SPIROCHAETES 



79 



product- were no longer individually visible even under tin- i 

 powers oi the microscope. The practical interest of this is in connexion 



with diseases due to " invisible germs." Thus Yellow Fever has been 

 pm\r<l to be due to a germ : the germ has never been seen but yet it 

 has been roughly measured, for by filtering infected fluid through a 



