CHAPTER III. 

 ILLUMINATION OF OBJECTS; ILLUMINATING DEVICES. 



Illumination and Illuminating Devices. Of even greater 

 importance 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 accom- 

 plish the objects of his investigations. 



For convenience of discussion the modes of illuminating 

 objects for microscopic study may be grouped under the follow- 

 ing heads: 



a. Transmitted axial light. 



b. Transmitted oblique light. 



c. Reflected axial light. 



d. Reflected oblique light. 



e. Oblique dark ground illumination. 



/. ''Orthogonal illumination" (Siedentopf Slit Ultrami- 

 croscope). 



g. Differential color illumination. 



h. By means of ultraviolet light, thus causing certain sub- 

 stances to become fluorescent. 



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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