A SIMPLE DESCRIPTION OF THE MICROSCOPE 25 



amount of sucli spreading is relatively small and need not be 

 considered. If the object is a regular periodic structure, like a 

 series of dots and lines, tbe spreading due to this cause may be 

 very considerable, and such an object may not require so large 

 a beam to use the full angle of the object glass. 



This is very noticeable in the case of the fine periodic structure 

 of diatoms, where the structure may often be shown when the 

 illuminating cone of light is considerably less than that required 

 to fill the whole of the aperture of the microscope. In such cases 

 it will be observed, if the eyepiece of the microscope is removed, 

 that the central direct beam illuminates the central portion of 

 the back lens of the object glass, but the rest of the lens may be 

 illuminated almost as strongly by the large amoimt of difEracted 

 light scattered by the periodic structure of the diatom. 



For the use of the microscope with any but the lowest 

 magnifying powers, a substage condenser should be used in order 

 that the natiire of the illumination may be completely varied at 

 wiU. 



