52 THE ELEMENTS OF VEGETABLE HISTOLOGY 



direction. In working with temporary mounts the 

 specimen should be allowed to stand for a few minutes 

 before observation, as diffusion currents in the liquid 

 cause rapid motion of the specimen. When viewing 

 very small objects in temporary mounts, one will 

 observe that they are in constant motion. This 

 phenomenon is termed Brownian movement, and is 

 exhibited by fine particles of solid materials in suspen- 

 sion or colloidal solution. In examining specimens 

 having a curved or irregular surface, one must make 

 full use of the fine adjustment so as to gain clear views 

 of the elevations and depressions in the specimen. 

 The novice will often mistake air bubbles for cells 

 or cellular material; but their behavior upon opening 

 and closing the diaphragm serves to identify them. 

 If, while looking at an air bubble, one closes the dia- 

 phragm opening, the wall of the bubble will increase 

 in thickness, and if the diaphragm is opened, the wall 

 will become thinner (Fig. 25). The walls of cellular 

 elements are not affected in this way by the opening 

 of the diaphragm. 



Drawings.— The representation of microscopic 

 images may be accomplished by photography, by the 

 use of mechanical accessories and by free-hand draw- 

 ing. The apparatus necessary for microphotography 

 consists essentially of a microscope to which is attached 

 the bellows and shutter of a camera (Fig. 26). Sepa- 

 rate lenses for the camera are not used, as the ocular 

 serves to project the image. While microphoto- 

 graphs give exact representations of the object, they 

 are not suited for many purposes or for many classes 

 of specimens. The object must be specially prepared 

 and one cannot, as is sometimes desirable, give promi- 



