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NOTE ON THE MEASUREMENT OF MAGNIFYING 



POWERS. 



By W. M. Bale, F.R.M.S. 



{Read in Title October '2Srd, 1917.) 



In this note, which, as will be apparent, is addressed mainly to 

 those who, like myself, are imskilled in the science of optics, it is 

 proposed to explain, somewhat more fully than is usual in the 

 textbooks, how to construct a table of magnifying powers ; and, 

 further, to set forth the simplest possible means of ascertaining 

 what power may be provided at any moment when, owing to 

 alteration of the tube-leng"th, the normal arrangement is departed 

 from. If opticians always stated, as they should do, the initial 

 magnification of each ocular and objective, it would only be 

 necessary to multiply the two together to get the power of the 

 combination, but neither this nor a complete table of powers 

 would meet the case of those who, like many of us, possess a 

 number of objectives and eye-pieces from a variety of makers. 



Even if we have a completely fitted instrument with an accom- 

 panying list of magnifying powers, we must verify the latter, as 

 these lists are sometimes very incorrect ; indeed there are some 

 respecting which it can be seen at a glance that they cannot 

 possibly be correct. For whatever may be the powers of a series 

 of objectives with a given eye-piece, it is obvious that by substitut- 

 ing a higher eye-piece the powers of all the combinations must be 

 increased in the same ratio ; e.g. if we have three objectives 

 giving with an A eye-piece the respective powers of 50, 100 and 

 200, then if the B eye-piece increases the first power to 75, we 

 know that it must increase the others to 150 and 300, and if an 

 optician's list fails to maintain these ratios it must necessarily be 

 wrong. To quote a case in point — I recently examined a cheap 

 but good instrument with an accompanying table in which the 

 powers of a one-inch and a quarter-inch with the lowest ocular are 



JouRN. Q. M. C, Series II.— No. 81. 24 



