288 M. A. AINSLIE ON A VARIATION OF CHESHIRE'S APERTOMETER, 



of a card diagram placed on the stage, is constructed as follows- 

 (% 7): 



A series of radial lines are drawn from a common centre, 

 making equal angles with one another ; the precise number is- 

 immaterial, but it has been found convenient to divide the circle 

 into sixteen equal parts. One of these (preferably that lying^ 

 horizontally) is selected as a zero, and points are marked off 

 along the others at distances equal to a constant length (usually 

 25 mm., or 1 inch) multiplied by the tangent of the semi-angle 

 of aperture ; i e. the tangent of the angle whose sine is the- 

 numerical aperture. This is done for every (H of N.A., and 

 a spiral curve drawn through the points thus obtained ; this. 



Fig. 7. 



curve being repeated, turned through 180. The curves are 

 shown with fair accuracy in fig. 7. 



The diagram is used precisely as the Cheshire Apertometer : 

 either the objective is focused on the upper surface of a cube of 

 wood as in the Cheshire instrument ; or else a pinhole in the 

 centre of the diagram is focused, and the body racked back 

 25 mm., or 1 in., this being measured easily enough with a 

 scale. This latter method is preferable for objectives of high 

 aperture. A |low-power eye-piece is employed. On examining 

 the Ramsden disc with a hand lens (a watch-maker's eye-glass 

 does well) the appearance in fig. 8 is seen, and the method of 

 estimating the value of the N.A is fairly obvious ; we have only 

 to start from the zero and count in the direction of the spiral, 



