CH. 7] MICROSCOPE AND ACCESSORIES 13 



I 25. Parachromatic, Pantachromatic and Semi-apochromatic Objectives. — 

 These are trade names for objectives, most of them containing one or more lenses 

 of the new glass (§ 22). They are said to approximate much more closely to the 

 apochromatics than to the ordinary objectives. 



\ 26. 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 ap- 

 proximated and thereby increasing it. 



\ 27. Projection Objectives. — These are designed especially for projecting an 

 image on a screen and for photo-micrography. They are characterized by having 

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



\ 28. Illuminating or Vertical Illuminating Objectives. — 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 concentrated 

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

 thus enabling one to see a clear image. 



\ 29. 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. VII, on mounting), 

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

 passing from the object (Fig. 57). To compensate for this the position of the sys- 

 tems composing the objective are closer together than they would be if the object 

 were uncovered. Consequently, in non-adjustable objectives some standard thick- 

 ness 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. 



