INTRODUCTION 19 



crude lenses of his own manufacture, chanced to observe the 

 presence of motile and motionless microorganisms in the tartar 

 from teeth and in various decaying organic materials. His 

 correspondence with the Royal Society of London and the figures 

 published leave no doubt but that he actually observed bacteria. 

 These drawings are of such historic interest that they are here 

 reproduced. 



Each advance in the efficiency of the microscope was followed 

 by an advance in our knowledge of the microorganisms, although 

 speculation frequently outran the ability to see clearly. The com- 

 pound microscope has proved to be indispensable in the study of 

 these forms. Since the introduction of this instrument the degree 

 of magnification, the clearness of definition, and the mechanic ar- 

 rangements for accurate focusing have been gradually improved 

 until at the present time the homogeneous oil immersion objective, 

 the compensating ocular, and the Abbe condenser are in constant 

 use in the laboratory, and enable us to secure readily magnifica- 

 tion to 1500 diameters or more. During the last several decades 

 there has been little increase in magnification, due to two reasons. 

 The greater the magnification the more convex and consequently 

 the smaller must be the lenses used in the objectives, and the more 

 difficult becomes their grinding and adjustment. Furthermore, 

 the physicist tells us that a clear view, with determination of the 

 size and shape of microscopic objects, cannot be obtained when the 

 objects examined are smaller than one-half the wave length 

 of the rays of light in which they are examined. There is thus 

 a seemingly insurmountable barrier set to an indefinite increase 

 in magnification. 



A recent advance has been made through the development of 

 the ultramicroscope. This has made visible objects much smaller 

 than those which had been previously observed. A bright gleam 

 of light from an arc or similar source is passed across the darkened 

 field of the microscope, and the light is reflected to the eye from 

 any particles that may be in suspension. These objects are 

 seen in the same manner that minute particles of dust are made 

 visible in a bright ray of light that enters a darkened room. The 

 use of the ultramicroscope has not as yet added many facts of 

 value to our knowledge of the bacteria. 



