The reliability and accuracy of the visual method of interpreting electron-aiffraction photographs, consisting in the correlation of values of (4 pi sin nu/2)/lambda obtained by visual measurement of rings of apparent maximum and minimum intensity with x values from a simplified theoretical curve (Eq. (7) or (8)), have been tested in the following ways: (a) The measurement and interpretation of "artificial electron-diffraction photographs" of bromine; (b) the comp'lrison of electron-diffraction and bandspectral values of interatomic distances for bromine. chlorine and iodine chloride; (c) the study of microphotometer records for benzene, and comparison of results with those of the visual method; (d) the study of micro photometer records for carbon tetrachloride, and compClrison with the visual method. It is concluded that the visual method when carefully applied leads to values of interatomic distances accurate to about 1 percent (probable error), or to ! percent in favorable cases. The regular plane hexagon model of the benzene molecule is verified, the carbon-carbon distance in the ring being determined as 1.390 +/- 0. 005A. The carbon-chlorine distance in the carbon tetrachloride molecule is determined as 1.760 +/- 0.005A.A brief discussion of the methods and results of other investigators is given.
