47 SIZE OF SCREENS [Cn. XII 



In this example 



f is unknown, (i. e., the focus of the objective). 



the height of the object is 7 cm. (2^4 in.). 



d the distance of the screen is 14.3 meters (47 ft.). 



1 the size of the image is 2.9 meters (9.5 ft.). 

 Substituting the values in the formula we have : 

 For metric values : 



or f = -^- = 34.5 cm., focus of the objective. 

 7 2.9 2.9 



For English values : 



or f = -^ = 1 3. 6 in., focus of the objective. 



2-75 9-5 9-5 



An objective of 13.6 in. or 34.5 cm. would have to be specially 

 constructed. Those on the market and easily procurable were of 

 30 cm. (12 in.) and 38 cm. (15 in.). The shorter focus objective 

 gave considerably too large a screen image and could not be used, 

 therefore the one of longer focus was taken, and a correspondingly 

 long focus condensing lens used. 



Second example. In another lecture room the lantern must be 

 16.75 meters from the screen and the screen could not exceed 3.35 

 meters in width, what should be the focus of the objective and the 

 second element of the condenser to meet these conditions? 



Applying the formula : 



- = '-^- whence f = 37.5 cm. the focus of the objective needed. 

 7-5 3-35 



In English measure : 



- = - whence f = 1 5 in. That is, a 1 5 inch objective is demanded. 

 3 ii 



640. Size of screen and screen images for micro-projection. 



Here the law holds, that to be satisfactory, the details to be 

 shown must be large enough so that they can be seen with ease. 

 The microscopic specimens vary so greatly in character that no 



