Fig. 292.
a. The electric light. b. The lens.
Bearing these elementary truths in mind, it will not be difficult to follow out a complete set of illustrations explanatory of the construction and use of various popular optical contrivances.
CHAPTER XXIII.
REFRACTING OPTICAL INSTRUMENTS.
I. The Magic Lantern.
No other optical instrument has ever caused so much wonderment and delight, from its origin to the present time, as this simple contrivance. For a long time its true value was overlooked, and only ridiculous or comic slides painted, but its educational importance is now being thoroughly appreciated, not only on account of the size of the diagrams that may be represented on the disc, but also from the fact that the attention of an audience is better secured in a room when the only object visible is the diagram under explanation. The lenses it contains are a "bull's eye" or plano-convex, nearest the light, and a double convex glass, for the purpose of focussing the picture which is inverted and placed between the two lenses. (Fig. 293.)