28 ELEMENTARY CHEMICAL MICROSCOPY 



their Abbe condensers. When the apparatus is to be employed 

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

 as ray niters. 



Color of Microscopical Objects. 1 When the recognition of 

 the true color of an object is an important consideration, as for 

 example in microscopic qualitative analysis, it must always be 

 remembered that the image seen in the microscope of an object 

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

 reflecting light from the sky, may not infrequently appear of 

 quite a different color than the object appears to possess by 

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

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

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

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

 or gray, according to atmospheric conditions. If the micro- 

 scope is so placed that light cannot be obtained above the tree 

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

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

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

 light transmitted by an object may be very different from that 

 reflected by it, and the thickness of the preparation may greatly 

 change the character of the color as seen in the image in the 

 microscope, (c) We may be dealing in a preparation both 

 with absorption and scattering of light and thus draw faulty 

 deductions, (d) The presence of occluded or adsorbed sub- 

 stances may modify the colors transmitted, (e) Total internal 

 reflection may take place and the image appear in part gray 

 or even black. This phenomenon is seen in most crystals 

 under the microscope, when crystal faces meet at an angle 

 such that the illuminating light rays strike them at the 

 critical angle, are totally reflected and therefore unable to 

 pass through. 



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

 tals are normally clear, transparent and colorless will usually 



^ee Wood, R. W.; Physical Optics, Macmillan Co., N. Y., 1919, pp. 

 436-441; 630,635. Bancroft, W. D.: Sci. Amer. Monthly, May 1920, p. 461. 

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



