Apri 28, 1904] NATURE 611 
In fact, the object will take a form which is very like 
a tree-like prominence. 
It is therefore of great interest to examine the same 
region of a large spot as photographed at two different 
levels, as in Fig. 3. 
This illustration shows the region of the large spot of 
October 9 of last year, and the secondary slit was 
FLOCCULUS 
| DENSER. 
-DENSEST 
H LINE 
ea OF 
CALCIUM 
Gute. 
1 ~~ ‘ 
ei Ha : 
Pe Toe 
| are ; 
Fic. 4 —Diagram to illustrate the relation between the various portions of 
the H calcium line, the layers of different density, the resulting extent 
of the calcium flocculi and the probable appearance of their vertical 
sections. 
placed at the middle of the calcium H, level (lower 
picture), and at the calcium H, level (upper picture) ; 
the plates were exposed at 3h. 43m. and 3h. 30m. p.m., 
so that they may be taken to represent approximately 
the solar conditions at these two different levels for 
the same moment of time. The actual spot itself is 
best seen at the H, level, where the flocculi are not so 
extensive. At the H, level the spot is nearly com- 
pletely covered up by the flocculi, which are here far 
more pronounced and extensive. At this level the 
calcium vapour overhangs parts of both the umbra and 
penumbra of the spot. 
The general appearance of the disc of the sun taken 
in calcium light is shown in the accompanying figure 
(Fig. 5). Under good atmospheric conditions the 
whole disc is covered with minute structure resembling 
somewhat the granulation of the.  photosphere. 
Scattered along distinct zones in both hemispheres are 
large bunches here and there of flocculi. 
These masses of flocculi partake of the general 
movement of rotation of the disc like the spots. Not | 
only do their areas vary very considerably from time to | 
time, but the positions at which they make their first 
appearance change both as regards latitude and longi- 
tude. The enormous extent of these calcium flocculli 
in relation to the solar disc, and their variation in 
amount from time to time, a fact also known, suggest 
that here we have an indication of solar action that has 
only up to the present time been feebly shown by spots. 
It is the investigation of the amount and distribu- | 
tion of these flocculi from day to day and from year 
to year that makes the spectroheliograph such an 
important instrument at the present moment, for it is 
the only means existing of recording these important 
phenomena. 
Not only has Prof. Hale employed the calcium 
lines in this work, but he has used other lines, 
notably those of hydrogen. For this investigation the 
NO. 1800, VOL. 69] 
plane reflector used in the optical train of the instru- 
| ment is replaced by a grating, as larger dispersion of 
the spectrum Has to be obtained. By this means the 
hydrogen or other lines are rendered somewhat 
broader, and it then becomes possible to isolate them 
completely by the second slit, the width of which is 
adjusted to be less than that of the lines employed, in 
order to cut out the continuous spectrum on both 
sides. Numerous photographs were secured with 
each of the HB, Hy, and Hé lines, and comparisons 
were made with the calcium photographs. 
The striking point which this comparison at once 
showed was that where on the solar disc the calcium 
flocculi did not exist the hydrogen flocculi were most 
apparent (Fig. 6), or, as Prof. Hale says, “the 
hydrogen flocculi are, in general, dark, and that while 
they have a general resemblance in form to the bright 
calcium flocculi, the differences are in many cases very 
striking.” 
Perhaps a short digression may here be made, as it 
does not seem quite clear, at any rate to the writer of 
this article, what Prof. Hale really means by the terms 
“dark ’?? hydrogen or ‘‘ dark’? calcium flocculi. 
The principle of the spectroheliograph is that if a 
calcium line be chosen to work with, then the result- 
ing solar disc is built up of two kinds of markings, 
namely, (a) where the calcium exists (bright), and (b) 
regions where it does not exist (dark). Further, as 
has already been pointed out, it does not matter which 
part of the calcium lines is used, as both the dark and 
bright parts produce bright calcium flocculi on the 
completed positive. Again, if a hydrogen line be used 
we have a disc built up of two kinds of markings, (c) 
where the hydrogen exists (bright), and (d) where no 
hydrogen exists (dark). 
Since the existence of each of these substances on 
the sun’s disc is indicated by bright markings, it is 
not quite clear why Prof. Hale calls the dark ~atches 
Fic. 5.—The sun in calcium light (Hg level), showing the bright ca’cium 
flocculi, August 12, 1903 
dark calcium or dark hydrogen, as in these parts 
calcium and hydrogen respectively are, according to 
the very principle of the spectroheliograph, shown to 
be absent. 
