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one after the illumination had been corrected. In the latter procedure, the point representing the quality on the trichroic ratio graph was brought to the same location on the graph within a reasonable tolerance. There was one exception to this. In one case, the variation consisted of an intensity difference with the illuminance from a ribbon filament a t one twentieth that of the first pair. This was brought to the quality represented by the point further along the line representing variation of the reference point nith intensity level. I t was found that the same reference point was not valid for the tungsten arc which contains lines of the mercury spectrum superposed on the tungsten spectrum, at least Tvith the photocell filters listed for Type B films. The method was satisfactory for variations of quality produced by changing from tungsten filament to a 4.5-ampere and a 10-anipeie arc and from transparent to vertical illumination. ( A sheet of this test on Kodachrome film was shown at St. Louis as a color slide.) Probably the svstem can be used with a tungqten arc but with its own reference points on the graph; this has been tested by only a few trials. This relatively new system ha.: proved valuable in routine n ork;
it saves much time and justifies the confidence with which exposed film is sent off for processing. LITERATURE CITED (1) Carlile, J. d., J . BioZ. Phot. Assoc., 13, 187-95 (June 1945). (2) Committee on Colorimetry, J . Optical SOC.Am., 34, 254 (1944). (3) Eastman Kodak Co., “Photomicrography,” 14th ed., Rochester, N. Y., 1944. (4) Jelley, E . E., “Microscopy,” in Weissberger’s “Physical Methods of Organic Chemistry,” 1‘01. I, Chap. XI, pp. 435-530, New York, Interscience Publishers, 1945. (5) Jones, L. A , J . Franklin Inst., 227, 297-354, 497-544 (March and April 1939). (e) Loveland, R . P., J . B i d . Phot. Assoc., 13, 79-97 (December 1944). A revised reprint of this article is available. (7) Loveland, R. P.. J . Phot. SOC.Am., 10, 16 (January 1944). (8) MacPherson, H. G., J . Optical SOC.Am., 30, 189 (1940).
RECEIVED RIarch 9, 1949.
chemical mic?roecopw e~mpoeium
Electron Microscopy of Colloidal Systems JOHN TUKICEVICH, Princeton University,
AND
JAMES HILLIER, RCA Laboratories, Princeton, iV. J .
..isurvey of colloidal systems with the electron microscope reveals the following
morphological forms : amorphous small particles, small regular forms, fibers, and plates. By use of the electron microscope the size, shape, distribution of size, structure, and genetic relationships among the various morphological species can be determined. The information presented is typical of the substances examined.
C
OLLOIDAL systems mag be defined as t\
