Ch. I] OBJECTIVES FOR^ THE MICROSCOPE 21 



§ 21. Names applied to parts of objectives. — As objectives have 

 usually two or more combinations of lenses (fig. 21 A-C) it is con- 

 venient to have a name for each combination. 



(i) Front combination. This is the part of the objective nearest 

 the object. 



(2) Back combination. The combination of lenses farthest above 

 the object, and, hence, nearest the ocular. 



(3) Intermediate or middle combination. The lenses between 

 the front and back lenses. Sometimes there are two or more inter- 

 mediate combinations (fig. 21 C). 



Kinds of Objectives 



Depending on their construction or manner of use, objectives have 

 received special designations or names. 



§ 22. Dry objectives. — • These are objectives in which air is be- 

 tween the objective and the object or cover-glass (fig. 34). 



§ 23. Immersion objectives. — With these there is some Kquid 

 between the front of the objective and the object or the cover-glass 

 (fig. 21 B). Immersion objectives are usually designated by the name 

 of the hquid used. 



§ 24. Water immersion objectives. — With these there is water 

 between the cover-glass or the object and the front lens. 



§ 25. Homogeneous or oil immersion objectives. — The immersion 

 liquid in such objectives has the same refractive index (see Ch. IX) 

 as glass, hence the Hght suffers no refraction in passing from the glass 

 shde and cover-glass into the immersing hquid, and from that into the 

 objective. As the hquid used with these objectives is nearly always 

 thickened cedar-wood oil, they are more frequently called oil immer- 

 sion than homogeneous immersion objectives. 



§ 26. Achromatic objectives. — These are objectives in which 

 the image is practically free from rainbow colors. They are com- 

 posed of one or more combinations of convex and of concave lenses 

 (see Ch. IX, under chromatic aberration). AU good microscope 

 objectives are achromatic. 



§ 27. Aplanatic objectives, etc. — These are objectives or other 

 pieces of optical apparatus (oculars, illuminators, etc.) in which the 



