222 Original Articles. | April, 
which utilizes for the formation of the image only about one 900th part 
of the incident rays, and if a greater diminution be desired, it may be 
obtained by a polarizing eye-piece. I have reason to believe that this 
construction will ere long receive a full and satisfactory trial at the 
hands of one of our most distinguished solar observers and practical 
mechanists. In default of a glass-reflecting speculum suchas this con- 
struction requires, and of the prism recommended for a second reflexion, 
I have used (vide locum citatum) a plane glass, roughened at the back, 
interposed obliquely, so as to intercept the converging rays before form- 
ing the first image, and reflect them through the eye-piece of a New- 
tonian telescope with great advantage. Mr. Hodgson* has recommended, 
and used successfully, a similar contrivance, with a refracting one. 
Spots on the sun have frequently been seen with the naked eye, by 
taking advantage of its proximity to the horizon, or of the intervention 
of light clouds. Instances of the kind are recorded by the annalists 
before the invention of telescopes—in A.D. 807 and 1160; and since 
by Galileo himself, by D’Arquier (April 15, 1764 ; January 30, 1767 ; 
June 6, 1763), by Sir William Herschel (April 17, 1779, September 2, 
1792), &e. Only the bare existence of a spot, however, can be so dis- 
cerned. No details of course can be distinguished. When viewed with 
telescopes, the spots are seen to consist of two very broadly distin- 
guished shades of darkness: that of the interior and smaller portions, 
or umbre, being so dark as to be called in common parlance black 
(considerably less so, however, than the body of Mercury or Venus 
seen in transit, or the moon during a solar eclipse); the exterior and 
larger (which usually, but not always, completely surrounds the umbra) 
of what would be termed in painting a half-shade, and therefore called 
the penumbra. Occasionally, but rarely in large spots, this is alto- 
gether absent. But whenever it exists the line of demarcation between 
the shades is sharp and unequivocal. So, at least, I have invariably 
found it, and whenever a gradation of tint from one to the other has 
been thought to have been perceived by other observers, I am disposed 
to attribute it to the optical mixture of the images of the ragged edges 
of the penumbra with the black ground on which they are projected on 
the retina arising from imperfect definition. The point is of extreme 
importance in the physical theory of the spots. So marked a distinc- 
tion is altogether adverse to the idea of a luminous gas or fluid, inde- 
finitely miscible with, or soluble in, a non-luminous transparent atmo- 
sphere ; while it agrees with that of an aggregation of the luminous 
matter in masses of some considerable size, and some certain degree of 
consistency, suspended or floating at a level determined by their specific 
gravity in a non-luminous fluid ; be it gas, vapour, liquid, or that in- 
termediate state of gradual transition from liquid to vapour, which the 
experiments of Cagniard dela Tour have placed visibly before us ; and 
which, when we consider the high temperature throughout the solar atmo- 
sphere, and the enormous pressure at the surface of its solid globe (if it 
have any such) we cannot but believe to be realized on the grandest scale 
in solar physics. And this is strongly corroborated by a certain streaky 
* Royal Astronomical Society’s Monthly Notices, Dec. 8, 1854. 
