272 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
spots and of the structure of the solar surface. I might enlarge upon 
this subject, but time is hardly sufficient to permit me to do so. 
Now let us consider the case of the prominences. If we have 
available a small spectroscope like that admirable little instrument 
designed by Evershed, or the one made by Thorp,’ or a still simpler 
home-made instrument, and attach such a spectroscope to a 4-inch or 
6-inch telescope, we have an almost ideal equipment for the observa- 
tion of the solar prominences. As a matter of fact, an instrunient 
like the 40-inch is wholly unsuited for work of this kind. You will 
easily see why. If you wish to observe the entire prominence, its 
image in the focal plane of the 40-inch telescope is usually so large 
that the slit can not be opened wide enough to include the prominence 
without admitting too much light of the sky. Therefore, for a study 
of the general characteristics of prominences, the small instrument 
has a great advantage over the large one. It was practically out of 
the question with the 40-inch for us to do systematic visual work on 
prominences. When the conditions were peculiarly fine we could 
study the structure of certain prominences, and I never saw anything 
more remarkable than such details when they came out under the 
best seeing. But with the spectroscope available, and under ordinary 
atmospheric conditions, we could not make records of the general 
form and distribution of prominences that would compare in value 
with the records obtainable with small telescopes. 
It has remained for certain amateurs here in England very recently 
to show that objects upon the surface of the sun which escaped many 
of the earlier solar observers can be observed at any time when the 
conditions are favorable with a very small instrument indeed. For 
example, Mr. Buss and Captain Daunt, and, I believe, some others, 
have been observing the sun with such instruments, and have been 
able to see upon the disk dark regions in which the D, line is 
strengthened, which I think have never been recorded before in a 
systematic way. Observations of the dark D, line upon the face of 
the sun were formerly mentioned as unusual and rather remarkabie 
phenomena, and certainly, so far as I have ever seen in the literature 
of the subject, the dark hydrogen flocculi were never recognized upon 
the sun by the earlier spectroscopists; but they are seen, at times 
‘at least, by those gentlemen to whom I have referred. This I°can 
make quite certain from my own knowledge, because on one occasion, 
when Mr. Buss had described one of the very peculiar dark hydrogen 
flocculi—flocculi of this type appear very much darker than the ordi- 
nary ones photographed daily with the spectroheliograph—I looked 
Be Le ee ee 
“TJ wish to call special attention to the solar spectroscopes and other inex- 
pensive instruments made by Mr. Thomas Thorp, of Manchester. Oue of these, 
a polarizing helioscope, has done excellent service on Mount Wilson. 
