12 MICROSCOPE AND ACCESSORIES [CM. I 



wholly or nearly eliminated, and the curvatures are so made that the central and 

 marginal parts of the objective focus rays at the same point or level. Such pieces 

 of apparatus are usually achromatic also. 



22. Apochromatic Objectives. A term used by Abbe to designate a form of 

 objective made by combining new kinds of glass with a natural mineral (Calcium 

 fluorid, Fluorite, or Fluor spar). The name, Apochromatic, is used to indicate 

 the higher kind of achromatism in which rays of three spectral colors are com- 

 bined at one focus, instead of rays of two colors as in the ordinary achromatic ob- 

 jectives. At the present time (1901) several opticians make apochrornatic ob- 

 jectives without using the fluorite. Some of the early apochromatics deteriorated 

 rather quickly in hot moist climates. Those now made are quite permanent. 



The special characteristics of these objectives, when used with the "compen- 

 sating oculars" are as follows : 



(1) Three rays of different color are brought to one focus, leaving a small ter- 

 tiary spectrum only, while with objectives as formerly made from crown and flint 

 glass, only two different colors could be brought to the same focus. 



(2) In these objectives the correction of the spherical aberration is obtained 

 for two different colors in the brightest part of the spectrum, and the objective 

 shows the same degree of chromatic correction for the marginal as for the central 

 part of the aperture. In the old objectives, correction of the spherical aberration 

 was confined to rays of one color, the correction being made for the central part of 

 the spectrum, the objective remaining under-corrected spherically for the red rays 

 and <?zw-corrected for the blue rays ($9). 



(3) The optical and chemical foci are identical, and the image formed by 

 the chemical rays is much more perfect than with the old objectives, hence the 

 new objectives are well adapted to photography. 



(4) These objectives admit of the use of very high oculars, and seem to be a 

 considerable improvement over those made in the old way with crown and flint 

 glass. According to Dippel (Z. w. M. 1886, p. 300) dry apochrornatic objectives 

 give as clear images as the same power water immersion objectives of the old form. 



23. Non-Adjustable or Unadjustable Objectives. Objectives in which the 

 lenses or lens systems are permanently fixed in their mounting so that their rela- 

 tive position always remains the same. Low power objectives and those with 

 homogenous immersion are mostly non-adjustable. For beginners and those un- 

 skilled in manipulating adjustable objectives ($ 24), non- adjustable ones are more 

 satisfactory, as the optician has put the lenses in such a position that the most 

 satisfactory results may be obtained when the proper thickness of cover-glass and 

 tube-length are employed. (See table of tube-length and thickness of cover-glass 

 below, p. 14.) 



24. Adjustable Objectives. An adjustable objective is one in which the dis- 

 tance between the systems of lenses (usually the front and the back systems) may 

 be changed by the observer at pleasure. The object of this adjustment is to cor- 

 rect or compensate for the displacement of the rays of light produced by the 

 mounting medium and the cover-glass after the rays have left the object. It is 

 also to compensate for variations in "tube-length." See $29. As the displace- 

 ment of the rays by the cover-glass is the most constant and important, these ob- 

 jectives are usually designated as having cover-glass adjustment or correction. 

 (Fig. 23. See also practical work with adjustable objectives, Ch. II). 



