754 
Let us consider for a moment some of the 
questions in this field that are open for in- 
vestigation. The motions in the line of 
sight of all stars within reach of the largest 
telescopes have to be measured. This im- 
portant line of research has already been 
referred to. The relation has to be ascer- 
tained between the various classes of star 
spectra and the probable order of stellar 
evolution. It now appears practically cer- 
tain that all the stars are not made accord- 
ing to asingle pattern, and that they cannot 
be fitted into a single scheme of develop- 
ment. The Wolf-Rayet stars, the stars 
with banded spectra, the stars with bright- 
line spectra, the planetary nebule, the 
spectroscopic binaries, the variable stars, 
require the most careful attention. Vari- 
ables of the Mira class should be followed 
with the spectroscope as far as possible from 
their maximum, and the spectral changes 
which accompany the light variation of 
other stars, whether due to phenomena of 
emission and absorption, or to relative mo- 
tion of bodies in a revolving system, should 
be studied with the most powerful instru- 
ments. 
The discovery, by means of the spectro- 
scope, of binary stars which are far too close 
for resolution with our most powerful tele- 
scopes, and which are recognized in their 
true character by a periodic doubling of 
their spectral lines, has brought to our 
knowledge strange and wonderful condi- 
tions of orbital motion. Such a system as 
that of Spica, where two bodies like our 
sun revolve around each other like the 
balls of a gigantic pendulum, in a period of 
only four days, at a distance no greater 
than that which separates the sixth satellite 
of Saturn from its primary, must have re- 
mained forever unknown to the older 
astronomy. Between these spectroscopic 
binaries and the most rapidly revolving 
doubles visible in the telescope there is a 
wide gap, the cause of which is obvious. 
SCIENCE. 
[N. 8S. Von. VI. No. 151. 
The conditions favorable to discovery in 
the two cases are directly opposed, and 
doubtless a large class of stars lies at present 
just beyond the reach of either method. 
But this gap may be bridged over by 
means of such a great telescope as we see 
before us to-day, while the work necessary 
to accomplish this end will open up stil] 
another field for research. It has long 
been recognized that the position microm- 
eter and the spectroscope, taken together, 
are theoretically competent to determine 
the real orbits in space of the components 
of a double star; hence, also, the masses of 
the components and their distance from the 
earth. Until recently the question had 
only a mathematical interest. But the 
small velocities to be expected in the case 
of any double star whose components can 
be separately distinguished with the tele- 
scope are now almost, if not quite, within 
reach of the spectroscope, and the investi- 
gation of such doubles has acquired a 
physical interest. 
Here I must close my review of the im- 
portant questions before the astrophysicist, 
with the consciousness that it is most re- 
markable for what it leaves unnoticed. I 
have said nothing of questions relating to 
the photography of comets and their spectra, 
the rotation of the planets or the absorp- 
tion spectra of their atmospheres, the colors 
of double stars, the spectra of temporary 
stars, the measurement of obscure wave- 
lengths ; nothing about stellar photometry, 
the application of interference methods to 
spectroscopic research, the exploration of 
the infra-red spectrum. But I will not 
trespass further on your patience. In all 
the fields that I have mentioned there are 
noble problems, worthy of the best efforts 
that can be given to their solution. To 
realize their importance, think how ill we 
could spare what we have already won. 
What a blank would be left in our knowl- 
edge of the heavens if the results of astro- 
