146 



THE POPULAR SCIENCE MONTHLY. 



days when the sun was observed, there were only 372 days when spots 

 were not seen. 



The spots vary in size and number: sometimes but a single one ap- 

 pears, and as many as 80 have been visible at one time. Nor are they 

 equally distributed over the solar surface. The region of the spots 

 is generally confined to the part of the sun within about 30 of its 

 equator, beyond which they are rarely seen. 



The photosphere is the name given to the silver sea of light by 

 which the spots are surrounded, and in which they seem to swim ; it is 

 the immediate source of illumination, and, viewed through a glass of 

 low power, it appears flat and smooth, and of a uniform whiteness. 



But, when a telescope of high magnifying power is directed to the 

 sun, its aspect is greatly changed : the spots lose their simplicity, and 

 the photosphere its uniformity, and in both there are a revelation of 

 structure, a diversity of parts, and a variety of changes, which at once 

 provoke questions in the mind of the observer, as to the causes of this 

 diversified appearance, and the constitution of the body which presents 



Fig. 2. 



Solar Spots, showing Furrows in the Penumbra. 



them. The hypotheses put forth are ingenious ; but, while the facts of 

 observation are rapidly increasing, and there is a growing agreement 

 on many points, there is still profound uncertainty as to the interpre- 

 tation to be given to the leading phenomena. 



Spots upon the sun's surface have been frequently seen with the 

 naked eye. Galileo saw one at sunrise. It was the observation of a 

 solar spot with unassisted vision that so impressed Sir William Her- 

 schel as to determine his attention to the physical nature of the sun. 

 The eminent observer Schwabe has also seen many without the aid 

 of the telescope. Solar spots, therefore, are discernible without glasses 

 at a distance of 91,000,000 of miles; but Lockyer says that the finest 

 telescope enables us to see the sun as we should do with the naked 

 eye at a distance of 186,000 miles, that is, 52,000 miles nearer than the 

 moon. The sun is about 62,000,000 times larger than the moon, and 

 Brande states that the smallest space that can be discovered in the 

 eun's disk must subtend an angle of one second, which is equal to 



