654 



DEMONSTRATIONS OF NORMAL VISION 



[Cn. XV 



position (fig. 3). The vertical boards holding the lantern slide and 

 the trial lens holder should have means of centering them accur- 

 ately. This is provided for by the U-shaped opening at the lower 

 B end of the uprights where the set-screw is inser- 



ted to hold the uprights against the movable 

 blocks (fig. 159, 381). 



FIG. 380. METAL LENS HOLDER FOR FOUR TRIAL LENSES. 

 (Half Natural Size) 



A Side view of the lens holder showing the stem by 

 which it is held in place in the lens support (fig. 378). 



B Face view of the lens holder showing the four grooves 

 for containing the lenses, and in which they can be rotated. 



B 



919. Lenses, lens holder and white screen. For the refrac- 

 tive part of the eye (cornea and crystalline lens), the trial lenses 

 used by oculists answer very well. To hold these and to permit 

 of their rotation it is necessary to have a metal lens holder (fig. 

 380). A lens holder with grooves for four lenses is very desirable. 



To represent the retina of the eye there is needed a white screen 

 about one meter (3 feet) square. This screen is kept at the con- 

 stant distance of 5 meters (16 ft.) from the lens in all the experi- 

 ments. 



920. Demonstration of normal vision. While men have 

 always known that the eyes were for seeing the things in external 

 nature, the knowledge that the eye acts like an optical instrument 

 and produces an inverted, real image upon the retina came only 

 when Kepler in 1604 demonstrated, in the clearest possible manner, 



FIG. 381. SLIDE-CARRIER FOR THE TEST SLIDES. 

 This consists of: 



An object block sliding on the optical bench or the rods 

 (fig. 378-379)- 



A vertical board i cm. (y+ in.) thick with a U-shaped 

 opening in the lower end. A thumb-screw and washer 

 serve to hold the board in any position against the ob- 

 ject block. 



The lantern-slide carrier consists of a thin board or a. 

 piece of cardboard with an opening of the proper size and 

 height (object) attached to the vertical board. The try- 

 square shaped piece (H H) is to hold the test slide (G SI) 

 in position. (See also fig. 159, b4). 



Object Block 



