348 Transactions of the Society, 



• 

 of foci;sing and centring employed in the case of an objective 

 cannot be used with the condenser, because the light source 

 cannot be replaced by an eye-piece without considerable incon- 

 venience. An alternative method is therefore best employed, 

 which may be described as follows : — To the sloped reflecting 

 edge of the Abbe apertometer, and immediately above the cover 

 glass, is temporarily mounted the hypotenuse of^ a small right- 

 angled prism (for this purpose stiff cedar-wood oil, or, better, 

 bifocal cement, will be found satisfactory). The upper surface 

 of the small prism should now be parallel with the plane of the 

 stage, and therefore an image of the silvered strips of the aper- 

 tometer may be clearly observed by means of any low-power 

 objective with sufficient working distances, since the spherical 

 aberration introduced by the considerable thickness of glass does 

 not cause sufficient degradation of the image to matter for the 

 present purpose. The images of the strips having been sharply 

 focused, the image of the light source is sharply focused in the 

 same plane by adjusting the condenser. The right-angled prism 

 is now removed, and the sloped surface of the apertometer care- 

 fully cleaned, in order that the rays incident on it may be 

 reflected to the periphery of the plate without degradation. This 

 must be done without removing the apertometer from the stage 

 or spoiling its adjustment relative to the condenser. A more 

 elaborate method, which avoids any difficulty due to this cause, 

 necessitates the application of a semi-transparent coating of silver 

 to the sloped surface of the apertometer. Above this is per- 

 manently cemented the right-angled prism, so that the greater part 

 of the light is internally reflected at right angles, to spread out 

 towards the periphery of the apertometer, while sufficient light 

 passes through the silver and prism to the low-powered objective 

 in order that accurate focusing and centring may be effected. It 

 should be noted that such a procedure does not in any way spoil 

 the apertometer for more genei'al use, so that when much work 

 of this nature is to be done this latter technique should be 

 employed. 



BiBLIOGKAPHY. 



1. Abbe. — Journ. Roy. Micr. Soc, 1880, p. 20. 



2. Hartridge,— Proc. Roy. Soc, 1913, B, p. 128. 



3. AiNSLiE. — Quekett Journ., 1914, p. 287. 



4. Spitta. — Microscopy, 1907, p. 432. 



5. Cheshire. — Journ. Roy. Micr. Soc, 1914, p, 398. 



6. AiNSLiE. — Journ. Roy. Micr. Soc, 1914, p. 400. 



7. Spitta.— Microscopy, 1907, p. 97. 



8. Travis.— Journ. Roy. Micr. Soc, 1907, p. 362. 



9. Hartridge. — Journ. Roy. Micr. Soc, 1913, p. 363. 



