134

1908 Nov 30
Logic
I.i. 13

imaginable; so that it is by no means impossible that the helometrist
may prefer another year to use the photographic method,
especially if, like Prof. Hale and the Carnegie observers he has a miror of 175 feet
focal length, so that a second of arc would measure over a fifth of a
millimetre on his photographs. He would, thus transform himself into
a photographic astronomer, and would naturally be drawn into
other researches in sidereal astronomy.
One more remark about my use of the word science
ought to be made in this connection; though I shall not
fully explain my explanation of this bit of my usage must be
explained a little later. Astonomers have taught mankind many
things; but they did not teach men that the moon goes through a
series of changes every 29 1/2 days. On the contrary, this was one