CH. /] 



MICROSCOPE AND ACCESSORIES 



n 



W. The Working Distance, that is the distance from the front of the objective 

 to the object when the objective is in focus. 



18. 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 contain- 

 ing 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. 57) 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, 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 mirro* 

 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 ex- 

 amined with immersion or with dry objectives. 



Stage. Section of the stage of the microscope. 



\ 19. 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). 



20. Achromatic Objectives. In these the chromatic and the spherical aber- 

 ration are both largely eliminated by combining concave and convex lenses of dif- 

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

 each other." All the better forms of objectives are achromatic and also aplanatic. 

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



| 21. Aplanatic Objectives, etc. These are objectives or other pieces of 

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



