Chap, xxvi ii.] CORRECTION FOR ABERRATION. 359 



and will see a virtual image A'B'. Now it is to be 

 observed that while ab is a real image of AB, A'B' is 

 only a virtual image of ab. In other words, it is an 

 image of an image. If the lens LL' is an ordinary 

 one. the image ab will exhibit spherical and chro- 

 matic errors, and consequently the image A'B' will 

 exhibit these still more, since it is a magnified image 

 of ab. Errors, that is, made by the object-glass, are 

 all exaggerated by the action of the eye-glass, and the 

 more refracting the lenses the more striking are the 

 aberrations. Hence it is easily seen how difficulties 

 grow in the effort to get higher magnifying powers, 

 and how specially great are the difficulties in the way 

 of the development of compound microscopes. 



Between the lens LL' and the position in which 

 its image would be formed, there may be interposed 

 another convex lens, the effect of which will be to 

 refract to a greater extent the rays going to form the 

 image 6, and thus to produce a smaller image, the 

 whole of which will more easily come within the 

 range of the eye-glass. The eye-glass and this addi- 

 tional glass are placed in one tube at a proper distance 

 from one another, and their combination is called the 

 eye-piece, the lens next the eye being still called the 

 eye-glass, and the distant one being the field-glass. 



Correction for aberration in microscopes. 

 The compound microscope was rendered practically 

 useless by reason of aberrations, till the discovery of 

 Hall and Dollond rendered it possible to correct a 

 lens so as to destroy its dispersive power without 

 abolishing its refractive power. It has been pointed 

 out (page 347) that a double convex lens of crown 

 glass properly adjusted to a plano-concave lens of flint 

 glass makes an achromatic combination for two colours, 

 but for only two. This is not, however, sufficient for 

 microscopic objects. By the labours of MM. Selligues 

 and Chevalier of Paris (1823), and those of Professor 



