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A VARIATION OF CHESHIRE'S APERTOMETER. 



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

 .(Read October 2Sth, 1913.) 



Figs. 7 and 8, 



Experience in the use of both the original forms of Cheshire's 

 Apertometer, and the modification thereof recently introduced 

 by Mr. E. M. Nelson, has revealed one or two difficulties in 

 connection 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 Mr. 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 Mr. E. M. 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 

 likelv to confuse the diagram and bewilder the observer. 



In either the old form or the new of Cheshire's instrument, 

 a count has to be made of concentric circles ; a thing which, 

 simple as it may seem, is peculiarly liable to confuse the eye ; so 

 that it is only after counting several times that one feels certain 

 that the number is, say, eight and not seven. In the present 

 instrument a totally different method of reading is adopted ; the 

 diagram is simplified, and the estimation of the second place of 

 decimals is merely the estimation of the point where a spiral 

 curve cuts the margin of the back lens of the objective, referred 

 to two points, one on each side, where radial lines cut the 

 same. 



The instrument, which consists, in the form for dry lenses, 



