6 5 6 



DEMONSTRATIONS OF NORMAL VISION 



[CH. XV 



8 A half convex lens of 4 diopters, and a half convex lens of 3 diopters 

 (+4, +3) in the same trial frame. 



A half convex lens of i diopter (-fi) and a half circle of plane glass (0), in 

 the same trial frame to serve as a correcting spectacle.* 



that whenever an object is seen, there must be formed an image on 

 the retina, and that this image, following the laws of optics, must 

 be inverted. A few years later, (1619), Scheiner showed by actual 

 experiment with the eyes of animals that such an inverted, real 

 image is formed on the retina; and in the year 1625, he showed that 

 the same is true of the human eye. 



1604 



Kepler: 



Retinal Image, 



Inversion, 

 Accommodation. 



FIG. 383. LANTERN SLIDE FOR THE EX- 

 PERIMENTS IN ACCOMMODATION. 



FIG. 384 TRIAL LENS FOR KEP- 

 LER'S EXPERIMENTS. 



For the demonstration of normal vision with the special projector 

 (fig. 378-379) are needed: 



(1) A3 diopter convex, trial lens (fig. 384). For the meaning 

 of diopter, see 356a. 



(2) A lantern slide for object (fig. 383). 



The screen representing the retina should be at a distance of 

 5 meters (16 ft.) from the lens and the lens should be 40 centimeters 

 (15.5 in.) from the condenser and the object 36 to 37 cm. distant 

 from the object (fig. 379). 



