11 



Possibly he merely made use of a type already in existence, 

 but this does not detract in any way from the value of the 

 contribution which he so modestly entitles "Eine Kleine Modifi- 

 cation." Actually it appears to have been one of those fortunate 

 inventions or adaptations, which despite lack of any funda- 

 mental novelty, nonetheless have very greatly advanced the 

 convenience of mankind. 



By comparison with the earlier apparatus, it simplified 

 bacteriological technique materially, obviating the need of 

 using any longer the cumbersome apparatus with which 

 Koch had made his transcendent discoveries. Fully to appre- 

 ciate the advantages of Petri's "kleine Modification," it will 

 be worth while to reproduce a brief description of the sup- 

 planted method, quoted from Muir & Ritchie, Manual of 

 Bacteriology, pages 60 and 61, 1913. 



Glass Plates (Koch).* 



When plates of glass are to be used, an apparatus on which they may 

 be kept level while the medium is solidifying is, as has been said, necessary. 

 An apparatus devised by Koch is used (Figs. 17, 18). This consists of a cir- 

 cular plate of glass (with the upper surface ground, the lower polished), 

 on which the plate used for pouring out the medium is placed. The latter 

 is protected from the air during solidification by a bell-jar. The circular 

 plate and bell-jar rest on the flat rim of a circular glass trough, which is 

 filled quite full with a mixture of ice and water, to facilitate the lowering 

 of the temperature of whatever is placed beneath the bell-jar. The glass 

 trough rests on corks on the bottom of a large circular trough, which catches 

 any water that may be spilled. This trough in turn rests on a wooden tri- 

 angle 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 outside and inside with perchloride of mer- 

 cury 1-1000, 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. 



To separate organisms by this method, three tubes, a, b, c, are inoculated 

 as in using Petri's capsules (p. 58). The hands having been washed in per- 

 chloride of mercury 1-1000 and dried, the plate box is opened, and a plate 

 lifted by its opposite edges and transferred to the levelled ground glass (as 

 in Figs. 17, 18). The bell-jar of the leveller being now lifted a little, the gel- 

 atin in tube a is poured out on the surface of the sterile plate, and while 



^ Muir & Ritchie — Man, of Bacteriology. Macmillan 1913, page 60, 61. 



