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 filters. 
Color of Microscopical Objects.^ 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 fight reflected from the sky varies greatly; when there 
are white clouds from which to reflect the fight, little difficulty 
is experienced, but at times the fight obtained is blue, or pink, 
or gray, according to atmospheric conditions. If the micro¬ 
scope is so placed that fight caimot 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 
fights which may give rise to multi-colored images, (b) The 
fight 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 fight 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 fight 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 
^ See 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. 
