IIJ 



OPTICAL PROJECTION 



pleasantly yet without shake; the screw- work screws and 

 tmscrews with accuracy and certainty. Thick and heavy 

 brass-work generally masks poor workmanship. 



What is best worth having in the way of objectives, has 

 been described in Chapter II. 



60. Focus, Distance, and Disc. It will be fully understood 

 from our first chapter, how the same objective which gives 

 a 12-ft. disc at 24 ft. away, will give a 24-ft. disc at 48 ft. 

 All details will be best gathered from the following table, 

 showing the distance required to get a disc of given diameter 

 with lenses of given focus, with slides of the usual 3 inches 

 diameter. The foci given are real or virtual foci, and in 

 double combinations what is called the ' back focus,' or dis- 

 tance of the back lens from the slide, will be less. 



Or the following formula may be useful, as giving the data 

 for any other slide than 3 inches. Calling the clear diameter 

 of the slides hi inches, slide, the objective focus in inches 

 focus, distance from screen in feet, distance, and the diameter 

 of disc in feet : then, 



If we want the distance the formula will be, 



disc x focus 



Distance = 



slide 



If we want the disc, the formula is, 



distance x slide 



Disc = 



focus 



