ut with components too close together to be distinguished
separately, the fringes behave differently. As the slits are moved
apart a point is reached where the fringes completely disappear,
only to reappear as the separation is continued. This effect is
obtained when the slits are at right angles to the line joining
the two stars of the pair, found by this method to be 0.0418 of a
second of arc apart (on December 30, 1919). Subsequent measures,
of far greater precision than those obtainable by other methods in
the case of easily separated double stars, show the rapid orbital
motion of the components of the system. This device will be applied
to other close binaries, hitherto beyond the reach of measurement.

[Illustration: Fig. 18. Ring Nebula in Lyra, photographed with the
60-inch (Ritchey) and 100-inch (Duncan) telescopes.

Showing the increased scale of the images given by the larger
instrument.]

Without entering into further details of the tests, it is evident
that the new telescope will afford boundless possibilities for
the study of the stellar universe.[*] The structure and extent of
the galactic system, and the motions of the stars comprising it;
the distribution, distances, and dimensions of the spiral nebulae,
their motions, rotation, and mode of development; the origin of
the stars and the successive stages in their life history: these
are some of the great questions which the new telescope must help
to answer. In such an embarrassment of riches the chief difficulty
is to withstand the temptation toward scattering of effort, and to
form an observing programme directed toward the solution of crucial
problems rather than the accumulation of vast stores of miscellaneous
data. This programme will be supplemented by an extensive study
of the sun, the only star near enough the earth to be examined
in detail, and by a series of laboratory investigations involving
the experimental imitation of solar and stellar conditions, thus
aiding in the interpretation of celestial phenomena.

[Footnote *: It is not adapted for work on the sun, as the mirrors
would be distorted by its heat. Three other telescopes, especially
designed for solar observations, are in use on Mount Wilson.]

CHAPTER II

GIANT STARS

Our ancestral sun, as pictured by Laplace, originally extended
in a state of luminous vapor beyond the boundaries of the solar
system. Rotating upon its axis, it slowly contracted through loss
of heat by r