UIST has in this way found _r_ = +0.85, a rather high value.

In order to facilitate the discussion of the relation between colour and
spectrum it is convenient to deal here with the question of the spectra
of the stars.

10. _Spectra of the stars._ In order to introduce the discussion I first
give a list of the wave-lengths of the FRAUENHOFER lines in the
spectrum, and the corresponding chemical elements.

_FRAUENHOFER line_ _Element_ [lambda]
A 759.4
B 686.8
C([alpha]) H (hydrogen) 656.3
D_1 Na (sodium) 589.6
D_3 He 587.6
E Fe (iron) 527.0
F([beta]) H 486.2
([gamma]) H 434.1
G Ca (calcium) 430.8
h([delta]) H 410.2
H([epsilon]) Ca(H) 396.9
K Ca 393.4

The first column gives the FRAUENHOFER denomination of each line.
Moreover the hydrogen lines [alpha], [beta], [gamma], [delta], [epsilon]
are denoted. The second column gives the name of the corresponding
element, to which each line is to be attributed. The third column gives
the wave-length expressed in millionths of a millimeter as unit
([mu][mu]).

On plate III, where the classification of the stellar spectra according
to the Harvard system is reproduced, will be found also the wave-lengths
of the principal H and He lines.

By the visual spectrum is usually understood the part of the radiation
between the FRAUENHOFER lines A to H ([lambda] = 760 to 400 [mu][mu]),
whereas the photographic spectrum generally lies between F and K
([lambda] = 500 to 400 [mu][mu]).

In the earliest days of spectroscopy the spectra of the stars were
classified according to their visual spectra. This classification was
introduced by SECCHI and was later more precisely defined by VOGEL. The
three classes I, II, III of VOGEL correspond approximately to the colour
classification into white, yellow, and red stars. Photography has now
almost entirely taken the place of visual observations of spectra, so
that SECCHI's and VOGEL's definitions of the stellar spectra are no
longer applicable. The terminology now used was introduced by PICKERING
and Miss CANNON and embraces a great many types, of which we