CH. IX] 



MAGNIFICATION IN MICRO-PROJECTION 



261 



392. Magnification and Screen Image of Various Objectives 

 as Found by Calculation (see 392a). 



LARGE TUBE (Fig. 121) 



392a. This table was derived by calculation from the optical law that: 

 The size of the image is to the size of the object as the distance of the image 

 is to the distance of the object from the center of the projecting lens or objec- 

 tive (fig. 209). In each case the objective's principal focus is marked upon it 

 by the maker, and the distance of the screen from the objective is known. 

 R~ef erring to the diagram (fig. 121) it is seen that the focus of the objective 

 represents approximately the distance of the object from the center of the 

 objective when the screen distance is relatively great. The focus of the objec- 

 tive and the screen distance being known their ratio is easily found. For 

 example, with the 20 mm. objective and a 5 meter screen distance, the object 

 will be 20 mm. from the center of the objective (fig. 209) and the screen image 

 is 5 meters (5000 mm.) distant, then the ratio is 250 to i (5000/20) and it 

 follows from the optical law given above, that the magnification in this case 

 is 250. 



The field in each case was determined by the use of a stage micrometer as 

 \vith39ia. From fig. 209 it is evident that the screen image of the entire 



