Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
it with water. A trough of this kind is very convenient for 
absorption experiments also. 
That lenses bend rays of light in the same way may be 
shown, by passing the same beam of light through the edge 
of a large lens, and also by the experiments described in 
Chapter I. 
160. Dispersion.—With a glass prism of 60°, or a pris¬ 
matic bottle filled with carbon disulphide, colour phenomena 
become conspicuous, while the deviation of the rays is far 
greater. The simplest arrangement for projecting the spec¬ 
trum is shown in fig. 149. A slit (from 1 to 3 mm. wide) is 
placed in the optical front, and focussed on the screen. The 
prism is placed just beyond where the rays cross, when the 
image of the slit will be greatly turned aside, and converted 
into a spectrum. 
Note on Deflected Projections.—Fig. 149 only gives the 
essential apparatus. The lantern may be turned aside on a 
bare table, and the prism-stand adjusted thereon in its place. 
But in this class of experiments, wThich include also reflected 
rays (as those from a soap-film later on), it is on the whole most 
convenient to employ such an arrangement as fig. 99, where 
the slit is first focussed on the screen, then the prism placed 
in position, and finally the whole arrangement, including 
the lantern, rotated until the spectrum comes again upon 
the screen. In this particular case all the apparatus would 
still be in the optic axis of the lantern ; but often a focussing 
lens has to be placed on one side of this, e.g. to focus the 
soap-film (fig. 179, p. 327). In this case the sliding cross-piece 
c d, of fig. 99, affords all the necessary accommodation. 
Another way of producing the spectrum is to project parallel 
rays through a vertical slit on the open front of the nozzle, the 
objective being removed ; and to focus it with the loose lens. 
Which plan is adopted will often depend upon the manner in 
which the lantern was left arranged from any preceding ex¬ 
periment—a consideration which will often vary precise details.


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