ON PROJECTION 



ii 



through the focussing lens ; and to understand this is most 

 necessary to any successful projection which may be the least 

 out of the beaten track. We shall see hereafter that, for 

 want of understanding this, even slides and diagrams are 

 often badly shown, and unnecessary expense is often incurred. 

 This understood, however, it is obvious that a much 

 better arrangement in several respects must be to interpose 

 the condensing lens between the slide or object and the light, 

 as in fig. 7. The otherwise wasted divergent rays E A and E B 

 are still bent in so as to pass through the focussing lens, L ; 

 but a large part of the heat is borne by the lens c, and the 

 slide or apparatus so far shielded ; and the focussing lens, L, 



,8 



FIG. 7 



has no distorting medium between it and the object. This, 

 therefore, is the arrangement now always used in instruments 

 of a serious kind. 



6. Management of the Rays. We now understand exactly 

 why the margin of our slide in the first experiment was not 

 illuminated upon the screen, until the light was drawn back 

 from its usual position. There was a condenser, and the rays 

 were sufficiently converged to pass through the lens say a 

 circle of two inches diameter in the usual manner. But 

 they were not sufficiently converged for any appreciable rays 

 passing through the margin of the slide, to pass through the 

 small central pinhole. By drawing back the light, they were 



