286 F. J. CHESHIRE, TWO SIMPLE APERTOMETERS FOR DRY LENSES. 



scale upon which N.A. values can be read directly. The scale 

 runs from 00 to 0*9 N.A. by steps of 0*05, i.e. the divisions 

 starting from the centre have the values 0, 0'05, 0"10, 015, - 20, 

 etc., of N.A. 



The short curved lines of the scale should strictly be hyperbolas, 

 but such curves are very difficult to draw accurately, and it was 

 not until my son, Mr. R. W. Cheshire, suggested to me that they 

 might be replaced by arcs of circles with curvatures equal to 

 those of the corresponding hyperbolas at their vertices that the 

 apertometer described became a practical construction. 



I may, perhaps, be allowed to avail myself of this opportunity 

 to say that in my opinion there are several objections to 

 Mr. Nelson's form of the Apertometer which was introduced 

 by me in 1904. These may be briefly indicated. In the first 

 place, no advantage can result from the use of the outer edges 

 of the lines, instead of the middles, as is usually done, as the 

 part of the lines from which distances and therefore NA.'s 

 must be estimated by eye. Further, in Mr. Nelson's form the 

 thickness of the lines varies in different parts of the diagram, 

 and has no assigned or stated thickness in terms of N.A. This, 

 I think, is a fatal defect, because when the thickness of a line 

 has a N.A. value of - 02, say, 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 sub- 

 division 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. 



Joum. Quekett Microscopical Club, Ser. 2, Vol. XII., No. 74, April 1914. 



