THE TYPES OF OBJECTIVE LEXS. 23 



All recent microscopes have the equivalent focal lengths of 

 the objectives marked in the metric system, which is prefer- 

 able. Thus the ^ inch will be represented by the 16 m.m., 

 the J inch by the 4 m.m., the T ^ inch by the 1.9 m.m. 



Th'e working distance of the microscope that is, the dis- 

 tance between the objective and the object is always less 

 than the equivalent focal distance of the objective. 



THE TYPES OF OBJECTIVE LENS. 



1. Dry and Immersion Objectives. 



In the dry objectives, nothing intervenes between the objec- 

 tive and the object to be examined except air : all low-power 

 objectives are dry. 



In the immersion objectives, some liquid, such as water, 

 glycerin, or oil (homogeneous immersion objectives), must be 

 placed upon the cover-glass over the object and make contact 

 between the cover-glass and the objective. Such lenses are 

 known, respectively, as water-, glycerin-, and oil-immersion 

 lenses. In homogeneous immersion objectives the oil has the 

 same refracting index as the front lens of the objective. 



2. Non-achromatic objectives are objectives in which the 

 color-distortion is not corrected, and the image produced is 

 bordered by a colored fringe ; they also show spherical dis- 

 tortion. 



3. Achromatic objectives are those in which the color- 

 aberration is corrected. 



4. Aplanatic objectives are those in which the spherical 

 aberration is corrected. All better classes of objectives are 

 both achromatic and aplanatic. 



5. Apochromatic objectives are objectives in which rays 

 of three spectral colors combine at one focus instead of rays 

 of two colors, as in the ordinary achromatic. They are 

 highly achromatic objectives. 



6. Adjustable objectives are objectives in which the distance 

 between the front and back combinations may be regulated 

 by means of a milled-head screw. This is useful in dry or 



