pictures. In certain cases, however, panchromatic films are to be preferred: for example, in photographs of the cell structure in vitrain, where there may be no yellows and whites, and where contrast may be desired.
In 1941 Martin-Duncan showed a group of infrared photomicrographs of coal sections at the Exhibition of the Royal Photographic Society. Some of them are reproduced in Figure 58. They clearly show fine detail of structure in relatively thick dense dark-brown sections of fossil stems of plants of the Carboniferous period from the coal strata of Yorkshire and Scotland.
Infrared photography was applied with a certain measure of success to the investigation of the fossils found by Weigelt in the lignite of the Geisel valley. Photographs of fossil frogs from this region have been published by Eggert,17,18 Fröhlich and Luft,19 in which details were reproduced by infrared which were invisible, or were visible only with difficulty in pictures made by red light. Eggert suggests that the success of the infrared method is due to the ability to penetrate the overlying lignite layer, and this would appear to be in accord with the observations which have been made on thin coal sections by other investigators.
Other references will be found at the end of Chapters IX and XI.
1. ALBERS V. M., and KNORR H. V., "Fluorescence and Photo-Decomposition of the Chlorophylls and Some of Their Derivatives in the Presence of Air," Cold Spring Harbor Symposia Quant. Biol., III, 1935, 87-97.
2. ARTHUR J. M., "Some Infra-Red Effects on Green Plants," ch. XXV of DUGGAR B. M., Biological Effects of Radiation, McGraw-Hill, New York and London, 1936, 841-52.
3. ATKINS W. R. G., "The Transmission of Light and Total Radiation by Leaves," Proc. Roy. Soc., B., 1937, 122, 26-9.
4. ATTA G. R. VAN, "Filters for the Separation of Living and Dead Leaves in Monochromatic Photographs with a Method for the Determination of Photographic Filter Factors," J. Biol. Phot. Assoc., 1936, 4, 177-91.
5. BABEL A., "Infrarot-Photographie im Pflanzenschutz," Angew. Botan., 1935, 17, 43-53.