Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
ago the remedy for this, in the shape of a ‘ condenser '—viz., a 
lens used in this case merely to bend in the illuminating rays, 
till they are brought to strike within a given area, or as nearly 
so as possible. Taking the same case as before, suppose we 
place immediately after the slide, s, a large convex lens, 0 
(fig. 6), which more than covers it. If this lens is of suitable 
focus, the outer part of the luminous cone, which before 
diverged uselessly in the directions ba,rb, is bent in so as 
to pass through the focussing lens, l. The consequence is 
that there is now a bright image of the whole of the slide. 
This is not the best position for the condenser, because it 
both impairs the sharpness of the image (which has in a 
manner to be focussed through it) and it leaves the slide so 
near and exposed to the heat of the light. But it is men¬ 
tioned first, because it actually was the position first given to 
the condenser, and still used in those ‘ toy ’ lanterns which 
are copied by toy-makers from generation to generation ; and 
still more, because it shows us that the common view of the 
function of a condenser is a mistake. It is usually stated in 
books upon this subject that this function is to ‘condense’ 
the greatest number of luminous rays upon the slide. That 
is not so at all ; this object could be obtained, as in the above 
arrangement, by placing the slide itself near enough to the 
source of light. It is not to condense rays upon the slide, 
but to converge the luminous rays so that they shall pass


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