140 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
graph will be seen the corona, so called, a pearly object stretching 
out in beautiful forms to a considerable distance outside the sun. A 
ereat change, however, apparently occurred in its form between the 
year 1900 and the year 1905. This change is shown by other eclipse 
observations to be characteristic, and to always accompany the 
change from sun-spot minimum conditions to sun-spot maximum 
conditions. At sun-spot minima the solar corona extends in long 
equatorial streamers, while at sun-spot maximum the corona, though 
somewhat brighter, is not so extensive in any particular direction, 
but stretches almost equally in all directions. 
Close up to the border of the sun there are also seen, at times of 
solar eclipses, bright red flames, called prominences, which are due 
to the gases hydrogen and calcium, with sometimes an admixture 
of other chemical elements. These beautiful objects sometimes reach 
above the surface of the sun as much as 500,000 miles, and ih some 
instances they have been observed to shoot up to such immense 
heights as this within 10 minutes of time. I say within 10 minutes 
of time, which implies that they may be seen at other times than 
during total eclipses. A method of observing them by aid of the 
spectroscope was devised indeperfdently by Lockyer and Janssen 
immediately after the eclipse of 1868, and nowadays many observa- 
tories examine them every day. <A beautiful prominence is shown 
in plate 3 as photographed by Slocum at the Yerkes Observatory. 
It would have seemed hardly credible to the contemporaries of 
Sir William, or even of Sir John Herschel, that the materials of which 
the sun and stars are composed could ever be known, but by aid of 
the spectroscope much is learned in this respect. White light may 
be thought of as a compiex mixture of vibrations of the ether, so 
called; that medium which is supposed to fill all space, including the 
interstices between the atoms and molecules of material bodies. 
When light passes through a prism of transparent substance, the com- 
plex vibrations are decomposed into their component parts, and we 
see the spectrum, in which the colors are arranged in the order, 
violet, indigo, blue, green, yellow, orange, red. The spectrum is by 
no means limited by the end of the visible red, or by the end of the 
visible violet, for rays which may be photographed, and which pro- 
duce heat when allowed to shine upon blackened substances, exist 
both beyond the red and beyond the violet. Those beyond the red 
are called infra-red, and those beyond the violet, ultra-violet. The 
ultra-violet rays may be readily photographed, and by specially 
staining photographic plates, with organic dyestuffs, it is possible 
also to photograph a limited region beyond the visible red. Further 
progress in that direction, however, must be made by delicate elec- 
trical thermometers or other heat-measuring instruments. 
