NOVEMBER 19, 1897. , 
quite different from those of physical as- 
tronomy. There a vast variety of highly 
complex phenomena are to be referred to 
the operation of a well-known and ex- 
tremely simple law. The mental discipline 
there obtained is of the highest order, and 
it is hardly necessary-to say that a training 
in the methods of the older astronomy 
should be regarded as an indispensable 
preparation for astrophysical work. Butin 
astrophysics, as in the sciences of chemistry 
and biology, there are difficulties which 
arise from an imperfect knowledge of the 
laws governing the phenomena observed. 
The discovery of unknown laws and princi- 
ples, as well as the explanation of phe- 
nomena by laws already known, is one of 
its most important objects. 
I have referred to the differences of 
opinion which usually exist with reference 
to the value of a new science. There may 
be some who view with disfavor the array 
of chemical, physical and electrical appli- 
ances crowded around the modern telescope, 
and who look back to the observatory of 
the past as to a classic temple whose severe 
beauty had not yet been marred by modern 
trappings. So mankind, dissatisfied with 
present social conditions, looks back with 
tender regret to the good old times of earlier 
generations, yet rushes forward with the 
utmost speed. May we regard the eager- 
ness of pursuit as a measure of the value 
of its object? That the importance of as- 
trophysical research, considered with re- 
spect both to its own ends and to its bearing 
on the advance of knowledge in other fields, 
is already great, and that it will grow 
steadily from year to year, is naturally my 
own belief. Ina general way I have con- 
sidered some of the reasons on which it is 
founded, and I now wish to eall your at- 
tention to a few specific cases which illus- 
trate my general remarks, and in which I 
think the importance of astrophysical sci- 
ence is manifest. 
SCIENCE. 
749 
Some of the most noteworthy advances 
in astronomy and in astrophysics have been 
made possible by the introduction of pho- 
tography. The photographic plate not only 
gives a permanent record of what the eye 
can see, but, by its integrating power con- 
tinued through long exposures, it builds up 
a picture from light impulses too feeble to 
affect the sense of vision. Thus it has been 
discovered that vast regions in the sky are 
filled with diffuse nebulee, which (since the 
apparent brightness of a surface cannot be 
increased by any optical device) must ever 
remain unseen. This information, which 
the photographic plate alone could furnish, 
is itself most wonderful and suggestive. It 
is, however, but a part of what the same 
plate may yield. Whoever has studied 
Professor Barnard’s admirable pictures of 
the Milky Way in Scorpio must have ob- 
served how accurately the distribution of 
the smallest stars corresponds to that of the 
extended nebulosity which fills this part of | 
the sky, and at the same time how strik- 
ingly the nebulous matter is concentrated 
around the brightest stars in the constella- 
tion. Bright stars, faint stars and nebu- 
losity are unmistakably physically related, 
and, hence, at the same order of distance 
from the earth; and from this it follows 
that the real sizes of the stars are of en- 
tirely different orders. Here is a fact hav- 
ing a most important bearing on the ques- 
tion of stellar distribution, brought out by 
the simplest possible means. It is perhaps 
beyond the reach of more elaborate methods. 
And in this case it is to be observed that 
the evidence would not be made clearer by 
any further treatment of the material. 
The positions of the stars and the density 
of the nebulosity might be measured, and 
the results might be tabulated, but all to 
no purpose ; for, if the data yielded by ob- 
servation were in the form of measurements, 
the first step toward their interpretation 
would be the construction of just such a 
