ASTRONOMICAL APPARATUS—HALE. 277 
because you can measure the position of a line on the photograph to 
much better advantage than you can do it visually at the telescope. 
They are also better for the determination of the relative intensities 
of the lines, especially the fainter ones. But when you have said that, 
you have said almost everything that can be said for the photographs, 
and you have left out of account many of the very important advan- 
tages of visual observation. These photographs represent the inte- 
grated spot spectrum, as it were. Even with a large image of the 
spot on the slit of the spectrograph (and you realize here that the 
principal point of our great focal length is to have a large image of 
the spot on the slit), we can not as yet satisfactorily record minute 
differences in the spectrum corresponding to small details in the 
spot. If we wish to study these very important differences in the 
spot, we must do so, at present at any rate, by visual means. For 
example, Mr. Newall, your president, told me the other day that he 
had found the spectrum of the outer edge of the penumbra of a spot 
to have the same characteristic strengthening of the lines that is 
observed in the umbra, which is a very difficult thing to explain from 
the standpoint of the hypothesis I have been favoring of late, viz, 
that the principal cause of the change of the relative intensities of 
lines in a spot is reduced temperature of the vapors in the umbra. 
I knew nothing about that; I had not been observing the spot spec- 
trum visually for many years, and in our photographs this phenom- 
enon is not recorded. You see, then, in such a case the decided advan- 
tage of visual observations. I might go on to speak of other advan- 
tages. For example, suppose there were a sudden change in the 
spectrum due to an eruption; the chances that one would get a 
photograph just at that time are small, whereas visual observations 
necessarily occupy a considerable period of time, during which erup- 
tions might be detected. Even a few results might be of extreme 
importance, and would probably be wholly missed in the photographs. 
Again, the extension of certain lines outside of the spot upon the 
photosphere is not recorded at all in our photogrphs, because of the 
method we usually employ of excluding from the plate all lght 
except that which comes from the umbra, and | erhaps part of the 
penumbra. We ordinarily get no trace of these extensions, but per- 
haps the conclusions drawn from the study of such phenomena may 
have much to do with the final views as to the nature of the spots 
themselves. 
To mention only one other thing, the reversals of spot lines which 
have been seen by some observers have not been photographed with 
@Tt is, of course, desirable to take photographs as often as possible, since a 
photographic record of a marked change in the spectrum, if fortunately ob- 
tained, may be much more valuable than the results of a few visual observations 
made hastily. 
