30 AIDS TO BACTERIOLOGY 



Limits of Microscopical Vision. There is a limit to 

 the visibility of microscopical objects. With the very best 

 optical appliances and the use of monochromatic violet 

 light it is impossible to see more than about 167,000 lines 

 to the inch, an object measuring less than about 0-14//, not 

 being perceptible. By means of transverse illumination, 

 ultra -microscopic particles may be rendered visible as 

 diffraction discs, and particles measuring far less than 

 half a wave-length of light can be made visible (Siedentopf ' s 

 apparatus). 



The Hot-Air Steriliser. An iron box, with double 

 walls, fitted with a door in front and supported on four 

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

 below, and the temperature of the interior is indicated by 

 means of a thermometer inserted through a hole in the 

 top. If necessary, a mercury-gas regulator may be 

 inserted through a second opening. 



The temperature in these ovens is by no means uniform ; 

 it therefore must be ascertained that the objects exposed 

 for sterilisation really reach the desired temperature. 



Test-tubes, Petri dishes, pipettes, etc., may be thoroughly 

 sterilised by exposure to a temperature of 150 C. for one 

 hour. The door must not be opened until the temperature 

 has dropped to 60 C. Neglect to observe this precaution 

 courts cracked glass. Inoculating-wires, forceps, etc., 

 are best sterilised by passing through the flame of the 

 Bunsen burner. 



The Steam Steriliser. This is a cylindrical vessel of 

 copper, about \ metre high by about 30 centimetres wide, 

 jacketed with non-conducting material, and provided with 

 a lid. The lid is covered with felt, and is perforated to 

 receive a thermometer. Inside the vessel is a diaphragm 

 or grating about two-thirds down which divides the 

 interior into two portions: the upper, or ' steam- chamber/ 

 and the lower, or ' water-chamber.' A water-gauge 

 indicates the water-level. The apparatus stands upon 

 three legs, and is heated by a large Fletcher burner, keep- 

 ing the water in vigorous ebullition, so that steam issues 

 freely from the top. A uniform temperature of 100 C. 

 is thus maintained in the apparatus. The steriliser is 

 fitted with a wire basket or metal rack for the reception 

 of test-tubes containing nutrient media. 



This apparatus is employed for sterilising media and 



