700 
chart as the photograph places ready in our 
hands. 
Of very great importance to the new as- 
tronomy has been the investigation of the 
conditions of maximum efficiency of its chief 
instrument, the spectroscope, by the meth- 
ods of physical optics. ‘The theory of re- 
solving power, introduced by Lord Rayleigh, 
and quite recently elaborated by Professor 
Wadsworth, has been especially fruitful. It 
has doue away with the old idea that the ef- 
ficiency of a spectroscope is measured by its 
dispersion, and may be trusted to destroy in 
time some musty traditions concerning the 
magnifying power and definition of astro- 
nomical telescopes. The theory hasalso been 
extended so as to include the spectrograph, in 
which the photographic plate takes the place 
of the eye at the observing telescope of the 
spectroscope. The designing of spectroscopes 
has thus been placed on a thoroughly scien- 
tific basis. At the same time the demands 
for accuracy in the practical construction of 
the instrument have been greatly raised. 
The objectives, the prisms, the fitting of the 
mechanical parts, must be the best possible. 
Hence the spectroscope has become an 
instrument of precision, worthy of a plaee 
among the most refined instruments of 
practical astronomy, and fitted for the class 
of work now most needed in astrophysical 
research. 
A familiar example of the mutual obli- 
gations of allied sciences is found in the 
first measurements of the velocity of light. 
Perhaps a somewhat parallel case may have 
to be recorded by the future historian of 
science. Spectroscopists have tested the 
validity of what is known as Doppler’s 
principle, by which the motion of a body in 
the line of sight is determined from the 
observed displacement of its spectral lines, 
and have at the same time proved the 
capabilities of their instruments, by means 
of the velocities of the earth and heavenly 
bodies furnished to them by astronomy. It 
SCIENCE. 
[N. S. Von. VI. No. 151. 
is not impossible that this also is a reversi- 
ble process, and that measurements of the 
velocities of bodies in the solar system may 
give one of the best methods of determining 
the dimensions of their orbits. 
Numerous cases could be mentioned in 
which astrophysical investigations have con- 
tributed to our knowledge of the chemical 
elements. Of these the first which naturally 
presents itself is one of the most recent. 
The element helium was discovered first in 
the sun (as its name implies), then in the 
stars, then in the nebule, and at last, by 
Professor Ramsay, it was ‘run to earth.’ 
It had animportant place in celestial chem- 
istry long before it was known to terrestrial 
science ; and, on account of its rare occur- 
rence and seeming inertness, it is quite pos- 
sible that but for the spectroscope of the as- 
trophysicist we should have remained for- 
ever ignorant of its existence. To the as- 
trophysicist, however, it was known only by 
the occurrence in its spectrum of one bright 
line. Laboratory investigations soon re- 
vealed its complete spectrum, and then the 
astrophysicists were able to recognize, as be- 
longing to helium, a large number of lines 
whose origin in the heavenly bodies they 
had been unable to discover. Our knowl- 
edge of the heavenly bodies may be greatly 
advanced when the properties of this re- 
markable element shall have been thor- 
oughly studied. 
It is not necessary, however, to seek il- 
lustrations in new elements. The complete 
series of hydrogen lines, to which belong 
the few lines that are ordinarily seen in the 
laboratory spectroscope, was discovered by 
Huggins in the spectra of the white stars ; 
and a new series, which had previously 
been seen by the eye of theory only, and 
which, so far as I know, has not yet been 
produced artificially, has recently been 
found by Pickering in the spectrum of the 
star Zeta Puppis. 
Another familiar element is calcium. Its 
