ESSAY ON THE VELOCITY OF LIGHT. 143 
of the car causes the rain to appear to fall more obliquely than it really does; 
if the wind blows the rain in a direction with that of the car, the obliquity 
with which it really falls appears diminished, and may, according to the 
circumstances, be annulled or even changed in direction; if the wind makes 
any angle whatsoever with the railroad, the drops of rain will always appear 
to fall behind the positions they would have if the car were at rest. In a word, 
the direétion according to which the observer seated in the car sees the rain 
fall is more than the real direction of the rain modified as it would be if blown 
by a wind in a direction contrary to the motion of the train. If the car runs 
over acircular road the influence of its movement on the apparent direction of 
the fall of the rain-drops changes progressively, so that the rain seems suc- 
cessively to come from different points of the heaven situate all round the 
point from which it really falls. 
If what has just been said is clearly understood, it will not be difficult also 
to understand the phenomenon of aberration discovered by Bradley, a phe- 
nomenon which presents the closest analogy with the apparent change in direc- 
tion of the rain-drops of which we have just spoken. Replace the rain by the 
light which comes from the stars, and the car moving on the circular railroad 
by the earth which annually moves around the sun in an orbit which differs 
little from a circle. The observer who perceives the light coming from the 
stars, and who is carried by the earth in its annual movement around the sun, 
receives that light from positions in space which differ from the directions in 
which it would come to him if he were at rest. so that to allow the light to 
traverse the axis of his telescope he is obliged to give it a direction different 
from what it would have if lhe were not himself translated with a certain 
velocity. In the example of the rain-drops observed from the interior of the 
moving car, if we placed near the window a tube so directed that a drop of 
rain entering the top would pass through the tube without touching the sides, 
that tube would have the direction of the lines the rain appears to describe; 
on the contrary, we would have to give it the true direction of the falling rain 
if the car did not move. This tube stands in the place of the telescope, which 
the observer so directs that the light coming from a star traverses its whole 
length to arrive at his eye placed at the other end behind the eye-piece. If 
the earth did not move, the telescope ought to point in the real path of the light 
coming from the star—that is to say, in the straight line drawn from the 
observer to the star; but the earth being in motion, the telescope has to be 
directed in the,apparent path of the light, a line which indefinitely prolonged 
into space would not pass through the star, but considerably in advance of it, 
when we refer its position to the motion of the earth. We therefore see the 
star in a point'where it does not exist, and that apparent deviation, which is 
but an illusion arising from the motion of the observer in space, changes 
gradually in direction as the earth moves over the different parts of its annual 
orbit, so that it returns to exactly the same conditions in the course of a year. 
Each star, therefore, seems to revolve around its real position, which point we 
do not see; and the angular distance which separates it from that real position 
depends not upon the distance which exists between us and it, but solely on 
the ratio which exists between the velocity of light and the velocity of the 
earth. If the two velocities were comparable, that is to say, if the ratio of the 
greater to the smaller was not a very large number of units, which is the case 
with the velocity of falling rain compared to the velocity of the train, the 
apparent deviation of which we have spoken would be very considerable, like 
the deviation we observe in the falling rain when we are in a ear, which from 
a state of rest commences to run rapidly over the rail. If, on the contrary, 
the velocity of light were incomparably greater than that of the earth, and as 
infinity compared with the latter, the apparent deviation of the rays of light, 
due to the velocity of the earth, would be extremely small, and might, on 
