CH. XV] 



DEMONSTRATIONS OF NORMAL VISION 



653 



opening a little smaller than the trial lenses to put just beyond the 

 trial lens. This cuts off any stray light falling outside the lens. 

 If large sources only can be used, then it would be an advantage to 

 have lenses of greater diameter than the trial lenses so that all 

 the light could be utilized and thus give a brighter screen image. 

 The trial lenses answer admirably for the arc light, however. 



918. Centering along one axis. As with the magic lantern 

 and the projection microscope, it is necessary to have all of the 

 elements of the projection outfit on one axis ( 51-58). 



Also as the light is liable to get out of the axis, it is a great advan- 

 tage to have fine adjustments on the arc lamp to bring it back in 



FIG. 379. PROJECTION APPARATUS WITH TWO-LENS CONDENSER FOR 

 DEMONSTRATIONS REPRESENTING VISION. 



Radiant. The arc lamp in the lamp-house. It has fine-adjustment screws 

 (L A, V A), and feeding screws for the carbons (F S) and the source of light in 

 the crater of the upper carbon (L). 



Lamp House, V The lamp-house and its ventilator. 



Rods The rods of this form of lantern. These serve as a kind of optical 

 bench along which the different parts can be moved. They should be long 

 enough to permit of a separation of the lens and the condenser of at least 40 

 cm. (16 in.). 



Cond, i 2 The two lenses of the condenser. Lens 2 should be of relatively 

 long focus, 25 to 30 cm. (10 to 12 inches). 



Object Block The block supporting the object, and by means of which the 

 object can be moved back and forth along the rods. 



Lens Block The block supporting the lens. It can be moved back and 

 forth along the rods. 



Image screen The white screen for the image. It is 5 meters (16 ft.) from 

 the projection lens. 



J7 Centimeters The distance between the lens and the object for a 3 diopter 

 lens. 



28 Centimeters The distance between the object and a 4 diopter lens. 



Ind i, Ind 2 A white strip of cardboard to serve as an indicator so that the 

 spectators can see when the object is moved toward or from the lens. 



