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THE MEASUREMENT OF MAGNIFYING POWER: A 

 NOTE ON MR. BALE'S PAPER.* 



By Maurice A. Ainslie, R.N., B.A., F.R.A.S. 



The method described by Mr. Bale of measuring the magnifying 

 power of a microscope is not by any means new ; it was described 

 by Mr. Nelson in a paper read before the Quekeftt Microscopical 

 Club [Journ. Q.M.C., Ser. 2, vol. xii. p. 239), and in the 

 English Mechanic for August 1st and 15th, 1913 (Nos. 2523 

 and 2525) will be found two letters of my own, giving several 

 methods of determining the diameter at 10 inches (or any 

 other specified distance) of the magnified field as seen through 

 the eye-piece. But Mr. Bale does well to call attention to this 

 method again, as it is without doubt one of the easiest as well as 

 one of the most satisfactory methods of arriving at the magnifying 

 power of the instrument as a whole, and also on account of its 

 having been hitherto so much, and so unaccountably, neglected 

 by writers on the microscope. The practical details of procedure 

 given by Mr. Bale are of great value ; but there are one or two 

 points to which I should like to draw attention, as being open to 

 objection. 



In the first place, Mr. Bale is not quite correct in stating that 

 '' by substituting a higher eye-piece the powers of all the com- 

 binations must be increased in the same ratio." This may be 

 the case, but it is not necessarily so. For in the first place 

 the substitution of one eye- piece for another very often alters 

 the tube-length ; since this latter is properly measured from the 

 upper focal plane of the objective to the image plane, which for 

 normal eyes coincides with the lower focal plane of the eye-piece. 

 At any rate, the magnification produced by the objective alone 



* The Hon, Editor is very much indebted to Mr. Ainslie for kindly 

 reading the MS. of Mr. Bale's paper and for adding the above note. 



