28 ELEMENTARY CHEMICAL MICROSCOPY 



:heir Abbe condensers. When the apparatus is to be employed 

 n photography, yellow-green glass disks are furnished to be used 

 is ray filters. 



Color of Microscopical Objects. 1 When the recognition of 

 :he true color of an object is an important consideration, as for 

 example in microscopic qualitative analysis, it must always be 

 •emembered that the image seen in the microscope of an object 

 Uuminated by light transmitted through it, by means of a mirror 

 •eflecting light from the sky, may not infrequently appear of 

 }uite a different color than the object appears to possess by 

 •eflected light. This difference may be due to a number of causes 

 a) the light reflected from the sky varies greatly; when there 

 ire white clouds from which to reflect the light, little difficulty 

 s experienced, but at times the light obtained is blue, or pink, 

 )r gray, according to atmospheric conditions. If the micro- 

 ;cope is so placed that light cannot be obtained above the tree 

 ops, a greenish tint is obtained from the leaves of the trees 

 md in the fall of the year trees with colored leaves yield colored 

 ights which may give rise to multi-colored images, (b) The 

 ight transmitted by an object may be very different from that 

 ■eflected by it, and the thickness of the preparation may greatly 

 :hange the character of the color as seen in the image in the 

 nicroscope. (c) We may be dealing in a preparation both 

 vith absorption and scattering of light and thus draw faulty 

 leductions. (d) The presence of occluded or adsorbed sub- 

 .tances may modify the colors transmitted, (e) Total internal 

 eflection may take place and the image appear in part gray 

 >r even black. This phenomenon is seen in most crystals 

 inder the microscope, when crystal faces meet at an angle 

 luch that the illuminating light rays strike them at the 

 :ritical angle, are totally reflected and therefore unable to 

 )ass through. 



The dendrites, skeletal forms, etc., of compounds whose crys- 

 ;als are normally clear, transparent and colorless will usually 



1 See Wood, R. W.; Physical Optics, Macmillan Co., N. Y., iqio, pp. 

 (.36-441; 630,635. Bancroft, W. D.: Sci. Amer. Monthly, May 1920, p. 461. 

 Bancroft, W. D.: J. Phys. Ch., 23 (1919), 365. 



