14 



MICROSCOPE AND ACCESSORIES 



[CH. I 



trary and does not, like the one above, give direct information concerning 

 the objective. 



{! 20. Air or 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 (Fig. 27). JMost objectives of low and medium power (i. e., } in. or 3 mm. 

 and lower powers) are dry. 



§21. 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 ppwer 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 

 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 disturbing effect of the cover-glass (Fig. 64) 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 objective ; and when the object is mounted 

 in balsam there is practically no refraction in the ray from the time it leaves ' 

 the balsam till it enters the objective. 



Fig. 28. Sectional view of an Immersion, 

 Adjustable 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, F C. The back, middle and 

 front combination of the objective. In this 

 case the front is not a combination, but a 

 single plano-convex lens. 



A, B. Parallel rays reflected by the mir- 

 ror axially or centrally upon the object. 



C. Ray reflected to the object obliquely. 

 I. Immersion fluid between the front of 

 the objective 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 examined with immersion or -with dry 

 objectives. 



Stage. Section of the stage of the micro- 

 scope. 



Non-Achromatic or Chromatic Objectives. — These are objectives 

 in which the chromatic 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. (\\ 8, 9, Figs. 12, 13. ) 



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