TEXT-BOOK OF BACTERIOLOGY. 27 



I. THE MICROSCOPE. 



Before going into the procedures at our disposal for preparing 

 the bacteria for microscopic examination, it will be well to direct 

 attention, first of all, to the microscope itself. 



The improvement of the microscope essentially contributed to 

 perfect our methods of procedure. 



In fact, it has been necessary to make a special study of the 

 handling of the microscope, and it has been requisite to provide it 

 with special auxiliary apparatus in order to make it suitable for 

 the purposes of bacteriology. 



The merit of having insisted on this point belongs to Koch. He 

 showed that the manner of using the microscope as was done for 

 histological researches did not suffice for the requirements of bac- 

 teriology, and by recommending the adoption of homogeneous im- 

 mersion and showing the right employment of Abbe's illuminating 

 apparatus in examining colored objects, he made a revolution in 

 the microscopic investigation of the bacteria from which a new era 

 dates. 



It is readily understood that for observing such extremely 

 small creatures as the bacteria it is necessary to use a very strong 

 magnifying power, and that we require all possible perfection in 

 the lenses to be employed. 



What is demanded, then, in a good, faultless system in these re- 

 spects ? 



Three things : First, it must sufficiently and very considerably 

 magnify the object under examination; secondly, it must give a 

 correct image, sharp and well defined; lastly and chiefly, how- 

 ever, the microscope must enable us to analyze the object into its 

 simplest component parts, to distinguish the finest combinations of 

 its lines, the arrangements of its substance, and this distinguish- 

 ing or resolving power contributes far more to the value of a lens 

 than its mere magnifying power. To ascertain the value of a sys- 

 tem the question should not be, How many times does it magnify ? 

 but, How does it define ? 



On what do these three chief points in a microscopic system de- 

 pend? 



The size of the image is in proportion to the focal distance of the 

 lens. For the compound lenses which are now almost exclusively 

 used we calculate from the focal distances of the component parts 

 an average or " equivalent " focal distance for the whole, an imag- 

 inary number which only gives the number of times which the sys- 

 tem would magnify if it had the given focal distance. 



