MICROSCOPE ACCESSORIES 219 



in turn acts as a mirror and transmits the view of the 

 drawing paper to the eye. At the same time views 

 of the microscope image are apparent through the 

 perforation in the silvered surface. 



The mirror should be adjusted at an angle of 45 

 degrees from the optic axis, and the drawing surface 

 should be horizontal and level. Where these adjust- 

 ments interfere with the proper projection of the image 

 the angle of the mirror may be slightly reduced. If 

 the angle of the mirror is varied from 45 degrees, the 

 outer edge of the drawing board must be tilted toward 

 the microscope, the drawing surface being raised 

 twice as many degrees as the mirror is depressed. 

 Certain types of instruments are equipped with slips 

 of slightly darkened glass which are interposed between 

 the prism and the mirror. As the drawing surface 

 is usually white it reflects an excess of light, and this 

 condition is overcome by use of the darkened glass 

 disks. 



POLARIZING APPARATUS 



Although the polarizing microscope is extensively 

 used in chemical microscopy, comparatively little 

 has been done in the adaptation of the apparatus 

 to the field of histology. In order to comprehend 

 the working of the polarizing apparatus one must 

 keep in mind certain physical concepts of light. Ordi- 

 nary light rays, emanating from the sun or other sources, 

 are in the form of innumerable waves or vibrations 

 of various and constantly changing direction. Certain 

 transparent and translucent materials possess the 

 property of separating light waves and permitting 

 only those which are parallel to a given plane to pass. 



