THE OBJECTIVE 125 



dry objective. An error in the choice of objectives for 

 routine work is sometimes the getting of objectives of a 

 too short focal length and too high aperture for the work 

 in hand, together with too poor a condenser. Thus an 

 oil-immersion 90 objective of 1.3 aperture, or a dry 40 or 

 dry 60 objective, are often advantageously replaced by 

 an oil-immersion objective of 0.85 or 1.0 aperture, and a 

 20 dry objective respectively; while the saving in cost 

 would perhaps enable a dry achromatic condenser to be 

 procured. 



The apochromatic objective of 1.4 aperture is not 

 always equal to its fellow of 1.3 aperture for all-round 

 work. It requires more careful centering and illumination, 

 and more attention to accurate tube length, and cover-glass 

 thickness. 



Achromatic and Apochromatic Objectives Compared. — 

 The apochromatic objectives, if equally well made, are 

 superior to the achromatic objectives because they are 

 spherically corrected for two colors and all zones; while 

 the achromatics are spherically corrected for one color 

 only, and differ in different zones. That the apochroma- 

 tics are chromatically corrected for three, instead of two, 

 colors, as in the achromatics, also makes the former superior, 

 both visually and for photographs, when yellow-green 

 screens are not used. The achromatic objectives can only 

 be employed to best advantage by using a more or less 

 perfectly monochromatic yellow-green screen. In practice, 

 the apochromatics are usually slightly superior to the 

 achromatics, even when a yellow-green screen is used for 

 both. Good fluorite objectives with a green screen, 

 however, are close to the best apochromatics in definition. 

 WTien employed with white light the apochromatics are 

 markedly superior. 



The writer has had occasion to use the achromatic 

 H2 objective and the apochromatic 2-millimeter, of one 

 maker for two years or so; and again the 2-millimeter 

 achromatic water-immersion and the 2.5-millimeter apo- 

 chromatic water-immersion of another firm have l)een 



