4 MICROSCOPE AND ACCESSORIES. 



\ 9. Dry Objectives. — These are objectives in which the space between the 

 front of the objective and the object or cover-glass is filled with air (PI. II, Fig. 

 12). Most objectives of low and medium power [i. e., }£th in. or 3 mm. and 

 lower powers) are dry. 



\ 10. Immersion Objectives. — An immersion objective is one with which there 

 is some liquid placed between the front of the objective and the object or cover- 

 glass. The most common immersion objectives are those (A) in which water is 

 used as the immersion fluid, and (B) where some liquid is used having the same 

 refractive and dispersive power as the front lens of the objective. Such a liquid 

 is called homogeneous, as it is optically homogeneous with the front glass of the 

 objective. It may consist of thickened cedar- wood oil or of glycerin containing 

 some salt, as stannous chloride, in solution. When oil is used as the immersion 

 fluid the objectives are frequently called oil-immersion objectives. The disturb- 

 ing effect of the cover-glass (§| 16, 63) is almost wholly eliminated by the use of 

 homogeneous immersion objectives. 



The course of the rays of light from the object to the objective with dry and 

 different forms of immersion objectives is shown in PI. V, Fig. 42, 43, 44. 



\ 11. Non-Achromatic Objectives. — These are objectives in which the chro- 

 matic aberration is not corrected, and the image produced is bordered by colored 

 fringes. They show also spherical aberration and are used only on very cheap 

 microscopes. 



\ 12. Achromatic Objectives. — In these the chromatic and the spherical aber- 

 ration 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 objectives are achromatic and also aplanatic 



(I 13). 

 § 13. Aplanatic Objectives, etc. — These are objectives or other pieces of optical 



apparatus (oculars, illuminators, etc.), in which the spherical distortion is wholly 



or nearly eliminated, as in \ 12. Such pieces of apparatus are usually achromatic 



also. 



\ 14. Apochromatic Objectives. — A term used by Abbe to designate a new form 

 of objectives made by combining new kinds of glass with a natural mineral (Cal- 

 cium fluoride, Fluorite, or Fluor-spar). The name, Apochromatic, is used to in- 

 dicate 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. 



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

 sating oculars" (§ 23), 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 ««<fer-corrected spherically for the red rays 

 and ozw-corrected for the blue rays. 



(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. 



