400 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



has no assigned or stated thickness in terms of N.A. This is a fatal 

 defect, because when the thickness of a line has a N.A. value of, say, * 02, 

 such thickness, especially when dealing with low-power lenses, provides 

 an invaluable standard of reference when estimating by eye N.A. values 

 intermediate to those represented on the scale. In apertometers of the 

 kind in question the further the subdivision of the scale is carried the 

 greater must be the complexity of the image presented to the eye — 

 the advantage of one is balanced by the disadvantage of the other. 

 Possibly, however, most people would prefer the simplicity of a diagram 

 with the larger divisions to the optical Hampton-Court-maze necessitated 

 by the smaller ones. 



Variation of Cheshire's Apertometer.* — M. A. Ainslie states that 

 experience in the use of both the original forms of Cheshire's Aperto- 

 meter, and the modification thereof recently introduced by E. M. Nelson, 



Fig. 38. 



have revealed one or two difficulties in connexion with the reading of the 

 instrument— that is, if any accuracy in the second place of decimals is 

 required — and the present instrument is an attempt at removing these. 

 The first difficulty is due to the fact that in Cheshire's instrument we 

 have to interpolate or estimate between two divisions on a scale, one of 

 which is not visible, being outside (apparently) the margin of the back 

 lens of the objective. This renders the estimation of the second place 

 of decimals in the N.A. uncertain, and although Nelson's modification 

 of the original instrument is somewhat better in this respect, yet the 

 very means adopted to improve the reading, namely, the introduction of 

 a large number of additional circles, is likely to confuse the diagram 

 and bewilder the observer. 



The author's instrument, which consists, in the form for dry lenses, of 

 a card diagram placed on the stage, is constructed as follows : A series 



* Journ. Quekett Micr. Club, xii. (1914) pp. 287-9 (2 figs.). 



