SPECIAL SENSES 



109 



Obtain a wooden box eighteen or twenty inches long and 

 about eight inches wide and deep. Leaving one side open, 

 paint the inside of the box black. Around the open side 

 tack a piece of black cloth large enough to cover the head 

 of the observer and shut out the light from the interior of 

 the box. At one end of the box cut a hole one inch in di- 

 ameter. Cut several black cardboard disks to fit this" aper- 

 ture, and perforate their centers with holes varying from 

 one-sixteenth to one-half inch in diameter. Mount a con- 

 vex lens in a movable holder which can be moved forward 

 and backward on the floor of the box, and which will bring 

 the center of the lens opposite the center of the hole. Mount 



Screen (Retina) 



FIG. 52. 



a piece of ground glass in the same way to serve as a screen. 

 Arrange all parts as in Fig. 52. 



The cardboard disks will then correspond to the iris with 

 its pupil; the walls of the box to the sclerotic; the black 

 paint to the choroid (what is its function?); the lens to 

 the crystalline lens, and the screen to the retina. A watch 

 glass placed on the aperture would resemble the cornea. 



A. Action of Parts. Darken the room and place a lighted 

 candle at a distance of three feet from the aperture. Place 

 in the aperture the disk with one-quarter inch perforation. 

 Cover head with cloth and place screen at the rear of the 



