5 MICROSCOPE AND ACCESSORIES. 



(4) These objectives admit of the use of very high oculars, and seem to be a 

 considerable improvement over those made in the old way with crown and flint 

 glass. According to Dippel (Z. w. M. 1886, p. 300), dry apochromatic objectives 

 give as clear images as the same power water immersion objectives of the old form. 



§ 15. Non-Adjustable or Unadjustable Objectives. — Objectives in which the 

 lenses or lens systems are permanently fixed in their mounting so that their rela- 

 tive position always remains the same. Low power objectives and those with 

 homogeneous immersion are mostly non-adjustable. For beginners and those un- 

 skilled in manipulating adjustable ($ 16) objectives, 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 thickness of cover-glass and 

 tube-length are employed. (See $ 17 and table and figure of tube-length and 

 thickness of cover-glass below ) 



\ 16. Adjustable Objectives. — An adjustable objective is one in which the dis- 

 tance 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 cor- 

 rect or compensate for the displacement of the rays of light produced by the 

 mounting medium and the cover-glass after the ra}'S have left the object. It is 

 also to compensate for variations in "tube length." See \ 17. As the displace- 

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

 jectives are usually designated as having cover-glass adjustment or correction. 

 (PI. II, Fig. 13. See also practical work, \ 63.) 



\ 17. Tube-Length and Thickness of Cover-Glasses. — "In the construction of 

 microscopic objectives, the corrections must be made for the formation of the 

 image at a definite distance, or in other words the tube of the microscope on 

 which the objective is to be used must have a definite length. Consequently the 

 microscopist must know and use this distance or ' microscopical tube-length' to 

 obtain the best results in using any objective in practical work." Unfortunately 

 different opticians have selected different tube-lengths and also different points 

 between which the distance is measured, so that one must know what is meant by 

 the tube-length of each optician whose objectives are used. See table. 



The thickness of cover-glass used on an object, (see Ch. V, on mounting), ex- 

 cept with homogeneous immersion objectives, has a marked effect on the light 

 passing from the object (PI. V, Fig. 41). To compensate for this the relative 

 positions of the systems composing the objective are different from what they 

 would be if the object were uncovered. Consequently, in non-adjustable objec- 

 tives some standard thickness of cover-glass is chosen by each optician and the 

 position of the systems arranged accordingly. With such an objective the image 

 of an uncovered object would be less distinct than a covered one, and the same 

 result would follow the use of a cover-glass much too thick {\ 63, Fig. 41). 



