30 OPTICAL PROJECTION 



diameter is a task of tremendous optical difficulty, and the 

 wonder is that such approximation to it has been attained. 



16. Focus and Diameter of Objectives. The proper focus 

 of an objective of course depends upon the size of disc which is 

 required to be covered by a slide at a given distance, and the 

 range of foci suitable in a set of objectives, will depend upon 

 the range of work, or variety in size of rooms, it is intended 

 to provide for. This is dealt with in detail in Chapter VIII. 

 Here it is only necessary to mention the matter in connection 

 with the diameter of objectives. We have seen that brilliance 

 of image depends upon sending the rays collected on the 



FIG. 19. Scattering of Rays 



object through the objective, and we know that only from a 

 radiant point could the rays be alike converged into a given 

 area at different distances from the condenser. With, say, a 

 luminous spot of f -inch diameter, the light cannot be con- 

 verged save into an image, proportionate in size to the 

 conjugate focus of convergence, as shown in fig. 19. Hence, 

 while all the light can easily be converged into a small lens 

 at a few inches from the slide, at twelve inches it is another 

 matter : the body of rays must spread out more, and require 

 a larger lens to utilise them in the image. Besides this, a 

 certain amount of the rays are irregularly ' scattered ' by the 



