Volltext: Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration

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slides into the body-tube carrying the objective, and must be 
guarded against any ‘ flare ’ or internal reflection from its 
sides. The effect of such a lens is to lengthen the focus of 
any objective in any given position ; hence its use also 
increases the working distance. For photographic purposes, 
the amplifier should be corrected ; but for ordinary projection 
a simple lens is sufficient. It is convenient to have a set of 
amplifiers of various foci, which give a great range of powers, 
and of working distances from the screen, enabling very similar 
results to be obtained under different conditions. 
The use of an amplifier lias another considerable advantage. 
Ordinary micro-objectives are corrected to a focal length of 
about 10 inches in England, and 6 or 7 inches on the Con¬ 
tinent. If such lenses are used to project an image, say, 
25 feet away, besides the mere greater magnification of any 
error, it is obvious that their ‘ corrections ’ can no longer be 
accurate. It is really wonderful that some lenses should 
perform, under such trying conditions, so well as they do. 
But now suppose some given lens is focussed on a slide, so as 
to project an image at its own proper distance of G inches. 
Our screen however is, we will say, 25 feet away ; a distance 
I have found to give on the average perhaps the best range of 
general results, though shorter and longer are better for some 
purposes. We can select a concave amplifier of a focus which, 
placed in a certain position, will bring the linage to a focus 
on the screen without altering the focus on the object, thus 
maintaining the ‘corrections’ of the objective at nearly their 
proper value. The late Colonel W. Woodward, U.fch, was, I 
believe, the first to point out this fact, which is of great use 
in photo-micrography. 
Any concave lens can be placed somewhere so as to focus 
a 6-inch or 10-inch focus on the screen, at any longer distance ; 
but practically we are limited to a certain range. I have 
found generally useful quite a low power, which does this at 
about four inches behind the objective, and amplifies eon-


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