760 THE SENSES 





 Construct a diagram showing the path of the light in the formation of 



the image in these cases. 



If the measurement in the above case is made through two parallel open- 

 ings or diaphragms about 5 mm. in diameter each, and separated by i or 

 2 cm., the point of focus can be more accurately determined (see Schemer's 

 Experiment, No. 14). Construct the mathematical figure showing the 

 course of both cones of rays in this test. 



10. Determination of Near and Far Limits of Vision. Support a 

 meter stick in a horizontal position at a comfortable level for the eye. Mount 



FIG. 482. Diagram of Experiment to Ascertain the Minimum Distance of Distinct Vision. 



a needle in a cork and set it on the meter stick about 25 cm. in front of the 

 eye. Make two pin-holes in a card at a distance of about 2 mm. from each 

 other. Hold this card with the pin-holes close in front of the right eye, and 

 bring the eye up to the end of the meter stick; cover the other eye. Ob- 

 serve that when the needle is brought nearer and nearer to the eye, at a 

 certain distance it becomes double. Determine this distance very accurately. 

 It is the near-point of accommodation for the right eye. Make the same 

 determination for the left eye. 



Hold the punctured card in front of the right eye, and move the needle 

 (it is better to use something larger) farther and farther away until it becomes 

 again double, if it does so. This is the far-point of accommodation. In 

 normal eyes infinity is the far limit. In practice an eye that has no far 

 limit under twenty feet is considered normal. This test should be made on 

 each eye. 



Test near vision with printed letters the size of those on this page. 

 Mount a test paper on a small cube and run it along the meter stick as 

 above. Test one eye at a time. When the printed letters begin to lose 

 their sharp black outlines the near-point is reached. To test the far 

 limit use letters large enough to be clearly seen at a distance of 20 feet by 

 the normal eye, see the test chart. 



ii. Inverted Image on the Retina. Dissect off a segment of the 

 sclerotic of a fresh ox eye, or use a fresh eye from an albino rabbit. Make 

 a tube of black paper of the size of the eye, and insert the eye in one end, 

 with the cornea directed into the tube. In the dark room examine the image 



