PROFESSOR ABBE'S APERTOMETER. 235 



diametrical edge. The centre of the semicircle is formed 

 by a small hole in a silvered cover a, cemented to the upper 

 face of the disc. Two indices of blackened brass Z>, b y sliding 

 on the periphery of the disc, afford visible marks for ob- 

 serving the aperture of any objective. On the upper face 

 of the disc there are two engraved scales, the outer indi- 

 cating the numerical aperture by direct observation. 



FIG. 206. 



In order to measure the aperture of any objective the 

 apertometer is placed upon the stage of a microscope, and 

 the objective, the aperture of which is to be measured, 

 roughly focussed to the small hole a, in the silvered cover. 

 In the case of an immersion lens, a drop of immersion fluid 

 must of course be applied. This done, the eye-piece of the 

 microscope is removed and the eye placed to the open 

 tube, the two indices of blackened brass b, b, being moved 

 to and fro until the points just touch the margins of the 

 illuminated field. The position of the indices is then 

 read off on the outer circle of figures, and half the sum of 

 both these readings will give the aperture of the objective. 



For lenses of higher power than the J-inch, a 2-inch 

 objective is provided. This is made to screw into the 

 lower end of the draw-tube and used with an ordinary A 



