Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
2 33 
as in fig. 120. A disö of microscopic cover-glass silvered, 
about £ inch diameter, should have attached to the back two 
morsels of beeswax and one of cobbler’s wax, and be so placed 
that the latter adheres to the spot on the wrist found to give 
the best pulse. A parallel beam, or preferably the focussed 
parallel beam from a small aperture in the pencil attachment 
(fig. 95), is reflected from the mirror a to the screen, and the 
reflected ray exhibits every beat by the oscillation of the bright 
This reflection is too simple to exhibit any characteristic 
pulse-trace, as taken by the sphygmograph. Some approxi¬ 
mation to it can however be obtained by receiving the reflection 
from A upon a tilting or rocking mirror, capable of giving a 
motion to the spot 
of light at right 
angles to that given 
by the pulse-motion. 
Then this rocking 
motion should be 
given, backwards and 
forwards, so that the 
motion synchronises with the pulse-motion ; when the break 
in the curve will be seen very fairly represented on the screen. 
It is true that this break can only be followed for one 
pulse at a time, and not for many beats together as in a 
tracing. But there is a general remark to be made here re¬ 
specting the true place and value in demonstration of direct 
projections. Exhibited thus, however roughly, they give a 
sense of vivid objective reality which can be imparted to a 
class in no other way, to ‘ tracings ’ prepared by any of the 
well-known methods, and which should immediately after¬ 
wards be projected upon the screen, using the tracings as 
ordinary scientific diagrams. To obtain such slides, either 
the revolving blackened cylinder so usual in tracing apparatus 
must be displaced for tracing on the flat, which is most 
Fig. 120.—Falbe Mirror


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