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. 



