In a paper communicated to this Society in March, 1903, it was shown that antimonite is fairly transparent to radiations of long wave-length and that its behaviour between crossed nicols is consistent with orthorhombic symmetry. At the same time it was pointed out that it would be of interest to determine the wave-lengths of the radiations transmitted and the principal indices of refraction of antimonlte for these radiations. On undertaking this investigation it was soon found that antimonite was quite sufficiently transparent to the rays at the extreme red end of the visible spectrum to enable visual observations to, be made, and in a verbal communication to the Society on March 22, 1904, it was stated that the indices of refraction for red light were 4.129 for rays vibrating parallel to the axis Z and 3.873 for rays vibrating parallel to the axis X.