach an extreme position, 

 observe the rapidly disappearing bluish spot surrounded by a yellow 

 halo. Obviously, this movement stimulates the retina around the 

 optic papilla in a mechanical way. Note that mechanical stimuli are 

 inadequate stimuli. While they produce visual sensations, they can- 

 not give visual concepts. Moreover, all retinal impressions are al- 

 ways projected into the opposite visual field, i. e., into that part of 

 the field with which that particular area of the retina is in functional 

 relation. 



5. The Field of Vision. Fasten a sheet of white paper 60 cm. 

 square upon a piece of pasteboard and make a small cross mark about 

 30 cm. to the right of the left margin of the paper. Let the subject 

 rest his chin upon an iron support, and adjust the paper in such a way 

 that his right eye is in a direct line with the cross mark. Move the 

 vertically placed paper close enough so that he cannot look beyond its 

 edges. Let the experimenter fasten a small piece of white paper to a 

 straw, move it horizontally inward from without, and mark on the white 

 sheet the point at which this object becomes clearly perceptible to the 

 person. Determine in this way also the boundaries of the visual field 

 of this eye in the vertical direction, and in two or three intermediate 

 directions. Outline the inner side of the field in a corresponding man- 

 ner. Connect the different pencil marks to form a continuous line. 

 The visual field of the left eye may then be mapped out in a similar 



