136 THE AMERICAN MONTHLY [Aug 



As 10 : 24 plus 10 : : 80 : x=272 diameters. This 

 shows clearly the advantage to be gained by using an eye- 

 piece. 



These are but rough and ready rules ; for very exact 

 work it is essential that a stage micrometer should be 

 used, and the enlarged image divided by the real meas- 

 ure gives the magnification. 



Modern Methods and Their Achievements in Bacteriology. 



In order to convey some concrete idea of the extreme 

 minuteness of bacteria, it has been mentioned that if a post- 

 age stamp f inch long and I inch wide (22-2 mm. by 

 19-05 mm.) were covered by a single layer of the typhoid 

 bacteria, placed end to end and side by side, 500,000,000 

 bacteria would be required ; and further, that the same 

 area, covered to the depth of one-tenth of an inch 

 (2-54 mm.) would accommodate no less than 2,000,000 

 million of these microscopic creatures. If beef-broth be 

 sterilized, and to the limpid liquid be added bacteria 

 known as Staphylococcus aureus, in the proportion of 246 

 per cubic centimetre of broth, and the whole maintained 

 at the temperature of the animal body (about 98 deg. F.) 

 for twenty-four hours, it will be found that the liquid has 

 become quite turbid, and calculation will reveal the pres- 

 ence of 20,000,000 bacteria in every cubic centimetre of 

 the solution. In other words, each original bacterium 

 has become 80,000. The bacterium which causes fowl 

 cholera, an epidemic disease which quickly decimates a 

 large fowl yard, is so abundant that the blood of an 

 infected fowl teems with them to the extent of 15,000,000 

 to each cubic centimetre. Indeed, if one-fiftieth of a 

 drop of this blood be injected into a healthy rabbit the 

 animal sickens and dies in twenty-four hours, and the 

 blood in its body contains about 12,000,000,000 bacteria. 



