130 BACTEEIOLOGY. 



rays, which leave the object (when in focus) and enter the 

 front lens. This ang-le, known as the angle of aperture, is 

 shown at a, in Fig. 18. This figure shows, in cross-section, 

 the lens system employed in the construction of an oil 

 immersion objective. The rays of light diverging from 

 the object enter the front of the small lens. It is obvious 

 that the more light admitted by an objective the more dis- 

 tinct the image. The angle of aperture, 

 therefore, conditions the resolving power of 

 an objective. 



In actual practice, the rays of light, 

 after they leave the object, pass into the 

 cover- glass (a denser medium) and from this 

 into the air, which is less dense. The result 

 is refraction, and, more or less, loss of 

 light by reflection. If the air is replaced. 

 Fig. i8. Arrange- by a denser medium, there will be less re- 



ment of lenses in an 



one-twelfth inch oil fractiou and heuce, it is possible to utilize 



immersion objective *■ 



^fe"f a°'erture'^~^" ^^^^ ^^ ^^^ light that would Otherwise be 

 lost. These considerations led to the intro- 

 duction by Amici, of the water immersion objective. In 

 this objective a drop of water was placed between the 

 cover-glass and the front lens. The ray of light passing 

 from the cover-glass into the water suffered less refrac- 

 tion than it would if it passed into air. Since the index 

 of refraction of water is 1.0 and that of crown glass is 

 1.52, it is evident that there was still room for improve- 

 ment. In 1878, Stephenson suggested that the water in 

 the immersion objectives be replaced by an oil having 

 the same index of refraction as crown glass. Abbe and 

 Zeiss, thereupon, introduced the homogeneous oil immersion 

 objective. The cedar oil, which is placed between the 

 cover-glass and the front lens of the object, has the 

 same index of refraction as crown glass (1.52). Conse- 

 quently, a ray of light after it once enters the cover-glass 

 passes in a straight line directly into the objective. The 



