APOCHROMATIC OBJECTIVES 55 



known as " chromatic difference of magnification." The 

 different coloured images are of varying size, the blue 

 larger than the red, and consequently they do not unite 

 to form a colourless final image. This is corrected by 

 using oculars specially constructed to magnify the 

 original red image more than the blue, and called 

 " compensating oculars " (see p. 72). In order that 

 these oculars may be used with the whole series of 

 apochromatic objectives, the same under-correction is 

 intentionally introduced into the lower powers. 



Superiority of Apochromats. The chief advantages 

 of apochromats may be stated thus : 



1 . Their corrections are so good, and the images they 

 produce so perfect, that a high eyepiece can be used to 

 further magnify them without showing defects. 



An eyepiece magnifying the image produced by the 

 objective 6 or 8 times is about the highest that can 

 be used with an ordinary achromat ; a good apochro- 

 mat will stand 18 or even as much as 27 times without 

 breaking down. 



2. They can be given a greater numerical aperture 

 (p. 160), and the aperture can be used to its limit, which 

 is not usually the case with an achromat. 



3. They produce a more brilliant image than an achro- 

 mat, and can therefore be used with higher eyepieces 

 without an undue loss of brightness. 



4. Their corrections are so good all over that the 

 colour defect, introduced into the high powers by the 

 hemispherical front lens, can be corrected by an eye- 

 piece much more perfectly than is possible in an achro- 

 mat. 



5. They form a perfect image right to the edge of the 

 field. This does not imply that the whole field can be 

 simultaneously focussed. In fact, it is frequently less 

 flat than * that of. an achromat. 



6. An object focussed by white light will still give 



