274 Tbaxsactioxs of the American Ixstitute. 



"When we view the sun tliroiii^h a telescope of moderate power, 

 provided witli a deep shade ghass, or witli a solar eje-piece, we see a 

 bright disc, in Avhich no measurement has _yet detected any variation 

 from a perfect circle. This circle is not of equal brightness, the 

 central portion being much more brilliant than the parts near the 

 circumference. Nor is the solar brightness ecpiable in otlier respects, 

 but the whole surface aj)pears mottled by small variations in bril- 

 liancy, which have been compared to the irregularities upon the rind 

 of an orange, and to the uneven surface of a stormy sea. 



Spots on the Sun. 



Usuall^y, too, several spots, or grouj)s of spots of irregular and 

 often fantastic shape, are to be seen variously distributed, but almost 

 always within a belt crossing the sun centrally, and not so wide as 

 one-half his diameter. These spots diifer in size from the smallest 

 visible one to a breadth of 1', which is one-sixteenth of the diameter 

 of the sun, and corresponds to seven times that of the earth ; and a 

 short period of examination suffices to show that their forms and 

 dimensions are undergoing continual and rapid changes. Ordina- 

 rily, and indeed, always, if of any considerable magnitude, they 

 consist of two distinct parts, each sharply defined, viz. : A dark 

 inner portion called the umbra or nucleus, and an extensive grayish 

 border, much brighter than the nucleus, and called the penumbra. 

 Sometimes, though not often, the sun is seen entirely free from spots, 

 while at other times they are extremely numerous, more than fifty 

 having been seen at one time. Not unfrequently they are visible to 

 the naked eye through a smoked glass. 



Finally, yet another appearance presents itself to the observer 

 with a'cfelescope of moderate power, viz. : Patches or streaks of light 

 more brilliant than the rest, and not to be confounded with the gene- 

 ral mottling of the surface. These are called faculns, and are gene- 

 rally, but not always, of an elongated form, and chiefly manifest in 

 regions near the limb or margin of the sun, where the fainter light, 

 and some other causes, render them especially conspicuous. Faculfe 

 may almost alwa^^s be seen in the close vicinity of the spots, and 

 generally to the left of them ; but they are not restricted to these 

 situations, and may be found in regions where spots are never seen. 



Wilson's Theoky. 

 To cxjjlain the phenomena of the spots, Domenic Cassini suggested, 

 in 1671, that the sun's surface must be an ocean of light, surrounding 



