to be fired
into a tapered hole in a great block of steel. The instantaneous
pressure, according to Cook, would be about 7,000 tons per square
inch, only 1/150000 of that possible through the collision of the
largest stars.

[Illustration: Fig. 41. Mount San Antonio as seen from Mount Wilson.

Michelson is measuring the velocity of light between stations on
Mount Wilson and Mount San Antonio. Astronomical observations afford
the best means, however, of detecting any possible difference between
the velocities of light of different colors. From studies of variable
stars in the cluster Messier 5 Shapley concludes that if there is
any difference between the velocities of blue and yellow light
in free space it cannot exceed two inches in one second, the time
in which light travels 186,000 miles.]

Finally, we may compare the effects of light pressure on the earth
and stars. Twenty years ago Nichols and Hull succeeded, with the
aid of the most sensitive apparatus, in measuring the minute
displacements produced by the pressure of light. The effect is
so slight, even with the brightest light-sources available, that
great experimental skill is required to measure it. Yet in the
case of some of the larger stars Eddington calculates that one-half
of their mass is supported by radiation pressure, and this against
their enormous gravitational attraction. In fact, if their mass
were as great as ten times that of the sun, the radiation pressure
would so nearly overcome the pull of gravitation that they would
be likely to break up.

But enough has been said to illustrate the wide variety of experimental
devices that stand at our service in the laboratories of the heavens.
Here the physicist and chemist of the future will more and more
frequently supplement their terrestrial apparatus, and find new
clues to the complex problems which the amazing progress of recent
years has already done so much to solve.

PRACTICAL VALUE OF RESEARCHES ON THE CONSTITUTION OF MATTER

The layman has no difficulty in recognizing the practical value
of researches directed toward the improvement of the incandescent
lamp or the increased efficiency of the telephone. He can see the
results in the greatly decreased cost of electric illumination
and the rapid extension of the range of the human voice. But the
very men who have made these advances, those who have succeeded
beyond all expectation in accomplishing the economic purposes in
view, are most emp