156 OPTICAL PROJECTION 



modified by sliding the condensers to and from the light, instead 

 of moving the light to and from the condensers. An adjustable 

 slit, and a circular revolving diaphragm of apertures, fit into 

 the front of this sliding tube, An ordinary front for showing 

 diagrams can be interchanged with the optical flange when 

 required. 



This arrangement of movable condensers is convenient 

 for purely optical experiments, and for focussing with the 



well-known Foucault and Duboscq 

 regulators, which would be difficult 

 to adjust in relation to a stationary 

 optical system, and which as a rule 

 are only employed intermittently. 

 But for anything like a continuous 

 arc light, the lantern itself is much 

 too small, and has occasioned many 

 very unpleasant burns : and optic- 

 ally the arrangement is unsuitable 

 to any other projections than 

 those with beams or pencils of 

 light, because to get a good disc, 

 in which to exhibit either dia- 

 grams or apparatus, is exceedingly 

 difficult with it. To put it in 

 another way, it is chiefly suitable 

 for experiments without an ob- 

 jective. For any range of demon- 

 stration, there is need for more 

 optical power than this lantern affords. 



In some cases this increase of power may be best obtained 

 by using two or more single lanterns, one of which might 

 have a front on the Duboscq plan, and another carry an 

 ordinary lantern stage and front for diagrams, or other fittings. 

 For powerful arc lights the bodies should, however, be much 

 larger. 



FiO. 83. Duboscq Lautcru 



