THE BOOK OF THE LANTERN. 79 



central as possible. When this is the case, tighten the screw 

 or screws (and two are better than one) which hold the jet 

 on the rod, so as to clamp it firmly in position. Now press 

 forward the tray, so that the light approaches the condenser, 

 and this will cause the sheet to become equally illuminated 

 with a sharply -defined margin all round. Now place a 

 slide on the stage of the lantern, and focus it as sharply as 

 possible. The best way to do this is to set the focussing- 

 screw so that it is at its middle position, then focus by 

 sliding in and out the flange into which the objective is 

 screwed. Get roughly the best focus which you can obtain 

 by this means, and then give a finishing touch by means of 

 the focussing-screw. 



If a double or biunial lantern be employed, the necessary 

 operations will be rather more complicated, for here we shall 

 have two lights, and two optical systems to control instead 

 of one. But, when once understood, the working of a double 

 lantern is both simple and easy, so much so that on many 

 occasions when a good assistant was not forthcoming I have 

 worked the lantern myself and lectured at the same time. 

 The two lanterns are connected by means of a dissolving tap, 

 such as that shown in fig. 3, which is a very good pattern. 

 This tap is so arranged that when the lever is upright both 

 lanterns have their full supply of the gases ; when the 

 lever is turned over towards the left-hand side, the lower 

 lantern only is provided for, whilst when in the reverse 

 direction the upper one is supplied with gas. The original 

 plan for dissolving was to move a couple of serrated 

 screens to and fro in front of the lenses, both jets 

 continuing burning during the entire exhibition. The 

 more modern method of cutting off the gas supply from 



