214 OPTICAL PROJECTION 



overcome with the Duboscq lantern, partly from the small size 

 of its condensers, and partly from the optical construction 

 already pointed out ; and to the former prevalence of this 

 lantern in lecture theatres, I attribute the little progress made 

 in this class of projections, until the Germans showed a more 

 excellent way. It arises from the fact that nearly all apparatus 

 has to be arranged, owing to its absolute thickness in the 

 direction of the optic axis, farther from the condensers, as 

 regards its ' average ' or practical focal plane, than a diagram 

 would be. We place the latter almost touching the front 

 condenser lens ; while in the apparatus the plane focussed 

 will probably be some inches away from it. Now this quite 

 upsets the uniform illumination of the disc ; a fact which 

 seems too often forgotten. If we are to have a good and 

 clear projection, the disc on which it appears must be as 

 carefully adjusted, as to centering of the light, and distance 

 of the light behind the condensers, as for a diagram. The 

 radiant has to be brought nearer the condensers, and even 

 then the cone of rays will not embrace so large a field as 

 the largest elide which can be shown. This is one reason 

 why the condensers of a demonstrating lantern, for exhibiting 

 solid apparatus, should be 4^ to 5 inches in diameter. 



In all work throughout the following chapters, and similar 

 experiments, the proper adjustment of the cone of rays from 

 the condensers, so as to embrace the apparatus, and finally 

 pass through the focussing lens, and give an evenly-illu- 

 minated disc over the plane of the object, is the chief thing 

 in order to obtain a good projection, and the objective itself 

 'focusses' better under these conditions. The adjustment 

 involves some trouble, and hence is too often neglected, as 

 re- arrangement may be needed from time to time. A fair 

 average for a series of apparatus can be pretty easily secured ; 

 but the reason will be evident why the same adjustment does 

 not suffice for apparatus and diagrams, and why, therefore, 

 some form of double lantern is so desirable where diagrams 



