52 Prof. Phillips Physical Aspect of the Sun. [Jan. 26, 



and 1864, I have several times noticed this persistence of the elliptically 

 contracted spot with its nucleus, equidistant from the borders, very near to 

 the edge, both coming on and going off, certainly within 10 of the edge, 

 from which it may be inferred that in that case the angle of inclination of 

 the edge of the spot to the general surface could not be greater than 10. 

 Taking the case of 10, and applying it to the spot now under considera- 

 tion, the difference of level in miles between *, the general solar surface, and 

 b, the ring terrace, might =a sin 10=300 miles, but could not exceed it. 

 This result is represented in the preceding diagram. 



Nor can the spot be sunk in a deep saucer-shaped concavity like 



for the same reasons. 



But, however deeply the spot may be sunk below the surface of the 

 sun, no notch could appear in consequence of that at the limit of the sun ; 

 for before reaching the limb the angle a E a', under which it is seen from 

 the earth (E), would become invisibly small, and the space a a' become 

 invisible. Even if the sides a and a' were very unequal in level, the leading 

 edge a' being depressed, this would make no visible notch on the limb 

 of the sun, except the spot were enormously large, as well as very deep- 

 much greater for instance than 40,000 miles. Except in very rare cases, 

 then, the sun's edge must always appear truly circular, notwithstanding the 

 depressions of the spots and the elevations of the faculse. 



Finally, I remark that the spots may appear black, dark, grey, &c., not 

 because they really are so dark as they seem, but that, being less luminous 

 than other parts of the disk, they acquire this relative darkness under the 

 operation of the optical apparatus, and the influence of contrast on the 

 sensation. An extremely good way of viewing the spots is to project the 

 sun's image on to a smooth porcelain screen, about a foot or 18 inches in 

 diameter ; very smooth white paper answers very well. Thus tried, every 

 imaginable degree of relative darkness appears in the spots, and the faculse 

 come out bright and distinct. In this experiment, the spots seem so dark 

 in the nuclei as to suggest the hypothesis that the parts of the sun to 

 which they correspond really emit specially heat-rays, below the range of 

 refrangibility which brings to our eyes light and the power of sight. Heat- 

 rays and light-rays come to the earth together, but that is no reason for 

 thinking they must spring in mixed pencils from every part of the sun 

 equally. In my way of considering it, this rather confirms the idea of the 

 deep black nuclei being the sun's body, the penumbrse that body partially 

 seen through the atmosphere, and the facular region transmitting to us rays 

 which have acquired a higher refrangibility than that with which they 

 started, by a peculiar change in the sun's atmosphere, which may justly 

 be called his photosphere. 



