36 ELEMENTS OF APPLIED MICROSCOPY. 



Spherical aberration may be detected by examining one. 

 of the test objects mentioned above. If the lens be under- 

 corrected, so that peripheral rays come to a focus nearer 

 the lens, the outer part of the object will be in focus in a 

 plane below the central portion. Chromatic aberration 

 may be detected in the examination of any object by the 

 appearance of colored rings surrounding it. 



7. Interpretation of Appearances. — The greatest care 

 must be taken in drawing conclusions as to the real 

 structure of bodies from their appearance under the 

 microscope. Right and left are reversed by the invert- 

 ing action of the objective lenses. Appearances in any 

 one plane may be very deceptive, as in the case of an air- 

 bubble at its upper edge. The medium in which an 

 object is mounted may completely alter its appearance, 

 as we shall see in Chapter III. It must always be re- 

 membered that contrasts of density are what give us mi- 

 croscopic pictures; only when these coincide with salient 

 differences in structure will such pictures be representa- 

 tive. The presence of dust or other foreign materials 

 may mislead the observer, and even air-bubbles have 

 sometimes caused confusion. If the preparation has been 

 treated with fixing agents, dyes, etc., as described in 

 Chapter III, great care must be exercised not to mistake 

 artificial conditions due to reagents for those normally 

 existing. 



Motion of bodies under the microscope is particularly 

 deceptive. Since the distance travelled is magnified, an 

 idea of rapid movement is conveyed when the actual 

 translation may be really very slow. Diffusion currents 



