CH. I] MICROSCOPE AND ACCESSORIES 15 



I 23. Achromatic Objectives. — In these the chromatic and the spherical 

 aberration are both largely eliminated by combining concave and convex 

 lenses of different kinds of glass "so disposed that their opposite aberrations 

 shall correct each other." All the better forms of obj ectives are achromatic 

 and also aplanatic. That is, enough of the various spectral colors come ap- 

 proximately to the same focus to give white light. (See also under apochro- 

 matics, g 25.) 



(S 24. Aplanatic Objectives, etc. — These are objectives or other pieces of 

 optical apparatus (oculars, illuminators, etc.), in which the spherical distor- 

 tion is 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. 



I 25. Apochromatic Objectives. — A term used by Abbe to designate a 

 form of objective made by combining new kinds of glass with a natural min- 

 eral (Calcium fluorid, Fluorite, or Fluor spar 1886*). The name, Apochro- 

 matic, is used to indicate the higher kind of achromatism in which rays of 

 three spectral colors are combined at one focus, instead of rays of two colors 

 as in the ordinary achromatic objectives. Some of the early apochro- 

 matics deteriorated rather quickly in hot moist climates. Those now made 

 are quite permanent. 



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

 pensating oculars " are as follows : 



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

 small tertiary 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 ob- 

 tained 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-cox- 

 rected spherically for the red rays and oz'^r-corrected for the blue rays, ($ 10). 



(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 apochromatic 

 objectives give as clear images as the same power water immersion objectives 

 of the old form. 



*According to F. J. Keeley (Proc. Acad. Nat. Sci. Philadelphia, lvi 

 (1904) p. 475 ; Jour. Roy. Micr. Soc. 1905, p. 103) a \ in. objective made by 

 Chas. A. Spencer in i860 contained a fluorite lens in one of the combinations. 



