VISION. 217 



(V) While the eye is focused steadily upon the near 

 point bring the distant point slowly up to a position 

 beside the near point. One of the images is trans 

 formed from an ill defined one to a clearly defined 

 one. Which image is it ? Does one note a similar 

 change in the definition of the image when he 

 moves the near point out to position beside the dis- 

 tant point while focusing steadily at the latter? 



(d} Sum up the results of the experiment into a con- 

 cisely formulated statement. 

 (2) Holding the two points side by side at a distance of 



30 centimeters note that the points appear equally 



well defined. 



(a) Direct the eye steadily at one of the points while 

 moving the other one nearer to the eye. Note the 

 number of centimeters which it advances toward the 

 eye before the outlines become ill-defined. Reverse 

 the act, moving the point back to its original posi- 

 tion beside the stationary point, noting that the 

 image of the receding point remains clear. 



() Continue to carry it farther from the eye, noting 

 that after it has been carried beyond the unmoved 

 focused point a certain distance the outline be- 

 comes again ill-defined. Note the number of centi- 

 meters between the two points in this position. 



(V) Make a similar experiment, using 50 cm. for 

 the distance of the stationary point, and note the 

 centimeters between the points at the limits of 

 clear definition. In this way one may observe and 

 measure the depth of focus of the eye. 



(//) Is the depth of focus greater at 30 cm. or at 

 50 cm. ? 



(e) Is the depth of focus greater at 1QO meters than at 

 one meter ? Demonstrate and explain. 



