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 
