es crossing it laterally
such as those we see in the spectrum of the sun. A cylindrical lens is,
therefore, placed before the eye-piece of the telescope, and as this has
the effect of turning a point into a line and a line into a band, the
narrow spectrum of the star is thereby broadened out into a luminous
band in which we can distinguish any details that exist. In other forms
of stellar spectroscope we require a slit which must be placed in the
focus of the object glass, and the general arrangement is similar to
that which we have described in the chapter on the sun, except that a
cylindrical lens is required.

The study of the spectra of the fixed stars made hardly any progress
until the principles of spectrum analysis had been established by
Kirchhoff in 1859. When the dark lines in the solar spectrum had been
properly interpreted, it was at once evident that science had opened
wide the gates of a new territory for human exploration, of the very
existence of which hardly anyone had been aware up to that time. We have
seen to what splendid triumphs the study of the sun has led the
investigators in this field, and we have seen how very valuable results
have been obtained by the new method when applied to observations of
comets and nebulae. We shall now give some account of what has been
learned with regard to the constitution of the fixed stars by the
researches which were inaugurated by Sir William Huggins and continued
and developed by him, as well as by Secchi, Vogel, Pickering, Lockyer,
Duner, Scheiner and others. Here, as in the other modern branches of
astronomy, photography has played a most important part, not only
because photographed spectra of stars extend much farther at the violet
end than the observer can follow them with his eye, but also because the
positions of the lines can be very accurately measured on the
photographs.

The first observer who reduced the apparently chaotic diversity of
stellar spectra to order was Secchi, who showed that they might all be
grouped according to four types. Within the last thirty years, however,
so many modifications of the various types have been found that it has
become necessary to subdivide Secchi's types, and most observers now
make use of Vogel's classification, which we shall also for convenience
adopt in this chapter.

_Type I._--In the spectra of stars of this class the metallic lines,
which are so very numerous and conspicuous in the sun's violet spectrum