the
carriage will mistake its true direction. A rifle fired through the
windows of a railway carriage by a man at rest outside would make its
perforations not in the true line of fire unless the train is
stationary. If the train is moving, the line joining the holes will
point to a place in advance of where the rifle is really located.

So it is with the two glasses of a telescope, the object-glass and
eye-piece, which are pierced by the light; an astronomer, applying his
eye to the tube and looking for the origin of the disturbance, sees it
apparently, but not in its real position--its apparent direction is
displaced in the direction of the telescope's motion; by an amount
depending on the ratio of the velocity of the earth to the velocity of
light, and on the angle between those two directions.

[Illustration: FIG. 78.--Aberration diagram. The light-ray L penetrates
the object-glass of the moving telescope at O, but does not reach the
eye-piece until the telescope has travelled to the second position.
Consequently a moving telescope does not point out the true direction of
the light, but aims at a point a little in advance.]

But how minute is the displacement! The greatest effect is obtained when
the two motions are at right angles to each other, _i.e._ when the star
seen is at right angles to the direction of the earth's motion, but even
then it is only 20", or 1/180th part of a degree; one-ninetieth of the
moon's apparent diameter. It could not be detected without a cross-wire
in the telescope, and would only appear as a slight displacement from
the centre of the field, supposing the telescope accurately pointed to
the true direction.

But if this explanation be true, it at once gives a method of
determining the velocity of light. The maximum angle of deviation,
represented as a ratio of arc / radius, amounts to

1 1
------------ - .0001 = ------
180 x 57-1/3 10,000

(a gradient of 1 foot in two miles). In other words, the velocity of
light must be 10,000 times as great as the velocity of the earth in its
orbit. This amounts to a speed of 190,000 miles a second--not so very
different from what Roemer had reckoned it in order to explain the
anomalies of Jupiter's first satellite.

Stars in the direction in which the earth was moving would not be thus
affected; there would be nothing in mere approach or recession to alter
direction or to make itself in any way vis