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



surrounding tracts of the sun's surface. It will be observed that the 

 nucleus is black, not quite uniformly so, however, but with some inequality 

 of tint ; and that it is branched or notched at the edges, in this respect 

 exhibiting some differences at different dates, which I do not think are due 

 to any error or uncertainty in the sketches. The ring space round the 

 spot, though not quite uniform, presented no inequalities that I could mark ; 

 the border was grey, striated, and unevenly extended into the surrounding 

 tracts. These unevennesses appeared to me to be little varied, during 

 several days. The striations and extensions in a radiating direction corre- 

 spond to what Mr. Dawes calls by the picturesque name of " thatch" a 

 name singularly appropriate, if this border overhangs, as is often supposed, a 

 cavity or depression. This " thatch," according to Mr. Nasmyth, is formed 

 by the concurrent outlines of his "willow leaves"; but Professor Airy, in 

 commenting on Mr. Stone's recent communication to the Royal Astrono- 

 mical Society, is reported to have called attention to the fact that the rice- 

 like aggregations were " quite distinct from the thatching so graphically 

 described by Mr. Dawes in the penumbrse "*. 



Until the opportunity shall have occurred to me of observing and drawing 

 the edges of penumbrse and nuclei under a sufficient variety of circum- 

 stances, I must not offer to reconcile these apparently different opinions ; 

 at present my impression is that the " thatched " edges of the penumbra 

 are only broken parts of the general groundwork of the body of the sun ; 

 which may be, or rather must be supposed to be very unequally luminous 

 in different parts, the depressed and granulated parts emitting the least 

 light. 



The spot No. 4 on the Diagram for March 29 was first seen near the 

 edge of the sun, surrounded by faculse of great brilliancy t. This spot 

 had the elliptical outline due to its position; the black nucleus was 

 central ; the border was shaded and striated ; between the border and the 

 nucleus was a clear bright space, not then appearing grey or dusky. As 

 the spot moved on toward the central part of the disk, the oval became less 

 and less eccentric ; the nucleus remained central, and the border retained 

 its dusky and striated aspect ; but the ring round the nucleus lost its 

 brightness, and took up more of the grey tint which belongs to the general 

 surface of the sun when a very dark glass is employed . 



If these circumstances be well considered, they appear sufficient to prove 

 that the spot taken as a whole is not sunk very much below, or raised very 

 much above, the general level of the region. For were its interior part 

 sunk very much below the border, it would have presented to the eye in 

 passing from near the edge toward the centre the appearances sketched in 



* ' Reader,' 2nd April, 1864. 



f The drawings are made with the solar eyepiece, placed as in a Newtonian reflector, 

 on the western side of the telescope. They do not, therefore, correspond with drawings 

 or photographs taken by the ordinary arrangement without diagonal reflector. 



{ The full opening of the telescope (6 inches) being employed for the sake of exact 

 definition, a very dark glass is required if the sun be clear. 



