to OPTICAL PROJECTION 



ago the remedy for this, in the shape of a * condenser ' viz., a 

 lens used in this case merely to bend in the illuminating rays, 

 till they are brought to strike within a given area, or as nearly 

 so as possible. Taking the same case as before, suppose we 

 place immediately after the slide, s, a large convex lens, 

 (fig. 6), which more than covers it. If this lens is of suitable 

 focus, the outer part of the luminous cone, which before 

 diverged uselessly in the directions B A, R B, is bent in so as 

 to pass through the focussing lens, L. The consequence is 

 that there is now a bright image of the whole of the slide. 



This is not the best position for the condenser, because it 

 both impairs the sharpness of the image (which has in a 



FIG. 6 



manner to be focussed through it) and it leaves the slide so 

 near and exposed to the heat of the light. But it is men- 

 tioned first, because it actually was the position first given to 

 the condenser, and still used in those ' toy ' lanterns which 

 are copied by toy-makers from generation to generation ; and 

 still more, because it shows us that the common view of the 

 function of a condenser is a mistake. It is usually stated in 

 books upon this subject that this function is to ' condense ' 

 the greatest number of luminous rays upon the slide. That 

 is not so at all ; this object could be obtained, as in the above 

 arrangement, by placing the slide itself near enough to the 

 source of light. It is not to condense rays upon the slide, 

 but to converge the luminous rays so that they shall pass 



