METHODS OF RESEAECH I APPARATUS 99 



the labyrinth. This cost only a small sum and has proved 

 very satisfactory. 



The electric bulbs for the red lights are outside but may be 

 turned on or off without leaving the dark-room, by means of a 

 double push button at Z (Fig. 61). 



USES OF CULTURE-MEDIA 



Culture-media are needed every day in the routine work 

 of the laboratory and are required for several distinct purposes : 

 (1) for the isolation of organisms from mixtures or directly from 

 diseased tissues; (2) for the long-continued growth of organisms 

 without loss of virulence; (3) for differential descriptive pur- 

 poses; (4) for cultures adapted to chemical analysis. 



For the first purpose we must study the nature and needs 

 of the various parasites, and when they differ from the common 

 sorts must cater to them, devising media exactly suited to their 

 requirements, or at least better adapted to the needs of the para- 

 site than to those of the accompanying saprophytes. This 

 frequently requires considerable study, as to range of toleration 

 of acids, alkalies, salts, N.-compounds, sugars, alcohols,- etc., 

 but standard media should be tried first. For first isolations I 

 always try +15 peptone-beef-agar poured plates. If this fails, 

 then +7 beef -peptone agar may be tried, and other media, such 

 as steamed potato and special agars, e.g., dextrose potato agar 

 or whey agar. 



The second kind of media varies a great deal with the or- 

 ganism and can be discovered only after prolonged study of the 

 parasite on a variety of substrata. Some observations on such 

 media will be found under the various diseases described in 

 Part III. 



The third sort need not be, and in fact cannot be, media of 

 universal value. They are good only for the particular purpose 

 in mind, and the future will see a large increase in their number. 

 What we seek here are media that will bring out not necessarily 

 good growth, or any growth at all, for that matter, but differ- 

 ences in behavior when a variety of bacteria are tested in it, that 

 is, changes in gross appearance, morphology, pigmentation, pre- 

 cipitates, pellicles, crystals, weak vs. dense clouding, medium 



