128 PHOTO-MICROGEAPHY 



front, it must be remembered one is dealing with a linear magnification of 240 

 diameters, and, therefore, any shake is itself magnified that number of times m all 

 directions. 



It is time now for us to enter into and discuss a phase of our subject, which, 

 although it is of a most engrossing nature and one which has a most important bear- 

 ing upon things photo-micrographical, yet is very frequently overlooked, and perhaps 

 with some photographers very slightly understood. 



It has been plainly pointed out that the apochromatic objective can stand eye- 

 piecing to almost any reasonable extent, and the effect or outcome of this excessively 

 useful property is that magnification of tlie same amount may be obtained by very 

 different combinations of objective and eye-piece, putting aside for the moment the utility 

 of different camera lengths. As an instance, just for example' sake, take the following 

 illustration. It is required to magnify an object about 250 diameters. This can be 

 obtained in the following manner : 



1. With the I inch apo. IST. A. -3 and a 4 eye-piece and 60 inch camera. 



2. „ I ,, N. A. -3 „ 24 



3- i M N. A. '65 „ 10 



4- „ i N. A. -95 „ 4 



5- » i „ N. A. 1-40 „ 3 



6- n tV n A. 1-40 „ 2 



Which, then, is the best ? Here we have to pause and ask ourselves, first, what 

 is required to be represented in the photograph ? 



(a) Is it to show the finest details that can be possibly effected in one plane, dis- 

 regarding altogether the relation of any one part to the whole ? or is it 



(/3) That a general view with as much detail as possible is required of the large 

 objects, but not so much of the small ? or 



(7) Merely a general idea of an object with just enough of the leading points to 

 make a recognisable picture and to show as many planes in focus at one time as 

 possible ? 



Before returning an answer to these questions, we must remind the reader that we 

 have explained in a recent chapter, when speaking about lenses, that although 

 "penetrating power" varies inversely as the square of the magnifying power, still it 

 also varies inversely as the N. A. of the objective. Now with respect to the magni- 

 fying power in the present case before us, we have nothing to say, as with each com- 

 bination the amplitude obtained is practically the same ; but it is to the effect 



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