It is only within a few months that my attention has been (at first accidentally) called to this matter. For, though I owe to the kindness of Prof. Klein a copy of his and Sommerfeld's Theorie des Kreisels, I had passed over, in reading the work, the “Digression on Quaternions” which it contains. But Prof. C. N. Little, in the course of correspondence about his remarkable paper on Knots (whose passage through the press I was looking after), referred me for a numerical detail to an article by Prof. Klein on the progress of publication of Gauss' Gesammelte Werke. Shortly afterwards Prof. Joly called my attention to the same article from another point of view. These references have led me to write the present paper; whose somewhat puzzling title is explained in the first section below.