4 TWO NEW WORLDS 



instrument as far superior to the microscope as the 

 microscope is superior to the naked eye may not 

 reveal further worlds of hitherto unsuspected life, 

 which may possibly be more difficult to destroy 

 than the minute organisms of the pond? 



The smallest object which can be distinctly 

 perceived by the normal human eye at a distance 

 of 10 in. is trrv m - m diameter; or, more strictly 

 speaking, when two black objects are separated 

 by a bright interval ^ in. wide, they are perceived 

 as separate objects. 



Now a good microscope magnifies about two 

 hundred times, so that the limit of resolving power 

 is brought down to TOSTO m - This is sufficient for 

 practically all purposes for which the microscope 

 is usually intended. A human blood corpuscle is 

 rejnj- in. in diameter, and is therefore a very large 

 object for a powerful microscope. Of yeast-cells 

 there are 3000 to the inch, and the amoeba is of 

 the same size. The spores of some fungi are as 

 many as 6000 or even 8000 to the inch, whereas 

 the spores of the anthrax bacillus are not more 

 than sT^ou- in. in diameter. The markings of 

 diatoms are considered somewhat exacting test- 

 objects for good microscopes, and 30,000 of them 

 have been counted to the inch, and the same size 

 must be assigned to the bacilli of tuberculosis. 



Small as these objects are, they do not represent 

 the extreme limits of microscopic vision. 



