370 De la Rue and Celestial Photography. [ July, 
careful attention to this class of phenomena. ‘This is not the place 
to discuss the questions which have arisen with regard to the nature 
of solar spots. Still it is important that a general idea should be 
given of the conclusions to which Mr. De la Rue and other astro- 
nomers have arrived with respect to them. 
It was first stated by Dr. Alexander Wilson, of Glasgow, in 
1773, that certain phenomena appeared to indicate that spots are’ 
cavities in a luminous photosphere which surrounds the sun. Mr. 
Dawes has shown that we have often connected with the same 
phenomenon—the formation of a sun spot—not less than five 
degrees of luminosity:—1. The facule, or bright streaks of 
irregular direction; 2. The ordinary photosphere; the /wminous 
envelope of the sun; 3. The penumbra, or shaded portion of a 
spot.; 4. The borders of the umbra; and 5, the very dark central 
nucleus. The term winbra is used by Mr. Dawes to denote a region 
of a spot intermediate in darkness between the nucleus and that 
designated as usual by the term penumbra. 
It should be understood that the photographs of the solar surface 
show with great distinctness those five degrees of illumination. 
Indeed, these are so decidedly delineated on the collodion tablet, 
that by calling in the aid of the electrotype process Mr. De la Rue 
has been enabled to obtain plates, from which any moderate number 
of copies can be printed off. 
The Kew photographs have already confirmed the results of 
some previous observations, and established some new and important 
facts. We have only space to state these as concisely as possible. 
1. These photographs prove the central portion of the solar dise 
to possess a higher degree of luminosity than the borders. 
2. They have shown that the wnbra of a spot is nearer to the 
sun’s centre than the penumbra. ‘That is, it is at a lower level. 
The solar spots are, therefore, cavities in the photosphere. 
3. It appears fair to conclude from the examination of all the 
sun pictures in which the facule# are copied, that “Solar faculee 
consist of solid or liquid bodies of a greater or less magnitude, 
either slowly sinking or suspended in xquilibrio in a gaseous 
medium.” 
Another paragraph must, from its importance, be quoted —*“ The 
idea that facule are portions of the photosphere raised above the 
general surface, appears to be confirmed by stereoscopic pictures of 
spots obtained by Mr. De la Rue, where the facule appear as 
elevated ridges surrounding the spots. Accepting this conclusion, 
we next remark that facule often retain the same appearance for 
several days together, as if their matter were capable of remaining 
suspended for some time.” 
The deductions to be drawn from a careful study of those solar 
autographs are, that the lower parts of a solar spot are of a lower 
