282 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
vapor in the chromosphere is recorded photographically, it acts as 
hydrogen does, and gives dark absorption phenomena, due to the high 
level H, line, and not to be confused with the bright calcium flocculi 
due to H,. 
I see that I must rapidly draw to a close. I might mention various 
other methods of employing spectrohelographs, and if anyone present 
should be interested at some future time to take them up I shall be 
delighted to discuss them in detail. I may remark in passing that with 
a Littrow spectrograph, or any long focus spectrograph, and a fixed 
solar image, one can undertake other work of various kinds, such as a 
determination of the solar rotation, along some such plan as Dunér or 
Halm followed, but using different lines in the spectrum, and bene- 
fiting from the advantages of photographic methods. In all such 
work, cooperation with other investigators is greatly to be desired, be- 
cause it might otherwise frequently happen that two men would be 
doing the same thing, whereas it would be just as easy for them to 
supplement each other’s work instead of duplicating it. 
One other phase of the subject which I should like to have time to 
discuss, but can not, is that of stellar spectroscopy. You will see that 
for stellar spectroscopy a large telescope in general does have an ad- 
vantage. The more light one can collect and concentrate in a stellar 
image the more dispersion can be employed in the spectroscope, and 
the users of large apertures therefore do have an advantage in stellar 
spectroscopic work. But the fact remains that small instruments can 
be used to very great effect in this field also, provided that one intel- 
hgently plans his investigations. I know of no better example of 
this than one which I am permitted, by the kindness of Father Sid- 
greaves, to illustrate. Here is a photograph of the spectrum of 
oCeti, made with a refractor of 4 inches aperture, with a prism of 
224° awole placed over the object glass. The focal length of the tele- 
scope is 4 feet. 
The slide shows the spectrum of Omicron Ceti on the 29th Novem- 
ber, 1905, and on the 1st December, 1906, and brings out with great 
clearness the remarkable changes which occurred during that period. 
If this spectrum had been photographed with such an instrument, 
let us say, as the Bruce spectrograph of the Yerkes Observatory 
attached to the 40-inch telescope, there would have been some advan- 
tages, but there would also have been some disadvantages, because 
the entire region covered by the photographs made with that instru- 
ment (when three prisms are used) is a limited one here in the blue. 
All of these remarkable flutings in the less refrangible region would 
not have appeared in the photographs, and nothing would have been 
known, if one had been confined with such an instrument to a short 
region of the spectrum, about the very interesting changes shown 
