30 APPLIED BACTERIOLOGY 



when in tissues. Failing bright dayhght, the most satisfac- 

 tory source of light for microscopical work is, in our opinion, 

 the Welsbach incandescent gas-light. This gives very 

 brilliant illumination combined with perfect steadiness. 



Micro-organisms are somewhat difficult to observe in 

 liquids and tissues, being only visible through the shadows 

 caused by the differences in the refractive power of the 

 various structures. Hence but little light should be used, 

 and consequently the hole in the diaphragm must be as 

 small as possible. In the case of stained specimens, how- 

 ever, an open diaphragm can be used, and the preparation 

 examined with the full power of the Abb^ condenser. 



After using the oil immersion objective, the cedar-oil 

 used must be removed from the lens with soft filter-paper, 

 and then wiped with a silk handkerchief. Should the oil 

 have been allowed to dry on at any time, it is best re- 

 moved by placing on a little fresh oil and allowing to stand 

 a short time ; this will soften the hardened oil, when the 

 whole may be cleaned off together. 



The Hot-air Steriliser. — This is an iron box fitted with 

 double walls, with a door in front. The whole is supported 

 on four legs. It is heated by means of a rose gas-burner 

 from below, and the temperature of the interior is in- 

 dicated by means of a thermometer inserted through a 

 hole ia the top. If necessary, a mercury gas-regulator 

 can be inserted through a second opening. 



It must be borne in mind that the temperature in these 

 ovens is by no means uniform ; care, therefore, must be 

 taken that the objects exposed for sterilisation really 

 reach the desired temperature. 



Test-tubes, dishes, plates, cotton-wool, etc., may be 

 thoroughly sterilised by exposure to a temperature of 150° C. 

 for one hour. The cotton-wool is put loosely into a 

 beaker, and placed with the tube or plates that are being 



