MEDIUM-POWER PHOTO -MICROGRAPHY 



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now explained how the photographer can ascertain how much magnification 

 an object has received at his hands. The explanation is given here as perhaps the 

 most suitable place, but it must be understood the remarks that follow equally 

 well obtain with high power or critical photo-micrography. 



The reader must frequently have read, or heard it said, that an object is 

 " magnified so many diameters," and we must all have heard also of a microscope 

 being able to magnify so many " hundreds of times." To explain the difference 

 implied by these two terms, consider Fig. 59. 



An enlarged image of the object A B C D, we will say is cast upon a receiving 

 screen at a given distance, and shown as E F G H. If, now, a pair of compasses 

 be taken, it will be seen that E G is twice A B, G H twice B C, F H twice D C, 

 and E F twice A D. It is not difficult to understand the dotted lines, L K M and 

 0 K P are drawn equally dividing E G and F H, and E F and G H respectively. 



Fig. 59 Fig. 60 



cc 



Then E L and L G each equal A B, G P and P H each equal B C ; and so on 

 with the other sides. A little more attention and it is evident that there are four 

 squares, each equalling A B C D ; so this object is said to be magnified four areas 

 or four " times." But it is equally obvious that A B C D is only magnified twice 

 in each direction — twice from above downwards, and twice from side to side. 

 Hence, when speaking in what is termed linear measure, the object is said to be 

 magnified two " diameters." This holds good for any magnification. Consider, for 

 example. Fig. 60. Here the little square A is magnified five diameters in the 

 amplified picture by its side, but this large picture has twenty-five little squares 

 in it, so it is said to be twenty-five times as large. 



For reasons which need not here be discussed, scientists always speak in linear 

 measurement, employing the term "diameters" in preference to using terms of 



