ASTRONOMICAL APPARATUS—HALE. 2838 
in this particular case. The next slide shows another photograph 
taken by Father Sidgreaves, in this case with a somewhat different 
instrumental arrangement—a direct vision prism at the focus of a 
15-inch equatorial. But you will see the great range of spectrum 
included on the plate, and remember again that almost all the spec- 
trum, except a very small region, would be missing on photographs 
taken with such instruments as the Bruce or Mills spectrographs, or 
other three-prism instruments employed for the investigation of 
stellar motions in the line of sight. You will notice the remark- 
ably interesting and important fact that the He line of hydrogen 
is absent from the picture, probably, as Mr. Newall suggested, cut 
out by the absorption of the H line of caleium—the broad H, band; 
perhaps in another star lying nearer to us than the star which gives 
the bright lines of hydrogen. This serves to illustrate the great im- 
portance of the work that can be done with an instrument of very 
small size indeed, even in this field of stellar spectroscopy, which 
seems peculiarly to belong to telescopes of large aperture. As I said 
before, in general the investigator with a telescope of large aperture 
does have an advantage in stellar spectroscopic work; but there are 
various investigations of this sort—and of the kind Professor Picker- 
ing has taken up in his very extensive surveys of the whole sky with 
objective prisms—which are of extreme importance, and which can 
not be carried out with large refractors of great focal length. 
I might go on to speak of the possibilities of work on variable stars, 
but they are familiar to most of you. The observation of many wide 
double stars, my friend Burnham tells me, has been neglected since 
the time of Herschel, because the large instruments, and even the 
small ones, have been devoted to closer objects, so that in revising his 
great catalogue Burnham had to measure with the 40-inch a great 
many wide doubles which had not been looked at perhaps since 
Herschel discovered them more than a century before. Important 
double-star work is always open to men with small instruments, if a 
micrometer is available. 
Then I might go on to the case where a man has no telescope at all, 
and still wants to make contributions to astrophysics. I do not now 
speak of such splendid work as Anderson did when he discovered 
Nova Persei with the naked eye; but if one were convinced that the 
overcast sky of London would never open again, he could still work 
in his laboratory and make important contributions by identifying 
lines and bands in spot spectra, as Professor Fowler has been doing of 
late, or by researches in a score of other fields. 
I will close with a few practical suggestions. One reference to the 
matter of atmosphere. Perhaps some of us feel that if we could only 
ascend into the upper regions we could get results very much better 
than are obtainable in London. But if we stop to think of the men 
