CHAPTER II. 
ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES. 
Illumination and Illuminating Devices. — Of even greater 
irnportance than the selection of the correct combination of 
objective and ocular for the study of a preparation is the matter 
of proper illumination. The earlier in his work the student 
appreciates the importance of Illumination and the more thought 
and care he expends upon this phase of microscopic methods, 
the fewer errors he will make and the more easily will he become 
expert in the interpretation of the images seen. 
For convenience of discussion the modes of illuminating 
objects for microscopic study may be grouped under the follow¬ 
ing heads: 
a. Transmitted axial light. 
h. Transmitted oblique light. 
c. Reflected axial light. 
d. Reflected oblique light. 
e. Dark-field illumination. 
/. “ Orthogonal illumination ” (Siedentopf Slit Ultramicro¬ 
scope) . 
g. Differential color illumination. 
h. Illumination by means of ultraviolet light, thus causing 
certain substances to become fluorescent. 
i. Polarized light. 
a. Transmitted Axial Light obtained by means of the mirrors 
with or without a condenser may be said to be the usual or most 
frequently employed method of illuminating transparent and 
translucent objects. With low power objectives and objects of 
coarse structure no condenser is necessary, but when the object 
to be studied presents a fine structure and delicacy of tracery 
and when its refractive index lies close to that of the mounting 
medium, structural studies become difficult, if not impossible, 
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