20 



BACTERIOLOGY 



correction may be thanged by altering, the distance between these 

 component lenses, as, for example, in an objective equipped with 

 a correction pollar. Objectives corrected in respect to spherical 

 aberration are designated as aplanatic. Restriction of the size 

 of the field is also an important factor in making it appear flat. 

 Light of different wave lengths (different colors) is refracted 

 to a different degree by the simple lens, so that, for example, the 

 violet rays are brought to a focus earlier than the red rays, with 

 the remainder of the spectrum spread out between. This defect 

 is known as chromatic aberration. It is corrected to a very con- 

 siderable extent by combining biconvex lenses of crown glass 



Fig. 6.- 



-Microscope objectives showing the component parts o£ the objective lens 

 system. 



with plano-concave lenses of flint glass (achromatic objectives),, 

 to a still nicer degree by combinations of lenses of several different 

 kinds of glass together with a lens of fluorite (apochromatic ob- 

 jectives) ; and finally, when desired, chromatic aberration may 

 be wholly avoided by employing mono-chromatic light. 



A thifd defect of lenses is known as diffraction, which is a 

 phenomenon giving rise to a whole group of less luminous second- 

 ary images around the principal image. The influence of diffrac- 

 tion is most evident when the surfaces of the lens are roughened 

 by scratches or by presence of dust, but even the most perfect 

 lens systems are not wholly free from diffraction phenomena. 



