272 TECHNICAL MICKOSCOPY. 



But as these liquids, if the objects are not immersed in them, 

 exhibit the well-known phenomena of adhesion, and therefore 

 are very uneven on the upper surface, it is for optical reasons 

 necessary, with the higher amplifications, to employ cover- 

 glasses which render the upper surface flat. By their applica- 

 tion a too rapid evaporation of the liquid is avoided, as 

 also the immersion of the lenses. With lower amplifications the 

 preparation may remain uncovered, and it is frequently advisable 

 not to cover it, especially when a further dissection is contem- 

 plated. 



If the preparations are of such a nature that it may be desirable 

 to invert them and view them on the other side, they should be 

 placed between two rather large pieces of cover-glass, which may 

 then be turned about at will. This process is often of great 

 advantage in the study of superposed cell-layers or other objects 

 the structure of which is to be investigated. 



The thickness of the cover-glass is immaterial with low powers ; 

 the choice must therefore be made rather in accordance with other 

 considerations such, for instance, as the pressure which is to be 

 exercised upon the preparation. High powers, on the other hand, 

 are mostly constructed for thin cover-glass, and correction-adjust- 

 ment becomes necessary as soon as the thickness exceeds \ mm. 

 With very powerful objectives only the thinnest cover-glass can 

 be used, in consequence of the short focal distance. 



When it is necessary to protect the preparation from all pressure, 

 however slight, a suitable body (a hair, a quill, a strip of paper or 

 tin-foil, &c.) is placed under the cover-glass, and of course the object- 

 distance of the Microscope must then be taken into account. The 

 beginner frequently discovers that he cannot focus on the object 

 in consequence of the thickness of the layer of liquid over it ; this 

 may happen even to the practised microscopist in using high 

 powers. This difficulty may be avoided by applying the cover- 

 glass by one end and then pushing it further on until the object 

 arrives at a sufficiently thin part of the wedge-shaped layer of 

 water. 



It is obvious that the cover-glasses must be as clean as possible, 

 and must not therefore be touched by the fingers at the optically 

 effective part of the surface. 



