When a prism with three polished faces is used on a spectrometer, because of partial internal reflection, light eventually emerges from all three faces of the prism. It is shown that, in general, there are only two directions of emergence from each face. From the face of incidence one of these directions is uniquely that of the externally reflected light. The two emergent beams from any face are unequally dispersed, and in certain circumstances one may be undispersed (one of the beams from the face of incidence is always of this character). In such circumstances the undispersed beam coincides in direction with each component of the dispersed beam when that component is at minimum deviation. This property provides an accurate method of adjustment for minimum deviation. When this method is used in practice, there are important advantages to be gained by heavily silvering two of the prism faces and making observations on the spectrum observed in a telescope directed towards the unsilvered face of incidence.