281 



AN IMPROVED FORM OF CHESHIRE'S APERTO- 



METER. 



By Edward M. Nelson, F.R.M.S. 



(Exhibited and described by James Grundy, F.R.M.S., October 28th, 1913.) 



Fig. 4. 



Of the value of Mr. Cheshire's form of Apertometer there can be 

 no doubt. The aim of Mr. Kelson has been to enable the N.A. 

 of an objective to be read on the Apertometer with greater ease 

 and accuracy. 



Distinctness and clearness of reading have been effected by 



APERTOMETER OlAGRAM 

 /S. - 1 >nch. 



(ffcfj) 



Fig. 4. 



increasing the number of marked values of N.A. from 9 to 22, 

 without the confusion that overcrowding of the lines would entail. 

 To accomplish this, short arcs of circles are used instead of whole 

 circles. A valuable property 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. 



