408 MR. JAMES NASMYTH ON THE STRUCTURE 



average type of those of the larger class in their general 

 aspect. 



The chief object \Thich I have had in making this 

 drawing is^ to exhibit (so far as such a drawing can enable 

 me to do so) those remarkable and peculiar " willow-leaf^^ 

 shaped filaments of which I find the entire luminous 

 surface of the sun to be formed. 



The filaments in question are seen and appear well 

 defined at the edges of the luminous surface where it 

 overhangs "the penumbra/' as also in the details of the 

 penumbra itself, and most especially are they seen clearly 

 defined in the details of " the bridges " as I term those 

 bright streaks which are so frequently seen stretching 

 across from side to side over the dark part of the spot. 



I accompany the drawing with a diagram (No. 2) which 

 exhibits in a more definite and clear manner the exact 

 form of those remarkable structural details of the solar 

 surface. 



So far as t have as yet had an opportunity of estimating 

 their actual magnitude, their average length appears to be 

 about 1000 mileSj the width about 100. 



Diagram No. 2 conveys a pretty clear idea of the man- 

 ner in which these remarkable details are arranged, in 

 forming, as they do, the entire luminous surface of the 

 sun. 



There appears no definite or symmetrical arrangement 

 in the manner in which they are scattered over the surface 

 of the sun ; they appear to lie across each other in all 

 possible variety of directions. The thickness of the layer 

 does not appear to be very deep, as I can see down through 

 the interstices which are left here and there between them, 

 and through which the dark or penumbral stratum is ren- 

 dered visible. It is the occurrence of the infinite number 

 of these interstices, and the consequent visibility of a cor- 

 responding portion of the dark or penumbral stratum, that 



