As a general rule, the light of these stars goes through its
changes in a regular period, which is sometimes as short as a few
hours, but generally several days, frequently a large fraction of
a year or even eighteen months. Observations of these stars are
made to determine the length of the period and the law of
variation of the brightness. Any person with a good eye and skill
in making estimates can make the observations if he will devote
sufficient pains to training himself; but they require a degree of
care and assiduity which is not to be expected of any one but an
enthusiast on the subject. One of the most successful observers of
the present time is Mr. W. A. Roberts, a resident of South Africa,
whom the Boer war did not prevent from keeping up a watch of the
southern sky, which has resulted in greatly increasing our
knowledge of variable stars. There are also quite a number of
astronomers in Europe and America who make this particular study
their specialty.
During the past fifteen years the art of measuring the speed with
which a star is approaching us or receding from us has been
brought to a wonderful degree of perfection. The instrument with
which this was first done was the spectroscope; it is now replaced
with another of the same general kind, called the spectrograph.
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