MEASUREMENT OF MAGNIFYING POWERS. 



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of the field, as projected by the camera, when the field-diameter 

 so shown will be 7fV inches, corresponding to the line F G. 



The observer who proposes to calculate his magnifying powers 

 by the foregoing method must of course substitute for the distances 

 B C and D E distances representing the apparent fields of his own 

 eye-pieces, and must be careful to secm'e accuracy, as the correct- 

 ness of all future measurements will depend upon it. It is well to 

 record the field-diameters in inches as well as in millimetres, to 

 be handy for reference when using either micrometer. But for all 

 ordinary purposes it will suffice to use the following table, which, 

 it is believed, is correct to the nearest millimetre. The first two 

 columns show the field-diameters as seen in a camera-drawing, 

 the third the amount to deduct. The remainder is the proper 

 field-diameter to be used in calculating magnifying power : 



While I have not seen this matter referred to in any book 

 specially in connection with the measurement of magnifying 

 power, I note that Mr. A. C. Cole, in his Methods of Micro- 

 scopical Research, refers to the distortion in drawing with the 

 camera, and somewhat drastically proposes that no eye-piece 

 higher than an A should be used for drawing with. This rule 

 seems to limit one's use of the instrument unnecessarily. The 

 distortion is mainly due, not to the greater power of the eye-piece, 

 but to the greater actual size of its field ; therefore the ratio of 

 distortion will be no greater with a jB or eye-piece than with an 

 A , provided that we only utilise so much of the field of the former 

 as will correspond with the field of the A ; or better still (since 

 there is some distortion even with the ^4), if we confine our drawing 

 with all eye-pieces to, say, not more than 4 inches in the centre 

 of the field, shifting the paper from time to time so as to keep 

 within that area. Obviously the only way to get a camera-draw- 

 ing with equal magnification throughout a large field would be to 



