IN the current number of NATURE you were good enough to print a short article by me, announcing a discovery which I believed was new. My attention has now been drawn to the fact that it was published a few months ago by Rydberg (Wied. Ann., vol. lviii. p. 674), to whom the honour of the discovery therefore belongs. My excuse for being unacquainted with Rydberg's paper must be found in a prolonged absence from home last summer, and the large amount of unread scientific literature which I consequently found on my return home. There is, moreover, nothing in the title of Rydberg's paper which would indicate the important nature of its contents. If by writing to you on the subject I have drawn the attention of physicists to what I consider the most important fact yet brought to light concerning molecular vibrations, my article will have served some good purpose.