CHAPTER XI 



THE COVER-GLASS PROBLEM 



Loss of Efficiency with High Dry Objectives. — It 



has long been known to workers in biology or histology 

 who gave attention to the quality of the image in the 

 microscope, that there was often a loss of efficiency in 

 the use of the ordinary (medium or) high dry objectives. 

 This resulted from the fact that the high-power dry objec- 

 tives could only be suited by one definite thickness of 

 cover-glass (or cover-glass plus stratum of mounting 

 medium above object). The ordinary microscope is 

 often supplied with an 8 or 10 and a 40 dry objective. The 

 10 objective gives sharp, or fairly sharp, images under 

 most of the usual conditions (provided, of course, that its 

 surfaces are kept optically clean) ; while the 40 objective 

 may be often disappointing, since it may rarely give as 

 sharp images as the 10-times objective; and sometimes 

 may show images with distinctly blurred outlines, as com- 

 pared with the images given of the same object by an oil- 

 immersion objective of the same aperture. Now the 40 

 objective, of 0.65 or 0.85 aperture, was calculated by the 

 maker to give about as sharp an image as the 10 objective, 

 within the limits of useful magnification; and when it does 

 not, there is something wrong. 



Cover-glass Thicknesses. — A medium or high-power dry 

 objective requires for maximum sharpness and maximum 

 working aperture that there should be a diaphragm on 

 the source of light, the image of the source on the object 

 being circular and equal to (or less than) the field of view.^ 

 The omission of this adjustment, however, does not injure 

 the image fatally, since the lower condenser aperture which 

 has to be used does no more than entail a loss of working 



1 See Beck, "The Microscope," Part 2. 



110 



