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THE PARTS OF A LANTERN 23 however, is that the lenses are not ground thin at the edges ; nearly all lantern-makers being far too careless in this respect. Not only is a needlessly thick lens much more likely to crack, and more absorbent of light, but it is distinctly worse in optical performance. The lenses should be ground to as nearly knife-edges as possible, being only just edged down for fixing into their cells. Both lenses should be mounted so loosely in their brass cells, that they can be turned round with the fingers, else they may crack merely from expansion when heated, and they often become hotter than the hand can bear. Holes must be pierced in the margin of the cell, to allow of the escape of the aqueous vapour which always forms when the lantern is first lit. Only the back lens ever cracks from heat alone. If the crack be irregular, the lens must be replaced; but it often happens that the crack is quite straight across a diameter, in which case the cracked lens will answer perfectly well if the crack is arranged perpendicularly, and the operator may feel certain that his lens will never crack again. 12. Size of Condensers.—The best general size of con- densers is 4 inches diameter next the slide. The standard size of slides is 8J inches square, and 4 inches will cover 1 cushion' slides on such square of glass. Of course a diameter of 3^ inches is ample for the usual circular slides, which give a disc of 3 inches diameter. If it is desired to cover square slides of three inches, as the corners are never absolutely square, it is better to have 4| inches diameter than 4| inches, because practically only a certain angle of light can be taken up from the radiant, and the larger surface this is spread over, the more it is diluted, so that for the usual size of slides, less light is passed through them by a large condenser than a small one. Theoretically any angle might be taken up by a condenser, but practically it is limited ; first by the thickness of the glass, which would both crack, and lose as much light by absorption as was gained in angle; and also