The Microscope. 



9 



Fig. 3. — A part of a 



order to experiment effectually it is neces- 

 sary to have the culture abtJolutely pure, and 

 it is obtained in this manner : In the first 

 place take some of that treated as above and 

 place it in another tube. This is repeated ; 

 after 50 or 60 successive changes of this 

 nature a residuum is obtained which is called 

 pure culture, that is to say, it contains abso- 

 lutely nothing but the microbe which it is 

 desired to study. The pure culture of bacilli 

 of tuberculosis is represented in the photo- 

 graph which we have reproduced. Fig. 4, the 

 negative of which came from the Koch labor- 

 atory. It gives perfectly the idea of what 

 may be seen in the field of the microscope. 

 Each one of the black lines seen in the pho- 

 tograph represents a bacillus, that is, a 

 pathogenic organism, which is the cause of 

 the disease and which was discovered by 



culture tube containing Koch. It was upon these pure cultures that 

 the germs of bacilli of j{;och made his first experiments, to try upon 

 Koch's laboratory. each one a long series of chemical reagents, 

 of which the following are the principal : In the series of ethers, 

 etherized oil ; of the series of aromatics, B naphthaline, para- 

 toluidine, xylidine, fuchsine ; among the colors, gentian violet, 

 methylene blue, China yellow, anilin yellow, orimene; of metals, 

 tin, silver, and gold. He found the action of the last of these 

 the most energetic of all. 



It only required one or two millionths of chloro-cyanide of 

 gold to stop all development of bacilli of tuberculosis in pure 

 culture. It is seen what an enormous amount of time and 

 trouble was required to make all these tests. Koch saw not only 

 that all the substances which we have cited, but many others 

 with which he experimented — a list of which would be too long 

 to give here — have the power in a test tube of arresting the de- 

 velopment of bacilli of consumption. He had, therefore, fin- 

 ished the first part of his programme in searching for the sub- 

 stances which when mixed with pure culture of bacilli of tuber- 

 culosis were able to arrest their development. He passed on 

 then to the second part of the programme, viz., experiments 



