29 : 2/ Microscopy 



539 



on the grating. The integer n is called the order of the diffraction line ; 

 n = is the central, undiffracted beam. 



The light reaching any one of the bright lines in the focal plane of 

 the lens is in phase. Accordingly, if a screen is placed so that only the 



Real Image of Specimen 

 Formed at This Height by Objective Lens 



Rack and Pinion 

 Adjustment 



Fine 



Adjustment 



Screw 



Rear Focus of Objective Lens 

 Specimen 



Condenser 

 and Diaphragm 



Light 



+ Mirror 



Virtual Image of Specimen ^'" 



Formed at This Height by Eyepiece 



Figure I. Essentials of the ordinary light microscope. This 

 is referred to in the text as the bright-field light microscope to 

 distinguish it from other types of microscopes. 



