MICROSCOPE AND ACCESSORIES. 



[CIT. I. 



\ 16. 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 chlorid, 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 (Fig. 56) is almost wholly eliminated by the use of 

 homogeneous immersion objectives, as the rays undergo very little or no refraction 

 on passing from the cover-glass through the immersion medium and into the ob- 

 jective ; and when the object is mounted in balsam there is practically no refrac- 

 tion in the ray from the time it leaves the balsam till it enters the objective. 



Fig. 23. Sectional view of an Immersion, Ad- 

 justable Objective, and the object lighted with 

 axial or central and with oblique light. 



Axis. The principal optic axis of the objective. 



B C, M C, EC. The back, middle and front 

 combination of the objective. In this case the 

 front is not a combination, but a single piano con- 

 vex-lens. 



A, B. Parallel rays reflected by the mirror axi- 

 ally or centrally upon the object. 



C. Ray reflected to the object obliquely. 



I. Immersion fluid between the front of the ob- 

 jective and the cover-glass or object (O). 



Mirror. The mirror of the microscope. 



O. Object. It is represented without a cover- 

 glass. Ordinarily objects are covered whether ex- 

 amined with immersion or with dry objectives. 



Stage. Section of the stage of the microscope. 



§ 17. 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. {\\ 7, 8, Figs. 12, 13). 



3 18. Achromatic Objectives. — In these the chromatic and the spherical aberra- 

 tion are both largely eliminated by combining concave and convex lenses of differ- 

 ent kinds of glass "so disposed that their opposite aberrations shall correct each 

 other." All the better forms of objpetives are achromatic and also aplanatic (§ 19). 

 That is the various spectral colors come to the same focus. 



§ 19. 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, 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 appa- 

 ratus are usually achromatic also. (§ 7, 8). 



§20. 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 



