2O 



BACTERIOLOGY 



Points in the same plane in the object are imaged by the simple 

 lens on a curved surface, the segment of a spherical surface. 

 This defect is known as spherical aberration. It is diminished 

 to some extent by combining convex and concave lenses and the 

 correction may be changed by altering the distance between these 

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

 a correction collar. 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 



FIG. 6. Microscope objectives showing the component parts of the objective 

 lens system. (After Leitz.} 



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 

 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 third 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 dif- 



