bservatories,
especially that of Harvard, consists in photographing the spectra of
thousands of stars, and studying the peculiarities thus brought out. At
Harvard a large portion of this work is done as part of the work of the
Henry Draper Memorial, established by his widow in memory of the
eminent investigator of New York, who died twenty years ago.

By a comparison of the spectra of stars Sir William Huggins has
developed the idea that these bodies, like human beings, have a life
history. They are nebulae in infancy, while the progress to old age is
marked by a constant increase in the density of their substance. Their
temperature also changes in a way analogous to the vigor of the human
being. During a certain time the star continually grows hotter and
hotter. But an end to this must come, and it cools off in old age. What
the age of a star may be is hard even to guess. It is many millions of
years, perhaps hundreds, possibly even thousands, of millions.

Some attempt at giving the magnitude is included in every considerable
list of stars. The work of determining the magnitudes with the greatest
precision is so laborious that it must go on rather slowly. It is being
pursued on a large scale at the Harvard Observatory, as well as in that
of Potsdam, Germany.

We come now to the question of changes in the appearance of bright
stars. It seems pretty certain that more than one per cent of these
bodies fluctuate to a greater or less extent in their light.
Observations of these fluctuations, in the case of at least the
brighter stars, may be carried on without any instrument more expensive
than a good opera-glass--in fact, in the case of stars visible to the
naked eye, with no instrument at all.

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