i66 OPTICAL PROJECTION 



give a shock to the system the effects of which would be felt 

 for hours. Demonstrators, at all events, have reason to look 

 with dread and dislike upon the extension of the alternate- 

 current system of lighting. 



For high-class optical work, and especially microscopic 

 work, in which one millimetre of misplacement will make a 

 great difference in effect, even a Brockie lamp should be 

 mounted on a stand constructed like a slide-rest, with three 

 rectangular screw movements worked by milled heads. The 

 lantern represented in fig. 86, p. 160, is shown so mounted, 

 the extra cost of which, substantially made in gun-metal, is 

 about 10Z., but which is well repaid in the perfect facility 

 with which the incandescent crater can be brought back to 

 focus in any direction, or moved in the line of the optic axis 

 as required. The illumination in a lantern thus furnished 

 is magnificent, and perfectly steady. 



A switch for the current should be provided on the base of 

 the lantern itself. It should never have to be felt for, or 

 looked for, elsewhere. Any alternate-current lamp should 

 only be handled with gloves, for a shock from such, as already 

 hinted, is very serious. That from a continuous current is 

 simply unpleasant. A lamp always works best with a rather 

 larger current and a corresponding resistance added to the 

 circuit. A small gas-jet is a great convenience inside an 

 electric lantern. 



85. Vertical Projections. It is very often necessary to 

 throw the projection of fluid surfaces, &c., vertically upwards, 

 afterwards reflecting it to the screen ; and the same arrange- 

 ment is very convenient for working out a diagram before a 

 class. This is generally effected by some form of the vertical 

 attachment devised by Professor Morton, and shown in fig. 89, 

 a similar arrangement by Stohrer of Leipzig being shown in 

 fig. 116, on page 227. The lantern condense; s are arranged 

 so as to throw a parallel beam, which by a large reflector at an 

 angle of 45 is deflected vertically, where it passes through a 



