222 THE ELEMENTS OF VEGETABLE HISTOLOGY 



analyser and polarizer. The selenite disk should 

 be so placed that its plane of vibration is at an angle 

 of 45 degrees to, or midway between, the planes of 

 vibration of the crossed prisms. The field will now 

 appear purple-red in color, and the change caused by 



Fig. 74. — Adjustable Microscope Lamp. 



Low-voltage lamp used with rheostat or transformer. 

 (Bausch & Lomb.) 



rotating weakly polarizing substances will be more 

 apparent because of the greater contrast. 



MICROSCOPE LAMPS 



Gas or electricity may be used as a source of light 

 for microscopic illumination. To obtain satisfactory 

 illumination with gas, it is best to use burners equipped 

 with inverted mantels. Burners of this type give 

 a fairly steady light, free from shadows cast by fix- 

 tures or other interfering objects. The ordinary 

 open flame gas burners are unsatisfactory because 

 of the unsteadiness of the light. 



