102 . TEXT-BOOK OP BACTERIOLOGY. 



eight hours after the preparation of a gelatin plate containing i 

 ccm. of water from the Spree, a number of characteristic differences 

 in the appearance of the separate colonies can be noticed. 



Some are found which liquefy the gelatin strongly. Some form 

 cup-like circular depressions, the edge clearly marked oflf against 

 the solid substance, and the whole colony looking uniformly gray; 

 in others a thick, crumbly mass is ^een, which consists of heaps of 

 bacteria, lying at the bottom of the liquefied depression; others 

 distinguish themselves by secreting a strong coloring matter^ — not 

 only the liquid colony itself, but also for quite a distance around it 

 the gelatin is dyed with a peculiar yellowish-green color. Then, 

 too, bacteria are observed which liquefy the gelatin more slowly; 

 a somewhat more careful examination is necessary to see that the 

 centre of the colony is slightly sunk, the edges being irregularly 

 jagged. Then a colony may be seen looking like an entanglement 

 of roots spreading its white branches over the plate, and beside it 

 one transparent and of a beautiful purple color. 



Then, again, there are some which do not liquefy the gelatin. 

 Some show themselves as small white dots in the interior of the 

 transparent food medium; others stand out like thick, transparent 

 buttons of porcelain ; also there is one having a beautiful" phos- 

 phorescent green coloring substance, and one that looks like a dry, 

 dirty gray skin. 



They must be seen over and over again in order to become 

 familiar with the different appearances and to be able to recognize 

 them wherever they occur. To this end, nothing is wanted but an 

 attentive eye and the necessary practice. It will be seen from ex- 

 perience how exactly one can fix upon the origin of such a plate, 

 and how correctly the nature of the different colonies upon it can 

 be determined. 



All this, however, is rendered much easier, and the exactness of 

 our investigations become indisputable, as soon as we further utilize 

 the transparency of our food medium and examine our plates 

 directly with the microscope. ~ 



As a rule, we employ for this purpose objectives of low 

 power — Zeiss, 16 mm.; Leitz, 3; Bausch and Lomb, |, etc.— because 

 for our particular purposes a considerable space between the ob- 

 jective and the plate is desirable; the want of high magnifying 

 power we endeavor to compensate for by using strong eye-pieces. 

 As in examining colonies on the plate we have to deal with un- 

 stained objects, a very small aperture of diaphragm, not much 

 above the size of a pin's head, is necessary. In order to examine 

 the whole surface of the gelatin the microscope should have a good 



