ASTRONOMICAL APPARATUS—HALE. 279 
This investigation is a many-sided one, with applications to both 
solar and stellar phenomena. There is room here for many investi- 
gators, who can obtain results quite equal, and very likely superior, 
in value to any we can get at Mount Wilson. A large image of the 
sun is not required, because the effect is very appreciable at a distance 
of 2 or 3 millimeters from the limb on our 6.7-inch image. It is also 
a matter of no importance whether the definition of the solar image 
be good or only fair. The one essential point is that the spectrograph 
be fairly powerful, and this is a very simple thing to realize at mod- 
erate expense. I hope to see this subject taken up by several observers, 
who will determine the shifts and the relative intensities of the Fraun- 
hofer lines, seek for evidence of periodic changes, and work out an 
explanation of these remarkable phenomena which will harmonize 
with some explanation of the relative intensities of the same lines in 
sun spots and in the spectra of stars. 
Fic. 1. 
I may now touch upon another field of solar research, and consider 
the possibility of doing useful new work with the spectroheliograph, 
which is by no means so expensive and formidable an instrument as 
one might suppose. The slide shows the first spectroheliograph used 
on Mount Wilson, before we built the permanent one now employed ; 
and since the fact that we did substitute a permanent instrument 
for the temporary one might lead to the inference that the former 
did not give good results, I may add that the photographs made 
with the wooden instrument are even better than the later ones. 
They show only narrow zones of the solar surface, but for sharpness 
they have never been surpassed.* In the illustration the spectrohelio- 
graph is partly hidden under this spectrograph, and you can only 
get a rough notion of it. There is a rectangular wooden platform 
here mounted on a pier. 
2Tn the 5-foot spectroheliograph now employed, the dispersion is great enough 
for photography with the hydrogen as well as the calcium lines. For this reason 
the exposures are longer, and the definition somewhat less perfect, though quite 
satisfactory for practical purposes. 
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