METHODS OF STUDYING BACTERIA 31 



equivalent to the one-thousandth part of a millimetre. If 

 those readers who are accustomed to think in British inches 

 will look at a tape-measure or ruler divided into both the 

 British and the metric systems, they will see that an inch 

 contains roughly some twenty-five millimetres, so that a 

 thousandth part of a millimetre will be nearly equal to the 

 one twenty-five thousandth part of an inch. Now, the average 

 bacterium, say a micrococcus, is usually just a little less than 

 one micrvmillimetre in diameter, so that if one were, in journal- 

 istic fashion, to imagine a straight row of them side by side 

 across a halfpenny piece, which measures exactly an inch in 

 diameter, it would take a line of more than 25,000 of them to 

 stretch right across it a fact which gives one some idea of 



5 10 



I i i I I I I I I I 



b. 



. The two larger circles represent the size of a red blood- 

 corpuscle magnified about 2500 diameters, (a) Chain of nine 

 average-sized micrococcL (b) Eighteen average-sized bacilli 

 placed side by side across a red blood-corpuscle. Above 

 and below these are drawn an influenza and an anthrax 

 bacillus respectively. 



(Each small division on the scale represents a micromilli- 

 metre magnified 2500 times.) 



their extreme minuteness. For anyone accustomed to the 

 use of the microscope and who knows what a red blood- 

 corpuscle is like, another useful rough-and-ready standard of 

 comparison can be suggested. A red blood-corpuscle is a small, 

 flattened, disc-like object about seven or eight micromillimetres 

 in diameter, so that we can realise the average size of a coccus 

 by imagining eight or ten of them placed in a row across its 

 flat surface. It has been estimated that it takes 10,000,000,000 

 average-sized bacteria to weigh one milligramme, i.e. a 

 thousandth part of a gramme or one sixty-fourth part of a 

 grain in our measure ! 



Bacteria, of course, vary considerably among themselves in 

 size. The average rounded Bacterium or Coccus is, as we 

 have seen, about 0*8 or 0'9 fi in diameter; and an average 

 rod-shaped Bacterium or Bacillus is about 0'5 fi in breadth, and 

 about 2 or 3 jt in length. The influenza bacillus is much smaller. 



