MICROSCOPIC ANALYSIS. xxxiii 



sections require to be made, and these treated in the same manner. If soft, their elements 

 may be separated by the aid of needles; sometimes pressure will answer the same 



purpose. 



When the object is very minute, it will frequently be desirable to examine both sides 

 of it with high powers. Hence it must not be placed upon an ordinary slide, i m account 

 of the thickness of the latter, but it must be supported upon, and covered by thin glass. 

 The best plan is to keep a number of slides of thin wood or tin, each having a piece cut 

 out of the middle. A thin glass cover, rather larger than the aperture, should then be 

 cemented by marine glue or Canada balsam to the slide ; the thin glass cover is then 

 applied as usual. 



If the object be very small and its structure very delicate, it must be crushed, so 

 that some of the fragments may lie perfectly flat upon the slide, See also the article 



PREPARATION. 



The points to be determined in regard to the different parts of an object, however, may 

 be best treated separately. 



The examination of a microscopic object must comprise : — a, the microscopic analysis, 

 including — 1, the form ; 2, the colour ; 3, the structure of the surface ; and 4, the internal 

 structure: b, histological analysis, in the sense already explained : c, the qualitative chemical 

 composition : and d, the measurement. 



A. Microscopic Analysis. 

 1. The Form. — a. This is usually judged of from the outline as seen by transmitted 

 light, and often erroneously. Where a low power is used, the upper surface of an object 

 and its sides are mostly simultaneously visible ; but under a high power, only those parts 

 lying within a very limited vertical range, or in the same plane, are visible at one focus, 

 and the parts lying in planes above or below this can only be brought into -view by 

 altering the focus: hence the views of objects under high powers correspond to views of 

 transverse sections of the same objects made through various horizontal planes; and as the 

 margins of objects are usually more distinct by transmitted light than the upper surface, 

 spherical or rounded bodies frequently appear flattened. When several bodies of the same 

 kind are visible in the field of the microscope, some will almost always be found lying upon 

 their sides ; and even when the objects are greatly flattened, some will mostly be found 

 lying on edge, presenting the side view. 



b. But as there may be uncertainty in regard to the relation of these bodies to each other, 

 the only safe method in forming a conclusion is to cause them to revolve or roll over, so 

 that all their aspects may be distinguished. This is in general easily accomplished : if the 

 object be already immersed in liquid, the inclination of the stage will answer the purpose; 

 or a little benzole, naphtha, alcohol, or some other volatile liquid in which they are 

 insoluble, must be added. The currents produced by the evaporation of these will cause 

 the objects, especially such as are near the edges of the liquid, to move in all directions, 

 and their true form may be discerned. Sometimes moving the thin glass cover sidewise, 

 the object being kept in view, will answer the same end. 



c. In figures of microscopic objects, the side view should always be exhibited or 

 described. 



d. In the case of crystalline bodies, or such as present angular edges, their angles should 

 be measured with the goniometer, if their chemical composition be unknown, 



