Bauhaus-Universität Weimar

Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine
Gage, Henry Simon and Henry Phelps Gage
Ch. I] 
or starch process is one of their great drawbacks. Only powerful 
radiants give satisfactory screen images. 
§ 93. Shadow on the screen with water-cell. In case the water 
in tile water cell has evaporated in part there will be a very dis¬ 
agreeable shadow on the lower part of the screen (fig. 31). It is on 
the lower part of the screen although it is the upper part of the water 
cell that will be empty. This is due to the inverting action of the 
Fig. 31. Shadow on the Lower Part of the Screen when the 
Water-Cell is but Partly Filled. 
S Screen image with shadow on the lower side. The water is of course 
present in the lower part of the water cell, and absent from the upper part; 
but, owing to the inversion produced by the objective, the screen image shows 
the shadow on the lower part. 
Occasionally the water is entirely absent from the water-cell. 
Then there will be a very poor screen image, the entire screen being 
affected by the obscurities on the dry surfaces of the water-cell. 
Breaking of Condenser Lenses 
§ 94. It is usually the lens next the radiant that cracks or 
becomes shattered. This is due to the too rapid heating or cooling 
of the condenser lens, or to the mounting, which may be too rigid 
to allow of free expansion of the lens as it becomes hot. 
Condenser lenses are especially liable to break: (1) When too 
heavy currents are used; (2) when the lamp-house is not well and 
evenly ventilated; (3) when currents of cold air strike the hot 
condenser; (4) when the lens mounting is not provided with 
ventilating openings for free circulation of air between the lenses ;


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