Bauhaus-Universität Weimar

Titel:
Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Person:
Wright, Lewis
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit39439/68/
58 
OPTICAL PROJECTION 
half empty, the stretcher is lifted off the pin with the toe, 
and the front end of the board being gently lifted (there is 
hardly any weight on this end) the legs can be pushed back 
underneath in the same way, and the front gently lowered on 
to the ground. 
Skeleton boards are usually made with a base as in figs. 33 
and 37, the legs or support being hinged to the front of this 
at b, and also to a point some distance back at a, on the under 
side of the bottom board. When the front is elevated, this 
arrangement brings the back of the bottom board forward to 
c, where it is kept from slipping back by iron pegs slipped 
into a hole at each side. The base extending back to d, pro¬ 
vides security against an overturn ; when half empty the pegs 
Fig. 36 
Fig. 37 
are withdrawn by an assistant, and the boards gently lowered 
and slid back. 
To get a good light (and it is also an element of safety) 
fair pressure must be used. On a small screen a sufficient 
light may often be had with 56 lbs., adding another towards 
the close. But for a large disc, a pair of bags 36x24x24 
will require two \ cwts. to begin, and a third to finish with ; 
and larger bags—say 42 x 32, for larger orifices, will need 
three such weights to begin, and a fourth to finish. If gas 
is no object, even more may be used with gain to the light, 
but not in proportion. I believe the profitable limit to he, 
with proper jets, somewhere between 12 and 15 inches 
pressure. Of course, if jets are used with a lot of so-called
        

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