Ch. I]
TROUBLES WITH THE MAGIC LANTERN
57
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
objective.
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 ;