ASTRONOMICAL APPARATUS—HALE. 273 
up our photographs of that date, and there was the image recorded 
by the spectroheliograph precisely as it had been described. So that 
if I had previously been a little doubtful as to the possibility of see- 
ing these objects with such an equipment, I gave up all doubt after 
having made that comparison. One might say that it would hardly 
be practicable to observe such phenomena in any satisfactory way 
with a large telescope. A small one is very much more advantageous 
for work of this kind. As soon as possible we are going to set up a 
small equatorial for the purpose of seeing these objects and compar- 
ing them with our photographs, after having derived the knowledge 
of the possibility of observing them from the work done by these 
men in England. But we will not undertake systematic work in this 
field, as I hope the valuable observations now in progress here will 
be continued. No records are made with the spectroheliograph of 
the D, image of the sun at present. We have tried experiments, but 
so far they have not been successful. We ought to be able to photo- 
graph the sun through the D, line, but we have not done it yet. The 
only existing records are those made by the members of the British 
Astronomical Association. These observations should be made in 
conjunction with other solar observations, as in fact is being done at 
the present time. The characteristics of the hydrogen lines are being 
observed at the same time that these D, images are being recorded, 
so that any relationship between the two may be discovered. I can 
not dwell upon this very interesting subject. There is a great oppor- 
tunity here for further work of high importance. 
I must now pass to the question of sun-spot spectra. I need hardly 
tell those who are present that observations of sun-spot spectra made 
visually are sometimes more valuable than those which can be made 
by photographic methods. Take, for example, the lines in the green 
region of the spectrum. This photograph will suffice to show them. 
Here is the 6 group in the spectrum of a sun spot and also in the 
spectrum of the photosphere. We see in the spot a large number of 
fine lines, long ago observed by Young and Maunder, and now being 
studied with great care. Most of these fine lines shown by a powerful 
instrument photographically can be seen visually with a small spectro- 
scope attached to a 6*inch or probably a 4-inch telescope, and many 
other phenomena which can not be photographed at all can be seen 
with a similar equipment.’ There is a certain advantage in observing 
such spectra with a larger telescope, provided that the spot under con- 
@As I understand the matter, only the more conspicuous dark flocculi can be 
observed visually. 
> Since the lecture was given much better photographs of spot spectra have 
been made with the 380-foot spectrograph and ‘ tower’ telescope on Mount 
Wilson. It still remains true, however, that visual observers of spot spectra 
can obtain various important results not yet within the reach of photography. 
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