392 
of a comet’s tail from hour to hour, 
due to its apparent change of position 
in the sky from hour to hour, is not— 
astronomically speaking—an ‘ ap- 
parent change” of curvature, but is, 
im fact, for the most part, an OPTICAL 
ILLUSION. 
There is, however, an ‘‘ apparent 
change ” of curvature of the tail from 
hour to hour, due partly to real 
changes of figure in the tail itself, and 
partly to the real change of position 
of the whole comet in space. This 
“apparent change” of curvature is 
not noticed in the Article referred to. 
It is the ‘‘apparent curvature,” 
and ‘‘ apparent change” of curvature 
of comets’ tails due to their real 
curvature, and real change of curva- 
ture or real change of position in 
space, which are referred to in works 
Correspondence. 
(July, 
on Astronomy. It seems incredible 
that all writers* on Astronomy up to 
the present time should have con- 
fused the cause of supposed visible 
curvature or change of curvature 
from hour to hour—which is here ex- 
plained to be a mere OPTICAL ILLU- 
s1oN—with the “ apparent curvature” 
and “ apparent change” of curvature 
(due to real curvature and to real 
change of curvature or real change of 
position in space). 
* An extract is given in the Article 
referred to trom Herschel’s ‘‘Outlines of 
Astronomy,” Art. 557, as ‘‘showing some 
obscurity in the minds of former writers 
onastronomy on this subject of the curves of 
comets’ tails.” It seems needless to say, 
however, that Herschel there offers a 
physical explanation only of the veal curva- 
ture. 
