INVERTED IMAGE ON THE RETINA 707 



meter stick which is provided with a movable diaphragm or screen, and a 

 holder for a lens. Measure the focal distance of lens number i as fur- 

 nished from the optical set. Put the lens in its holder and focus the image 

 of the sun or of an electric bulb on the screen, moving the screen back and 

 forth until the sharp focus is determined. If the lens is accurately ground, 

 the focus will be at a distance of one meter, which is the refractive power of 

 a one-diopter lens, by definition. In the same way determine the refractive 

 power of lenses numbers 2, 3, and 4. 



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 4 or 

 5 mm., the point of focus can be more accurately determined (see Scheiner'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. 



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 



