746 
high development has in fact been so re- 
cent that its name is found in only our 
latest dictionaries. It is closely allied on 
the one hand to astronomy, of which it 
may properly be classed as a branch, and 
on the other hand to chemistry and physics ; 
but it assumes wide privileges, and it is 
ready to draw material which it can use 
with profit, from any source, however dis- 
tant. It seeks to ascertain the nature of 
the heavenly bodies, rather than their po- 
sitions or motions in space—what they are, 
rather than where they are ; and for my own 
convenience I shall use the terms astro- 
physics and astronomy to denote the scien- 
ces of which these aims are respectively 
characteristic. Yet here again the line of 
demarcation cannot be sharply drawn, since 
the measurement of celestial motions that 
cannot be dealt with by the methods of the 
older astronomy is one of the most impor- 
tant tasks of the astrophysicist. That 
which perhaps is most characteristic of as- 
trophysies is the special prominence which 
it gives to the study of radiation. The 
complex nature of white light, in particular, 
is never lost sight of, and its consequences 
are thoroughly exploited. 
That the older astronomers made no ef- 
forts systematically to study the nature of 
the heavenly bodies, is to be ascribed to the 
seeming hopelessness of such an attempt in 
their day, rather than to a lack of interest 
in the subject, or a slight appreciation of its 
importance, on their part. They did, in 
fact, seek explanations of such phenomena 
as they could observe, and the beginnings 
of astrophysics are to be found far back in 
the past. But the curious speculations of 
Sir John Herschel on the structure of the 
sun’s photosphere show how inadequate 
was the supply of facts to serve as a basis 
for a science of solar physics in Herschel’s 
time. The conception of living organisms 
a thousand miles long, floating about on 
the sun’s surface, and shining with the in- 
SCIENCE. 
LN. S. Vou. VI. No. 151. 
tense brillianey of the photosphere, seems 
to us extraordinary, and even grotesque. 
To lose its strangeness it has to be consid- 
ered with reference to the contemporary 
state of knowledge. But the fact that only 
fifty years ago it was regarded as an ad- 
missible supposition by one of the most 
eminent of astronomers helps us to realize 
how rapid has been the advance of astro- 
physical science. It was only after the dis- 
covery was made, that the light which re- 
veals to us the existence of the heavenly 
bodies also bears the secret of their consti- 
tution and physical condition, that the 
basis for a real science was obtained. The 
spectroscope placed new and hitherto un- 
dreamed-of powers in the hands of men. 
It is to the astrophysicist what the gradu- 
ated circle and the telescope are to the as- 
tronomer. 
The study of astrophysics does not at 
present seem to have a very direct bearing 
on the practical affairs of everyday life. If 
to this statement the objection should be 
made that the study of solar radiation is 
likely to lead to a practical method of util- 
izing the sun’s heat as a source of mechan- 
ical power, I should say that such a dis- 
covery (if it is ever made) is much more 
likely to be the result of an ingenious appli- 
cation of principles already known. What 
the future may have in store we cannot 
tell, but at present the statement I have 
made holds good. With respect to practi- 
cal usefulness, therefore, astrophysics does 
not possess the same claims to consideration 
as astronomy, which has obviously im- 
portant applications in furnishing standards 
of time, and in surveying, geodesy and 
navigation, and in addition to these, an im- 
mense indirect influence on thousands of 
ordinary affairs. Yet on such grounds it 
is not probable that any astronomer would 
care to base a claim for his science. As- 
tronomy long ago reached that state of per- 
fection which sufiices for the practical ends 
