24 OPTICAL PROJECTION 



by such loss from reflection at the edges of the first lens, owing 

 to the high angle of incidence, that the edge of the disc would 

 be less illuminated than the centre. Practically the angular 

 pencil is thus limited to somewhere from 65 to 70 in lanterns 

 for exhibiting slides. 



18. Triple Condensers. Condensers of three and even four 

 lenses have been used, especially in America. Theoretically, 

 they admit of more perfect correction for aberrations, and a 

 larger angle of light ; but this is complicated by the number 

 of extra reflecting surfaces. I have perfectly satisfied myself, 

 that for exhibition lanterns they afford no gain whatever. 

 For large condensers, as five inches and over, and where the 

 light is to be condensed upon a small surface, as for the 

 projection microscope, they are of advantage, and allow us to 

 use a pencil of 90. Such a condenser will be described in 

 connection with the instrument just named. 



14, The Slide-stage. Little need be said here about this. 

 For a single lantern only used to exhibit simple slides and 

 diagrams, it does not matter much how it is constructed. 

 When, however, experiments may have to be made in the 

 stage as in a chemical tank, and for some ' effects ' with 

 slides as the ascent of a balloon, it is important that the stage 

 be open at the top, with the exception of the pillars needful to 

 carry the objective mount. Also, for anything like general 

 work, it is important that while the spring pressure-plate 

 allows the slide to be inserted easily, for which purpose the 

 edges should be carefully turned back to a smooth curve, the 

 slide should be held firmly when in place. If it be not so, the 

 working of any mechanism, as the handle of a chromatrope, 

 may move the slide about in a very unpleasant manner. Both 

 these requirements should be attended to, even in a cheap 

 lantern of japanned tin. 



15. The Objective. This is the lens directly employed in 

 the projection. A simple lens is never now used except in toy 

 lanterns, and for physical experiments, the latter for reasons 



