330 COMPOUND AND ELECTRON MICROSCOPES 



The high-power 4-mm achromatic objective discussed above is de- 

 signed so that its blue image is 0.65 per cent larger than its red image. 

 If the average compensation of a hyperplane eyepiece is 0.80 per cent, 

 this type of eyepiece will very nearly compensate the chromatic differ- 

 ence of magnification in this form of high-power dry achromatic objec- 

 tive. 



If the objective is designated as undercorrected (+0.65 per cent) 

 and the eyepiece as overcorrected ( — 0.80 per cent), then the residual 

 chromatic difference of magnification for this combination is —0.15 

 per cent. If a 16-mm undercorrected chromatic objective is chosen in 

 which the percentage ratio of the magnification of blue to red image is 

 +0.25 per cent and if the image is viewed with a Huygens eyepiece of 

 average compensating power equal to —0.25 per cent, the image seen 

 through this eyepiece will be quite free from primary color. A slight 

 secondaiy spectrum will be present, however. 



Compensating eyepieces are designed particularly for use with all 

 apochromatic objectives. Unless they are employed in this combination 

 the margin of the field will be quite chromatic. 



The residual chromatic difference of magnification for the proper 

 combination of objective and eyepiece in modern microscopes is never 

 greater than 0.5 per cent, even at highest resolution. It should be 

 completely absent in combinations of 16-mm achromatic objective with 

 Huygenian eyepiece, or 4-mm apochromatic objective with compensat- 

 ing eyepiece. 



Area Visible on Slide 



With a 7.5 X Huygenian eyepiece (draw-tube length 160 mm), and 

 virtual-image distance 250 mm, the diameter of the unmagnified area 

 seen as a magnified virtual image is 4 mm when a 4 X , 32-mm achromatic 

 objective is used. The diameter of the area is 1.9 mm with a 10 X, 

 16-mm objective; it is 0.68 mm for a 21 X, 8-mm objective, and only 

 about 0.2 mm for an 1.8-mm oil-immersion objective. 



THE ELECTRON MICROSCOPE 



A considerable increase in the resolving power of optical microscopes 

 or optical micrographic projectors can be obtained if it is possible to sub- 

 stitute for the visible or ultraviolet light, with which the object is illu- 

 minated, a form of radiation of much shorter wavelength, which on 

 passing through a deviating medium would come to a focus and produce 

 images. 



