324 COMPOUND AND ELECTRON MICROSCOPES 



that some color is present which becomes disturbing when high-power 

 eyepieces and oblique illumination are resorted to. The residual color 

 which exists in the images produced by achromatic objectives is known 

 as the " secondary spectrum." Its presence is due to the difficulty, if 

 not impossibility, of finding two kinds of glass in which the dispersion 

 is exactly proportionate. By using three glasses it is possible to remove 

 the secondary spectrum. 



Apochromatic Objectives 



These objectives are designed to reduce the secondary spectrum 

 characteristic of achromatic systems. Apochromatic objectives con- 

 sist of several fluorite lenses and contain correcting lenses which remove 

 the secondary spectrum. They are made with large numerical apertures 

 giving the same approximate initial magnification as the achromatic 

 objectives. They are also corrected for spherical aberration for all 

 colors. Much higher-power eyepieces can, therefore, be used so that 

 higher total magnification can be obtained. 



Immersion Objectives 



The lower faces of immersion objectives are immersed in a liquid 

 instead of air. They are usually designated by the name of the liquid 

 used. Water-immersion objectives have water between the cover glass 

 or the object and the face of the lower lens. Homogeneous or oil- 

 immersion objectives are immersed in a liquid whose index of refraction 

 must be the same as that of optical glass so that light suffers no refrac- 

 tion in passing from the glass slide and cover glass into the immersing 

 liquid and thence into the objective. The medium below the lens is 

 therefore homogeneous, as illustrated in Fig. VIII-4. Thickened cedar- 

 wood oil has proved very satisfactory. Canada balsam of moderate 

 thickness in connection with white light gives good, though not correct, 

 results. See Table VIII-2 for comparative indices of refraction. 



Proper Illumination 



In working with objectives of numerical aperture greater than 1.0 an 

 important factor must not be overlooked. The cone of light entering 

 the objective must be equal to the numerical aperture of the objective, 

 or a narrow diagonal pencil of light entering at that angle may be used 

 if the full resolving power is to be obtained. For instance, no matter 

 if the numerical aperture of the condenser is greater than 1.0, it cannot 

 send light into air, in contact with its upper surface, at an angle greater 

 that that corresponding with 1.0 N.A. 



