62 Modern Microscopy 



the wave-frequency of light used, equals twice the numerical 

 aperture. 



These calculations are based on the assumption that 

 annular or some other form of oblique illumination is used. 

 With a solid cone of illumination equal to the numerical 

 aperture of the objective no fine detail is visible ; it 

 becomes blurred, and, in practice, when using solid cones 

 of illumination, it is usual to make them fill | only of the 

 back lens of the objective. This will be found treated on 

 page 97, in connection with condensers. 



Under these conditions the number of lines per inch 

 that will be resolved by the objective will be ascertained by 

 multiplying the wave-frequency of the light used by f , and 

 then multiplying the product by the numerical aperture.* 



The Apbetometbb. — To enable the numerical apertures 

 of objectives to be taken without a calculation, Professor 

 Abbe devised the apertometer. It consists of an almost 

 semi-circular plate of glass, having the diametrical edge 

 ground to an angle of 45°, while the circumference is a 

 polished cylindrical surface. It is shown in Fig. 17. 



The centre of the semi-circle is marked by a silvered disc, 

 a, having a very small central aperture, and on the upper 

 surface on the periphery it is provided with a scale of 

 divisions, indicating both angular and numerical apertures. 

 The manner of using the apertometer is as follows : The 

 microscope is placed in a vertical position, and the aperto- 

 meter is laid on the stage, with the diametrical edge 

 towards the limb of the instrument. The objective that 

 it is desired to take the aperture of is then screwed 

 on, and the objective focused on the plain centre of the 

 silvered disc. It is well now to fix the apertometer to the 

 stage, either by springs or an elastic band, to prevent its 

 moving. The two pointers, b, are then set on the edge of 

 the circle to read zero. The draw-tube and eyepiece with 

 which the silvered disc has been set are removed, and at 

 the lower end of the draw-tube a special objective of low 



* E. M. Nelson, Journal of the Eoyal Microscopical Society, 1893, p. 15. 



