i6 



MICROSCOPE AND ACCESSORIES 



[CH. I 



''/. 26. 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 homogenous immersion are mostly non-adjustable. For beginners 

 and those unskilled in manipulating adjustable objectives ($ 27), non-adjusta- 

 ble 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 table of tube- 

 length and thickness of cover-glass below (\ 34). 



j! 27. 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 

 light 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". 

 See \ 32. As the displacement of the rays by the cover-glass is the most con- 

 stant and important, these objectives are usually designated as having cover- 

 glass adjustment or correction. (Fig. 28. See also practical work with 

 adjustable objectives, Ch. II.) 



\ 28 . Parachromatic, Pantachromatic and Semi-apochromatic Objec- 

 tives. — These are trade names for objectives, most of them containing one or 

 more lenses of the new glass [\ 25). They are said to approximate much 

 more closely to the apochromatics than to the ordinary objectives. 



§ 29. Variable Objective. — This is a low power objective of 36 to 26 mm. 

 equivalent focus, depending upon the position of the combinations. By means 

 of a screw collar the combinations may be separated, diminishing the power, or 

 approximated and thereby increasing it. 



Fig. 29. An objective in section, showing the differ- 

 ent combinations formed of concave and convex lenses. 

 Cut loaned by Voigtldnder & Sohn, A. G. 



I 30. Projection Objectives.— These are designed 

 especially for projecting an image on a screen and for 

 photo-micrography. They are characterized by having 

 a flat, sharp field brilliantly lighted. (See Ch. IV, IX.) 



I 31. Illuminating or Vertical Illuminating Ob- 

 jectives.— These are designed for the study of opaque 

 objects with good reflecting surfaces, like the rulings on 

 metal bars and broken or polished and etched surfaces of metals employed in 

 micro-metallography. The light enters the side of the tube or objective and 

 is reflected vertically downward through the objective and thereby is concen- 

 trated upon the object. The object reflects part of the light back into the 

 microscope thus enabling one to see a clear image. For a figure see Ch. VIII. 



