73G 
DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
be aimed in front of the object. Tlie shot will go straight along the barrel produced, 
but the hole in the target will indicate a gun in front of its true position ; this 
error being aberration proper. 
Diagram of shot fired from a moving cannon; piercing a target, at Y if stationary, at Z if moving at 
same pace as gun. ABCD is the locus of successive shots, but is not the line of tire. 
If both gun and target are travelling at the same speed everything occurs as if 
they were at rest, unless a stagnant medium has to be taken into account. Relative 
motion of the medium causes windage, as is well known, 
Since motion of the medium causes a shift of the line of fire, it may be expected 
to produce a miss, but this is not a true aberration, it only appears to be such 
because of the fire being limited to one line ; suppose instead of a single gun a broad¬ 
side of guns or a number of guns firing from a turret, then the effect of a cross-wind 
is, indeed, to displace all the shots, but not to prevent the target being hit by one 
which would otherwise have missed it, and the hole in the target will indicate the 
] 30 sltion of the gun really firing the shot.^ Hence, even on a corpuscular theory, a 
wind across the line joining source and receiver, will not cause any efiective aberra¬ 
tion. Neither can a steady tail wind deliver a stream of bullets from a inachine-gmi 
more frequently than they are emitted. 
If guns are fired from a I'evolving turret, the paths of the shot will not be radial, 
but will be skewed by an amount depending upon the peripheral velocity. 
Watching the beams of a revolving lighthouse, tracking their way to a distance 
and brandished rapidly round, it is not at once quite evident whether the shape of 
those beams is not a spiral of enormous pitch (see below). We see, however, that 
on the corpuscular view the paths wdll be straight, though not radiating from the 
precise centre ; for instance, the rays from the Sun, whose peripheral velocity is 
nearly .5000 miles an hour, would if regarded as projectiles, be inclined to their radius 
at an angle of lurevsiro-o radian, or about Ig- seconds of arc ; and the Sun’s centre 
would be, apparently displaced through a fraction of this angle, equal to Sun’s 
radius/Sun’s distance ; i.e., through about the of ^ second. 
11. But nowq proceeding to look at the matter from the point of view of waves, 
there are many differences ; principally depending on the fact that there is no ques¬ 
tion of initial velocity of projection about a wuive : it crawls through the medium, 
* As these projectile examples are only used for illustration, I simplify matters artificially by 
omitting all curvatures of path. The subject of aberration in general is illustrated more fully in a 
Royal Institution Lecture, ‘ Proc. R. I.,’ April 1, 1892; also reported in ‘Nature,’ vol. 46, p. 497 
