MAGNIFYING POWER : A NOTE ON MR. BALE's PAPER. 319 



of eye^piece employed. Again, the term " magnifying power " 

 may either mean the ratio of the linear dimensions of the image 

 (supposed projected to some conventional distance, e.g. 10 inches 

 or 250 mm.) to those of the object ; or it may mean the ratio of 

 the angles under which the image and object are seen, both being 

 supposed at this conventional distance. If the latter meaning of 

 the term be adopted, it would seem that there is no real dis- 

 tortion of the kind mentioned by Mr. Bale ; and the only dis- 

 tortion would be that of an optical nature already alluded to ; 

 but if it is a case of linear ratio, then we must assume the image 

 projected on a plane surface, in which case the degree of flatness 

 of field of the microscope would enter into the matter. I do not 

 know that it really much matters, even at the edge of the field, 

 which way we look at the matter ; but it is worth while mention- 

 ing. As far as the centre of the field is concerned, there is of 

 course no difference. 



Mr. Bale states that, broadly speaking, the method he describes 

 is the only one available in the absence of knowledge of the 

 separate powers of the constituents of the microscope. It is 

 therefore, I think, desirable to. call attention once again to a 

 method which has as yet received but scant treatment from the 

 textbook writers ; I refer to the Measurement of the Ramsden 

 Disc, or spot of light seen just above the eye-piece. I have given 

 full directions for this method in a letter to the English Mechanic, 

 August 15th, 1913 (one of the letters already alluded to), and 

 also some details in a lecture before the Photomicrographic 

 Society (see their Journal, February 1916, vol. v.. No. 1). I will 

 therefore only mention here that the formula employed is 



M = 2D.A/d, 



where M is the magnifying power of the instrument as a 

 whole, D the conventional distance of the image from the eye 

 (10 inches, or 250 mm.), A the N.A. of the objective (which 

 for this purpose must have its full value, i.e. must not be cut 

 down by the substage iris diaphragm), and d the diameter of 

 the Ramsden Disc. The latter can be measured by various 

 methods ; e.g. Banks' "Dynameter," Berthon's " Dynamometer," 

 or a finely divided scale. As an example suppose D = 10 inches, 

 A=0-84, d=0-035 inch, thenM=20 X 0'84/0-035, or 480 diameters. 

 The great advantage of this method is that, given the N.A. of 



