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CH. X] DRAWING WITH THE MAGIC LANTERN 331
reduction of the picture can be determined by photographing a metric or other measure (fig. 178) on the same plate with the object or piece of apparatus.
§ 467. Size of condenser necessary for making drawings.—
When lantern slides, or negatives made on lantern-slide plates or other plates of that size are used, the condenser of any magic lantern will answer. Sometimes, however, it is desired to make diagrams or drawings from negatives of larger size. There are two ways of accomplishing this:
(1) A lantern slide can be made from the large negative by the aid of a photographic objective as described in Ch. VIII, § 329. This can then be used in the ordinary lantern.
(2) If the large negative is to be used direct, then the condenser of the magic lantern must be of sufficient size to illuminate the negative. That is, the condenser must have a diameter a little greater than the diagonal of the negative to be illuminated and drawn (see fig. 114).
§ 468. Drawing on a horizontal surface by the aid of the magic lantern. — This is easily accomplished by using a 45 degree mirror or a prism beyond the objective (fig. 192).
One must be careful to put the negative or lantern slide in the carrier in such a way as to give an erect image (§ 512).
If the negative or lantern slide or other object is too dense, so that the light is relatively dim, the image will be duplicated when a mirror silvered on the back is used, therefore, one must use a prism or a mirror silvered on the face for these dark objects. For very transparent objects the image appears single even with a mirror silvered on the back, the silver image being so much brighter than the glass image that the latter does not show.
One can use the magic lantern and separate table with a 45 degree mirror (fig. 182) or the mirror can be fastened to the projection table as in Dr. Riley's device (fig. 184) or the mirror may be close to the objective, and the adjustable drawing shelf used (fig. 183).