Any combination of the prisms may thus be employed. As is usual in such
an investigation, a great variety of difficulties have been encountered,
and the most important of them have now been overcome.

4. _Faint Stellar Spectra_.--The 28 inch reflector will be used for the
study of the spectra of the faint stars, and also for the fainter
portions near the ends of the spectra of the brighter stars. The form of
spectroscope mentioned above, in which the collimator and slit are
replaced by a concave lens, will be tried. The objects to be examined
are, first, the stars known to be variable, with the expectation that
some evidence may be afforded of the cause of the variation. The stars
whose spectrum is known to be banded, to contain bright lines, or to be
peculiar in other respects, will also be examined systematically.
Experiments will also be tried with orthochromatic plates and the use of
a colored absorbing medium, in order to photograph the red portions of
the spectra of the bright stars. Quartz will also be tried to extend the
images toward the ultra-violet.

5. _Absorption Spectra_.--The ordinary form of comparison spectrum
cannot be employed on account of the absence of a slit. The most
promising method of determining the wave lengths of the stellar spectra
is to interpose some absorbent medium. Experiments are in progress with
hyponitric fumes and other substances. A tank containing one of these
materials is interposed and the spectra photographed through it. The
stellar spectra will then be traversed by lines resulting from the
absorption of the media thus interposed, and, after their wave lengths
are once determined, they serve as a precise standard to which the
stellar lines may be referred. The absorption lines of the terrestrial
atmosphere would form the best standard for this purpose if those which
are sufficiently fine can be photographed.