Plano-convex lenses are generally used in magic lanterns, two to each condenser, with their convex sides towards each other. The smallest condensers used are tin. in diameter, and this is none too much, as the slide pictures are supposed to be I in. in diameter. A single lantern should have a condenser not less than 4-Jin. in diameter. Biunials and triples require 4 1/2-in. condensers to allow for the rolling of the curtain, and also a little margin to get the two or three discs coincident on the screen. For the objective, the following lenses are reqitired: -

The front combination consists of a double convex lens and a plano-concave leus cemented together. These should be l 3/8 in. to 1 3/4 in. in diameter. The back combina-1 ton has two lenses separated by a short space: the one est the front is a meniscus, with the convex side towards the front, and the other is a double convex lens of unequal curves, the curve with the longer radius being placed nearest the light. These lenses should be 2 in. in diameter. An objective of this description has a focus of about 6 in. and gives the best results. When, however, an objective of very long focus is required, a single achromatic answers nearly as well: but it must be sufficiently large to take all the rays of light. A single lens of 12-in. focus should be at least 3 in. in diameter.