QUEKETT MICROSCOPICAL CLUB. 343 



passed to Mr. Murray for his paper and to Mr. Scourfield for 

 giving them so good a resume of it. 



Mr. James Grundy described and exhibited "An Improved 

 Form of Cheshire's Apertometer." 



Mr. Grundy said that of the value of Mr. Cheshire's form of 

 apertometer there can be no doubt. The aim of Mr. Nelson has 

 been to enable the N.A. values of an objective to be read on the 

 apertometer easily and accurately. Distinctness and clearness of 

 reading have been effected by increasing the number of marked 

 values of N.A. from 9 to 22 without the confusion that over- 

 crowding of the lines would entail. To accomplish this, short 

 arcs of circles are used instead of whole circles. A valuable pro- 

 perty of these is the clear visibility of the ends or edges of the 

 arcs : they are seen more distinctly than complete circles would 

 be. The contrast between the white ground and the short black 

 lines favours this. The exterior edges of the arcs denote the N.A., 

 and thus give most convenient, accurate, and definite positions for 

 reading. 



Mr. F. J. Cheshire said it might interest members to know 

 that he described his apertometer before the Club some ten years 

 ago. When Zeiss first issued Abbe's form, it was marked to read 

 only to 005. In a paper defending this marking, read before the 

 R.M.S. in 1880, Abbe dealt with the accuracy it was necessary 

 to strive for. On the Zeiss apertometer it is possible to read to 

 | per cent. ; but blue rays alone will give a difference of 1 per 

 cent, over a reading taken with red light, so that the maximum 

 accuracy it was advisable to attempt to obtain was 1 per cent. 

 Mr. Cheshire thought that one point in Mr. Nelson's diagram 

 largely vitiates the advantages given by a greater number of 

 fiducial Hues that is, that the fiducial edge in the diagram is 

 the outer edge of the line ; and, again, the lines are of varying 

 thickness. There are twenty-two edges of lines on the diagram 

 with no fiducial value. He himself thought that his original 

 form was not capable of further accuracy. Mr. Cheshire then 

 described, and subsequently demonstrated, another method of 

 measuring N.A., which he considered an improvement on the 

 older form. 



A visitor Mr. M. A. Ainslie, K.N. said that experience in 

 the use of both the original form of Cheshire's apertometer and 

 the modification thereof recently suggested by Mr. Nelson has 



