The main absorption system of IC1 in the visible region extends from MATHS5730 upwards, and its vibration structure has been studied by various workers. The analysis of this is complete, and the existence of two isotopic molecules, IC135 and IC137, has rendered it possible to determine the vibrational quantum numbers unambiguously. The vibrational constants of the molecule in the normal and excited states have been evaluated, and approximate values of the rotational constants estimated. Precise values of the latter can only be obtained from a fairly complete analysis of the rotation structure. It was known that this would present difficulties, but since some obvious regularities were discernible on preliminary high-dispersion plates, it seemed to be worth attempting. The difficulties arise from two main causes, namely, the relatively high moment of inertia of the molecule, leading to a relatively close spacing of the lines of a branch, and the existence of the two isotope molecules, each giving rise to a complete system. In addition, the vibrational structure is relatively close and high rotational states are developed, with the consequence that successive bands of the same progression always overlap. Since also there are usually several progressions occurring in the same region, it will be realized that the complexity is considerable, and it was important, therefore, to select the most favourable point for the initial attack.