THE SENSES 



Construct 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 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 

 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 one eye, and bring 

 the eye up to the end of the meter stick; cover the other eye. Observe 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 eve. 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 



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



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

 normal eyes there is no 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. 



11. 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 

 of the candle flame as formed on the retina of the eye in the tube. In a 

 favorable experiment, a clear inverted image of the candle can be seen on 

 the retina through the semi-transparent membranes of the eye. The same 

 experiment can be demonstrated with the camera, or with a small lens, using 

 a ground-glass plate to make the image more apparent. 



12. Spherical Aberration. In physical optics it is found that it is 

 difficult to grind lenses so that they will refract equally in the center or 



