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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 watercell 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 lantern slide upon the screen. If the instrument is in proper adjustment the objective will transmit to the screen the rays of light from the condenser which pass through the lantern slide or other semitransparent 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).