240 



PROGRESS IN MICROSCOPY 



violet beyond the specimen. The fluorescent portions ot^ the specimen 

 show up brightly against a dark ground (Fig. 9.16). A dark-ground 

 condenser (Barer) may also be used to illuminate the specimen thus 

 preventing the incident beam from entering the microscope. Quartz 

 condensers or comprising mirror optical systems may also be used. 

 In the latter type, the surfaces are not to be silvered but, mstead, 

 aluminized. 



FiG. 9.16. Fluorescence of a cross-section of a stalk (Courtesy of Dr. Gabler-Reichert). 



Many biological substances are fluorescent under the effects of 

 wave-lengths below 3650 A and at which glass is transparent. In such 

 cases a standard condenser is suitable. 



When the specimen emits radiation within the visible light spectrum, 

 standard optical systems are used beyond the specimen. The microscope 

 is not to comprise semi-apochromalic or apochromatic objectives since 

 some of their elements are of fluorite whose fluorescence may set in 

 disturbances. Likewise, cedar oil, which is fluorescent, is not suitable 

 as immersion fluid. It can be substituted for non-fluorescent syn- 

 thetic oils. 



