THE BACTERIOLOGICAL MICROSCOPE. 69 



do not vary slightly in thickness, the most perfect definition 

 can only be obtained by adjusting for each separate cover-glass 

 preparation. 



Immersion system. All objectives were formerly used dry 

 that is to say, with an air space between the objective and the 

 specimen to be examined but high-power objectives are now almost 

 entirely made on the immersion system, a drop of liquid being 

 interposed between the objective and the cover-glass. 



About fifty years ago Amici observed that if a drop of water 

 intervened between the cover-glass or an uncovered object and the 

 lens the image was more brilliant. The passage of rays from the 

 object or the cover-glass into air, 

 and again from air into glass, caused 

 considerable loss of light. With 

 objectives of wide angle of aperture 

 the advantages were counteracted 

 by the reflection of rays falling ob- 

 liquely upon the lens. By inter- -. 



posing water more rays are bent 

 in. or refracted, and enter the lens 

 instead of being reflected and lost. 



Hartnack, Nachet, and others 



adopted the immersion svstem, and 



, . , . , FIG. 15. OBJECTIVE WITH COLLAR 



high-power water immersion lenses CORRECTION (6). 



were constructed with high angle 



of aperture.* It was found that there was less necessity for 

 correcting for covers of different thickness, as the aberration from 

 this cause was diminished. The lenses were corrected for an average 

 thickness of cover, and slight deviations produced hardly any 

 appreciable effect. 



Wenham, Stephenson, Abbe, and Zeiss carried the system to 

 perfection. They argued that the advantages obtained by water 

 immersion would be intensified if a liquid could be found of the 

 same refractive and dispersive power as crown glass. The media 

 would be optically uniform, and the residt a homogeneous immersion 

 system. 



* The angle of aperture is " the angle made by the most diverging of the 

 rays of the pencil issuing from any point of an object that can enter the lens, 

 and take part in the formation of an image of it." 



The numerical aperture is defined by Abbe as equal to " the sine of the 

 angle of aperture multiplied by the refractive index of the medium between 

 the object and the objective." 



