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The Sizes and Masses of the Stars 81
may be accurately measured, and this tells us how far
towards disappearance they are when the mirrors are set
at a given distance—say twenty feet—and hence at what
distance the mirrors would have to be set to give complete
disappearance, even though this may be far beyond the
mechanical limits of the apparatus. This will multiply the
power of the instrument three or four fold, if not more,
and may make it possible to measure a few of the white
stars, such as Sirius. This solution of a century-old prob-
lem, in its brilliancy, simplicity, and rapid advancement,
deserves to rank with the most noteworthy triumphs of
physical investigation, and would doubtless have brought
a swarm of honours upon the head of its inventor if he
had not already received almost all that the scientific world
has to give. With its aid one may reasonably expect to
have, within a few years more, a sufficient number of
measures of stellar diameters to tell us whether the theore-
tical predictions will be as good in other cases as they are
in these three. It may be mentioned in passing that the
estimates of temperature for the various classes, upon
which the calculations given above depend, are taken from
some notes of mine prepared about three years before
Michelson’s apparatus was projected. Indeed, before
Michelson’s results appeared, Wilsing in Germany, Edding-
ton in England, and Shapley and I in this country, had all
published predictions concerning the diameters of Betel-
geuse, none of which differed by more than about thirty
per cent, from the value later observed. Unless further
observations with the interferometer reveal larger discord-
ances than have so far appeared, it should soon be possible
to obtain thoroughly reliable values for the surface bright-
ness of the stars of the various spectral classes, and then
to calculate the diameters in miles of all stars of known

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