Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration (1906)

Record Details:

Title:
Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration
Date:
1906
Author:
Wright, Lewis, 1838-1905
Publisher:
London, New York, Bombay : Longmans, Green and Co.
Volume:
Optical projection: a treatise on the use of the lantern in exhibition and scientific demonstration
Leaf #:
00025
Language:
English
Collection:
Books
Format:
Books
Contributor:
University of California Libraries
Sponsor:
MSN
Coordinator:
Media History Digital Library
Description:
Date Added:
July 19, 2022
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to OPTICAL PROJECTION 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 B A, R B, 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 FIG. 6 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