KOCH'S METHOD OF PREPARING PLATES. 59 



which catches any water which may be spilled. This 

 trough in turn rests on a wooden triangle with a foot at 

 each corner, the height of which can be adjusted, and 

 which thus constitutes the levelling apparatus. A spirit 

 level is placed where the plate is to go, and the level of the 

 ground glass plate thus assured. There is also prepared a 

 "damp chamber," in which the plates are to be stored 

 after being made. This consists of a circular glass trough 

 with a similar cover. It is sterilised by being washed out- 

 side and inside with the perchloride of mercury i-iooo, 

 and a circle of filter paper moistened with the same is 

 laid on its bottom. Glass benches on which the plates 

 may be laid are similarly purified. Three gelatine tubes, 

 marked a, b, c, 1 are now liquefied by placing in a 

 beaker of water at any temperature between 25 C. and 

 38 C. Inoculate 

 a [ with the bac- 

 terial mixture. The 

 amount of the latter 

 to be taken varies. If 

 the microscope shows 

 enormous numbers of 

 different kinds of bac- 

 teria present, just as 

 much as adheres to 

 the point of a straight 

 platinum needle is 

 sufficient. If the 

 number of bacilli is FIG. 19. Koch's levelling apparatus, 

 small, one to three Hands shown in second position just as the 

 loons' of rhp mivl-nrp P late is lowered - to the ground glass surface. 

 ' By executing the transference of the plate from 



may be transferred to the box in this way, the surface which was 

 the medium. Shake undermost in the latter is uppermost in the 



a well, but not so as 



leveller, and thus never meets a current of air 

 which might contaminate it. 

 to cause many fine 



air-bubbles to form. Transfer two loops of gelatine from a to 



1 For marking glass vessels it is convenient to use the red, blue, or 

 yellow pencils made for the purpose by Faber. 



