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THE PROJECTION MICROSCOPE 207 protection from heat is required. I was obliged to adopt a trough at least three inches thick, and 5 inches in diameter; and were any instrument ordered with a specially powerful lamp, even another inch would be advisable, though three inches of alum solution is practically sufficient up to nearly 3,000 candles. It is also desirable that the brass front should be kept apart from the body of the lantern, if of metal, by washers of non-conducting material, to prevent heat passing by direct conduction, and to ensure that the alum be only heated by actual absorption of the rays. In spite of all protection, however, the heat from a powerful arc is so much greater than from a lime, that every precaution should be taken, and no slide at all ' green ' or unsafe left on for too long a period. Glycerine mounts are probably the most risky; old and set balsam mounts I have never known injured. The other change is advisable owing to the less handy character of many electric lamps, which cannot be adjusted so readily in focus as a lime-jet. The triple condenser is replaced by a double-piano, since no lens would bear an arc so near its surface as a lime is brought to the triple form. The double 5-inch condenser brings the arc 6 inches from its back surface, which is a convenient distance. The arc is then adjusted to give a parallel beam from the first lens ; and the second lens of the condenser is made to adjust by a rack and pinion, so that by its motion those changes are made in the cone of rays which in the previous instrument are made by moving the tray of the jet. When the lens is racked back a smaller cone of rays is brought on the parallelising lens, just as when the lime-light is drawn back. When Brockie's lamp and a focussing stand are used, however, a racked condenser is unnecessary. These alterations make it advisable to affix the microscope securely to the lantern, instead of sliding it into a nozzle. It can however be easily removed, and another front, adjusted to the same template, substituted for physical demonstration.