to astronomers; but when a
comet passes in front of a star the circumstances are widely different.
The star is indeed seen nearly as well through the comet as it would be
if the comet were entirely out of the way. This has often been noticed.
One of the most celebrated observations of this kind was made by the
late Sir John Herschel on Biela's comet, which is one of the periodic
class, and will be alluded to in the next chapter. The illustrious
astronomer saw on one occasion this object pass over a star cluster. It
consisted of excessively minute stars, which could only be seen by a
powerful telescope, such as the one Sir John was using. The faintest
haze or the merest trace of a cloud would have sufficed to hide all the
stars. It was therefore with no little interest that the astronomer
watched the progress of Biela's comet. Gradually the wanderer encroached
on the group of stars, so that if it had any appreciable solidity the
numerous twinkling points would have been completely screened. But what
were the facts? Down to the most minute star in that cluster, down to
the smallest point of light which the great telescope could show, every
object in the group was distinctly seen to twinkle right through the
mass of Biela's comet.

This was an important observation. We must recollect that the veil drawn
between the cluster and the telescope was not a thin curtain; it was a
volume of cometary substance many thousands of miles in thickness.
Contrast, then, the almost inconceivable tenuity of a comet with the
clouds to which we are accustomed. A cloud a few hundred feet thick
will hide not only the stars, but even the great sun himself. The
lightest haze that ever floated in a summer sky would do more to screen
the stars from our view than would one hundred thousand miles of such
cometary material as was here interposed.

The great comet of Donati passed over many stars which were visible
distinctly through its tail. Among these stars was a very bright
one--the well-known Arcturus. The comet, fortunately, happened to pass
over Arcturus, and though nearly the densest part of the comet was
interposed between the earth and the star, yet Arcturus twinkled on with
undiminished lustre through the thickness of this stupendous curtain.
Recent observations have, however, shown that stars in some cases
experience change in lustre when the denser part of the comet passes
over them. It is, indeed, difficult to imagine that a star would