NATURE of April 21, containing Mr. Thomson's letter, has just reached me, and I hasten to say that, had I known that his father had ever claimed to have originated the word “radian” I should, of course, have mentioned the claim in my communication to NATURE of April 7. As a matter of fact, Prof. Thomson never did so in my presence, and he certainly knew shortly after he came to Glasgow that I had on my own initiative proposed the word, and had made use of it for some years. One day when I met him accidentally he told me that he had found a college student who had been a pupil of mine using the word “radial” for a unit-angle, and that, while agreeing with me as to the need of such a word, he had doubts as to the suitability of the terminal syllable. My reply, as may be guessed from my recent communication, was that “radial”, “radian”, “rad”, had all something to be said for them, and I referred him to my letter to NATURE dated April 4, 1870. On at least two subsequent occasions we spoke of such things, and he supported the termination -an in this particular case, because of a supposed analogy with the geometrical term “median”. All this, you will see, does not preclude the possibility of an independent origination of the term by him in July, 1871, as stated by Mr. Thomson, and I therefore regret that here there is no chance of me having the satisfaction of seeing the printed word in the Calendar of Queen's College, Belfast, for 1873–4.