Principles of Microscopy 7 



verging to form a red image. The blue components are bent most and 

 therefore travel a relatively short distance before converging to form a 

 blue image. When these two images are seen in front view, the central 

 portion, in which all the colors are superimposed, still appears white. 

 The red image is, however, larger than the blue image and sticks out 

 around it. The edges of this image therefore show up as a color fringe 

 around the outside of the white image. 



Correction of Aberrations. Spherical aberration is theoretically easy 

 to correct since it is caused by the difference in distance between the 



MENISCUS IRIS DIAPHRAGM "CODDINGTON" DOUBLET 



5 6 7 8 



Figs. 5, 6, 7, and 8. Methods of correcting aberrations. The meniscus (Fig. 5) dimin- 

 ishes the difference in thickness between the center and the edge. An iris diaphragm 

 in front of the lens (Fig. 6) or a "Coddington" lens (Fig. 7) produces the same effect 

 at the expense of die aperture. The action of the doublet (Fig. 8) in diminishing 

 chromatic aberration is discussed in the text. 



center and the edge of a lens. The easiest method (Fig. 6) is to place 

 a diaphragm in front of the lens so that only the center is used. The 

 same effect (Fig. 7) is obtained from a cylindrical piece of glass with a 

 lens face ground on each end. This, which is to all intents and purposes 

 the center cut from a larger lens, is often sold as a hand "magnifying 

 glass." The last commonly used method (Fig. 5) is the so-called "menis- 

 cus lens" in which one face partially compensates for the other. Com- 

 binations of all three of these ways of correcting shape distortions are 

 used in microscope objectives. 



The correction of chromatic aberration is much more difficult. The 

 only solution so far discovered makes use of the fact that the relation 

 between "index of refraction" and "dispersion" differs in different kinds 

 of glass. 



Take, for example, the combination shown in Fig. 8. If the positive 

 lens at the left is made of a glass of high refractive index and low disper- 

 sion, it will bend the light a great deal and separate the colors very little. 

 The negative lens on the right, if made of a glass with a lower index of 

 refraction, will not bend the light out as much as the one on the left 

 bends it in, so that an image will be formed. But if the negative lens on 



