H6 OPTICAL PROJECTION 



is placed in the lantern, and the objective being slid into 

 approximate focus with the rack about half-way, is properly 

 focussed with the rack itself; then the slide is removed, 

 leaving nothing in the stage. The jet is now to be adjusted 

 according to the appearance of the bright blank disc on the 

 screen. Starting with the lime about 3 inches from the back 

 of the condensers, the disc may present any of the appear- 

 ances in fig. 64. If it resembles A, the lime must be moved 

 to the left; if like B, to the right ; like c, it must be lowered ; 

 like D, it must be raised ; always moving it to the side oppo- 

 site to the dark shade, till whatever light or shadow there 

 may be, is central on the disc. It may now resemble E, when 

 the lime must be moved nearer the condenser ; should the 

 centre be dark and the edge bright, it must be drawn back. 

 When properly adjusted the disc will be even and bright all 

 over, as at F. 



Amateurs often fail in this adjustment of the light, through 

 attempting to accomplish it with a slide in the stage, by which 

 it can never be done properly. The only way is that here 

 described. The lantern first adjusted had better be the 

 bottom one, which is first to be given its proper inclination, so 

 as to show its disc centrally on the screen. Should the lime 

 have to be moved much to or from the condenser, it may need 

 a little re-centring, as this movement will take it a little out 

 of the optic axis, which is inclined somewhat to the slide of 

 the tray. Before the other lantern is similarly adjusted, its 

 disc must be made to coincide with the other on the screen. 

 It is necessary to centre the limes with the pressure full on 

 which is to be used ; because if centred with a lighter pressure, 

 when more is used the luminous surface extends farther up 

 the lime, which is equivalent to raising the jet, and would 

 throw it out of centre. 



Both lanterns being adjusted, the apparatus is ready ; 

 except that when the condensers cannot be warmed previously 

 at a fire, dew will form upon them in cold weather, and time 



