1 8 MAGIC LANTERN WITH DIRECT CURRENT [Cn. I 



18. Water-cell. This is a vessel of water with parallel, glass 

 sides, placed in the beam of light from the lamp, before the light 

 reaches the lantern slide or other object. The water-cell absorbs 

 most of the radiant heat from the lamp and thus protects the 

 objects from over-heating (fig. 2-3). 



The water-cell is especially needed for opaque lantern slides like 

 those of dark scenes or colored slides made by the Autochrome 

 process. It sometimes happens that in an exhibition as many as 

 10 to 30 per cent, of the slides are cracked by the heat, if no water- 

 cell is used. 



Unfortunately the water-cell is oftener absent than present in 

 magic lanterns. (For a further discussion of the avoidance of heat 

 see 364, 854). 



19. Projection objective. This forms an image of the lan- 

 tern slide upon the screen. If the instrument is in proper adjust- 

 ment the objective will transmit to the screen the rays of light from 

 the condenser which pass through the lantern slide or other semi- 

 transparent object. These rays reflected from the screen to the 

 eye give rise to a picture with all the gradations of light and shade 

 and color of the lantern slide or other object used (see fig. 1,2, and 

 811). 



PERFECTION AND BRILLIANCY OF THE SCREEN IMAGE 



20. The quality of the screen image depends upon : 



1 . The accurate centering along one axis of the source of light, 



the condenser, and the projection objective (fig. 1,2). 



2. The amount and intensity of the light used. 



3. The excellence of the condenser. 



4. The aperture and perfection of the objective. 



5. The proper proportion of the objective and the condenser to 



each other and to the size of the room. (See fig. i, 2, 

 634-636). 



6. The perfection and transparency of the lantern slides or 



other objects imaged on the screen. 



7. The accuracy of the focus of the image on the screen. 



8. The reflecting qualities of the screen (sec 621). 



