EXPOSURE AND CONTACT. 169 



If uow we examiae these points upon our bread or 

 potato with a hand-lens of low magnifying power we 

 will be enabled to detect differences not noticeable to the 

 naked eye. In some cases we shall still see nothing more 

 than a smooth non-characteristic surface; while in others, 

 minute, sometimes regularly arranged corrugations may 

 be observed. In one colony they may appear as toler- 

 ably regular radii, radiating from a central spot ; and 

 again they may appear as concentric rings ; and if by 

 the methods which have been described we obtain from 

 these colonies their individual components in pure cul- 

 ture, we shall see that this characteristic arrangement 

 in folds, radii, or concentric rings, or the production of 

 color, is under normal conditions constant. 



So much for the simplest naked-eye experiment that 

 can be made in •bacteriology and which serves to furnish 

 the beginner with material upon which to begin his 

 studies. It is not necessary at this time for him to 

 burden his mind with names for these organisms; it is 

 sufficient for him to recognize that they are mostly of 

 different species and that they possess characteristics 

 which will enable him to differentiate the one from the 

 other. 



In order now for him to proceed it is necessary that 

 he should have familiarized himself with the methods 

 by which his media are prepared and the means em- 

 ployed in sterilizing them and retaining them sterile — 

 i. e., of preventing the access of foreign germs from 

 without — otherwise his efforts to obtain and retain his 

 organisms as pure cultures will be in vain. 



Exposure and Contact. — Make a number of plates 

 from bits of silk used for sutures, after treating them 

 as follows : 



