Evening star. [volume] (Washington, D.C.) 1854-1972, 1903-11-21
|eeMETls FIERY TAIL.
MECHANICAL FORCE OF LIGHT
May FxfLArN zrs BEHAVIOR.
Why Does the Brilliant Appendix
Always Point From
i the SUM-
From the New York Tribune.

From the New York Tribune.
THE persistence with which comets
tail points away from the sun, thus trav-
cling in advance of~the head when the lat
ter is travelling outward from the solar sys-
tenn" said Charles Vernon Boys, at the re-
cent meeting of the British Association for
the Advancement of Science, is still
mystery. Let me take the most recent ex-
PLANTATION, which was set forth only three
| months ago in the Astrophysical Journal
| in the United States Those admirable ex-
perimentalists, Nichols and Hull, have for
some years been investigating the back
pressure exerted by the action of light upon
bodies on which it falls. In this they have
followed the Russian physicist, Lebedew.
both in minuteness and delicacy of meas
urement, and in their successful elimlna-
ton of disturbances their results are un-
eQualed.
Having now definite and accurate con-
firmation of the existence of the force pro-
duced by the action of light, or rather ra-
diation, Nichols and Hull proceed to ex-
anime the question as to how Tar such re-
pulsion may be competent to overcome the
gravftative attraction OF the sun and drive
away the matter which pOurS out from the
comet. 1t is interesting to note here that
Kepler put forward this very idea, and that
Newton. the inventor of the corpuscular
theory of light, looked upon the suggestion
with some favor.
Repulsion by Light.
Coming now to this recent paper or
Nichols and Hull. we find first the consid-
GRAFTON of the relation Of the attraction by
gravitation, and the repulsion by light upon
particles of different sizes and densities
Density has no influence on the action Of
light. while 1t is favorable to gravitation.
and therefore unfavorable to tail forma-
ton Size Is favorable to both, but more
to gravitation than to light. for If the d.
ameter of particle be doubled. one is In
creased eightfold and the other only four
So size favors gravitatlonal attraction
Conversely, of course, smallness favors re-
pulsion by light, which relatively should
get greater and greater as the particles
diminish in size. At last, then, degree Of
smallneSs may be reached In which the re-
pulsion by light will actually be equal to
the attraction by gravitation, and such a
particle would remain in space its motion
unaffected by our sun. Let the dim1nut1oD
of size continue, and then the repulsion will
be in excess, and If the law were to con-
tinue 1t would with sufficient diminutlon
become relatively as large as we please,
Light Pressure and Gravity.
THE law, however, does not continue.
Schwarzschild has shown that when the
particles are small enough, light does not
act upon them in the same way. Owing to
diffraction, the effect of light is unduly
great for a certain very small size Of par
ticle, while it fails almost entirely when the
particle is made much smaller. Thus 1t is
that the indefinite increase in the repulsion
by light as compared with the attraction
by graVitation with diminutiOn of size of
particle is checked, and when, according to
theory. with a particular density of par
ticle, the light pressure is about twenty
times as great as graVitational attraction,
further dminution of size ceases to favor
the action of light, and 1t begins to fall off
again. The distance of the particle from
the sun has no influence upon the relation
between the two kinds of forces, for they
rise and fall together. Nichols and Hull,
therefore, while not denying other causes
may operate, believe that light pressure is
adequate to account for the phenomena,
and that where the material coming from
the head or comet proper is of two or three
kinds, whether of density or of size of par
ticle, the separation of the two or three
tails should naturally follow"