MATERIAL WITH WHICH TO BEGIN WORK. 237 



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 noth- 

 ing 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 tolerably regular radii, radiating from a cen- 

 tral spot; and again they may appear as concentric 

 rings; and if by the methods which have been de- 

 scribed we obtain from these colonies their individual 

 components in pure culture, we shall see that this char- 

 acteristic 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: 



11* 



