ASTRONOMICAL APPARATUS—HALE. 981 
of calcium, and this is the same region as photographed with the H, 
line of calcium. If somebody would go to work with such an instru- 
ment and let us know exactly what such photographs as these mean, 
they would at least confer a very great favor upon me, because hither- 
to I have been unable to determine with certainty the relative parts 
played by the continuous spectrum of the facule and the light of the 
H, line of calcium in producing the photographs. That question is 
still open, and many investigations will be required to settle it beyond 
doubt.? 
In this H, photograph we probably have a picture of the calcium 
vapor at a higher level than the level represented by the H, plates. 
You see, for example, this bridge of calcium vapor across the spot, 
which is not shown by H,. Many investigations of great interest 
could be carried on with such a spectroheliograph as I have de- 
scribed. I wish I had time to go into,them; there is only one I may 
mention, and that is the comparison of the calcium and the hydrogen 
images. Mr. Butler has asked me to explain to-night a point which 
I unfortunately failed to make clear in my talk here at the last 
meeting of the society. In speaking of the relative level of the ecal- 
cium and hydrogen flocculi, I said we found that the dark hydrogen 
floccul are shifted somewhat toward the limb of the sun as com- 
pared with the corresponding bright calcium flocculi. The natural 
conclusion to which I came was that the hydrogen absorption shown 
in this photograph is produced at a somewhat higher level, amounting 
to something like 1,500 miles, than the calcium radiation which gives 
us this photograph. Mr. Butler pointed out to me that the photo- 
graphs of the flash spectrum show the calcium vapor to rise to a 
higher level than the hydrogen gas, and that the difference is about 
1,500 miles. There is no question about the validity of this result, and 
the point is to show that it is compatible with my conclusion. I think 
the reason is simple enough, and lies in this fact: The floceuli photo- 
graphed with the H, line do not represent the highest calcium vapor, 
but a level considerably below that; whereas the absorption phe- 
nomena known as hydrogen flocculi apparently represent the upper 
hydrogen in the chromosphere, or in some cases the prominences 
themselves. The average level of the hydrogen absorption seems to be 
about 1,500 miles higher than the region from which the H, light of 
calcium proceeds. If, as occasionally happens, the highest calcium 
@Two photographs taken with ihe wooden spectroheliograph are reproduced 
in plate vi. They are not the ones referred to in the lecture. Since the above 
was written, photographs of vortices in the solar atmosphere have been made on 
Mount Wilson with the spectroheliograph, using the H@ line of hydrogen. A 
simple wooden spectroheliograph would suffice to give good photographs of these 
vortices, which are thus open to investigation by amateurs with limited equip- 
ment. 
