Part II, STEREOCHEMISTRY: With the rectification of errors which has marked recent progress in the chemistry of diphenyl, it has become evident that the various space formulae proposed for diphenyl do not adequately explain the existence of optical isomerides amongst the diphenic acids. From a study of the 4- , 5- , and 6-nitrodiphenic acids it is now postulated that the presence of substituents in the 2:2':6:6'- positions in the diphenyl nucleus imposes an obstacle to the free rotation of the nuclei and so gives rise to enantiomorphous forms. Attempts to resolve acids and bases with but two groups in the central positions have failed but the complete resolution of 6-nitrodiphenic acid has been achieved. The 'Obstacle Theory' now put forward is supported by experimental evidence on:- (a) The relative stability of the optically active diphenic acids. (b) Formation of anhydrides by diphenyl carboxylic acids. (c) The nitration of phenanthraquinone. Possible extensions of the theory are discussed. Fluorenone formation has been studied arid a possible mechanism is suggested.