My object in the present communication is not to discuss Professor Challis's theory, but to rectify some statements as to the experimental facts of the case, as well as one relating to the extent of some researches of my own. I have, however, on some points expressed opinions, respecting the justice of which it is only one who is familiar with certain classes of optical experiments who can feel the confidence that I entertain.

From the paragraph commencing at the foot of page 330, it is plain that Professor Challis has made some confusion between three perfectly distinct things : Sir David Brewster's controverted analysis of the solar spectrum by means of absorbing media; his discovery of the phenomenon of internal dispersion; and my own discovery, that a beam of rays of prismatic purity (whether belonging to the visible or invisible portion of the spectrum is indifferent) may, by their action on certain media, produce light which may be decomposed by the prism into portions extending over a wide range of refrangibility, and having colours answering to their refrangibilities.

As to the first, it was asserted by Sir David Brewster that light of prismatic purity may have its colour changed by passing through absorbing media. This has nothing to do with “internal” or “epipolic” dispersion, or “fluorescence.” Glass coloured blue by cobalt, for instance, has none of these properties, although it is one of the media which exhibit most strikingly the phenomena adduced by Sir David Brewster.