22 OBJECTIVES FOR THE MICROSCOPE [Ch. I 



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 apparatus are usually 

 achromatic also. 



§ 28. Apochromatic objectives. — By this is meant objectives in 

 which by means of special forms of glass and a natural mineral (Cal- 

 cium fluorid, Fluorite, Fluor-spar) the color and the spherical correc- 

 tions have been made especially perfect, that is, rays of three spectral 

 colors are combined into one focus instead of rays of two colors as 

 with the ordinary achromatic objectives. 



§ 29. Semi-apochromatic, parachromatic, pantachromatic objec- 

 tives are trade names for those containing one or more lenses of the 

 new forms of glass and are said to approximate more closely with 

 the apochromatic than with the older achromatic objectives. 



§ 30. Non-adjustable or unadjustable objectives. — Objectives 

 in which the lenses or lens systems are permanently fixed in their 

 mounting so that their relative position always remains the same. 

 Lower power objectives and those with homogeneous immersion are 

 mostly non-adjustable. For beginners and those unskilled in manipu- 

 lating adjustable objectives (§ 31), non-adjustable ones are more 

 satisfactory, as the optician has put the lenses in such a position that 

 the most satisfactory results may be obtained when the proper thick- 

 ness of cover-glass and tube-length are employed (Ch. IX). 



§ 31. Adjustable objectives. — An adjustable objective is one in 

 which the distance between the systems of lenses (usually the front 

 and the back systems) may be changed by the observer at pleasure. 

 The object of this adjustment is to correct or compensate for the 

 displacement of the rays .of hght produced by the mounting medium 

 and the cover-glass after the rays have left the object. It is also to 

 compensate for variations in tube-length (§ 134). As the displace- 

 ment of the rays by the cover-glass is the most constant and important, 

 these objectives are usually designated as having cover-glass adjust- 

 ment or correction. (See also practical work with adjustable objec- 

 tives, § 134). 



§ 32. Variable objective. — This is a low power objective of 36 

 to 26 mm. equivalent focus, depending upon the position of the com- 



