48 METHODS OF EXAMINATION. 



Koch insists that all tissues in which the presence of micro- 

 organisms is suspected should be hardened in large quantities of 

 absolute alcohol. Such a method, however, is extremely expensive, 

 and, except for small pieces of tissue, which should be kept as test 

 pieces, is scarcely necessary, as methylated spirit, or even a mixture 

 of two parts Miiller's fluid and one part of a two per cent, solution 

 of carbolic acid appears to answer equally well. In putting away 

 tissues to harden, the pieces should never be more than about half an 

 inch or three-quarters of an inch in diameter; and where absolute 

 alcohol is used they should be even smaller, otherwise the centre of 

 the cube is untouched by the reagent, which hardens Very rapidly 

 indeed (within a week). The other fluids harden the pieces in 

 from two to five weeks. Tissues hardened in Miiller's fluid and 

 carbolic acid should be transferred to methylated spirit at the end of 

 five or six weeks, after being washed in distilled water for twenty- 

 four hours. All sections, whether of fresh or of hardened tissues, 

 should be thoroughly washed in absolute alcohol before they are 

 stained (Gram), and sections to be kept for any length of time 

 before they are mounted are best preserved in the same medium. 



Method of Illumination. 



34. As some of the organisms to be examined are exceedingly 

 minute, it is only by the greatest care and with the aid of the most 

 perfect illuminating apparatus that many of them can be recognised, 

 and when their presence is recognised, their position in the tissues can 

 often be determined only by carefully modifying the light and other 

 conditions. The recognised combination in Germany is Zeiss' j^th 

 inch oil immersion lens and an Abbe's illuminator,, and this undoubt- 

 edly combines most of the requisite conditions. The condenser has a 

 very short focus, and must therefore be brought as near to the under 

 surface of the glass as possible. It may then be used as an ordinary 

 condenser if the lateral or greatly converged rays be cut oiT by 

 means of a diaphragm with a small aperture, placed below. If 

 a large aperture in the diaphragm be used, the greatly converged 

 rays of light are also allowed to play on the structures, which are 

 lighted from all points, all shadows disappear, and the " structural 



