1. Urey, H., Nature, 179, 556 (1957).
2. Fensenkov, V. G., Sci. Amer., 204, No. 1, 180 (1961).
3. Whipple, F. L., Nature, 189, 127 (1961).
4. Page, T., Science, 132, 1870 (1960).
5. Swings, P., and Haser, L., Atlas of Representative Cometary Spectra (University of Liège, Astrophysical Institute, Louvain).