410 MR. JAMES NASMYTH ON TH« STRUCTURE 



able details of which " the bridges " are composed will be 

 revealed to sight. 



You will also observe that the details of the penumbral 

 portion of the spot are slightly varied in brightness ; that 

 portion of the penumbra immediately under the bright 

 edges of the external luminous envelope is less bright 

 than the part of the penumbra next the dark centre of 

 the spot. This is not a mere effect of contrast, but an 

 actual variation in brightness. 



You will also notice that portions of the details of the 

 penumbra are in patches considerably brighter than the 

 rest. This effect appears to me to be due to such portions 

 of the penumbra^ or the filaments forming it, being more 

 elevated, and consequently brought up into more close 

 contact with the luciferous atmosphere which I am of 

 opinion surrounds the sun, and excites, by some peculiar 

 action, the willow-leaf shaped filaments into full lumin- 

 osity. This of course is only conjecture at present, but I 

 have some pretty strong grounds for entertaining this 

 view of the subject. 



I have also indicated in the drawing a portion of the 

 third luminous envelope, which you will observe like a 

 mist underneath some portion of the penumbra. This 

 mysterious object is very difficult to catch a glimpse of, 

 as its comparative brightness is of so very low an order 

 that it is but faintly distinguishable from the darkest 

 portion of the centre of the spot. 



I do not as yet feel warranted to hazard any conjecture 

 as to the nature and special functions of those remarkable 

 willow-leaf shaped details of the solar surface which I 

 have discovered, and have attempted to describe to you. 

 I hope however to pursue the investigation of this most 

 interesting suoject with all due assiduity this summer, 

 and trust I may be fortunate enough to obtain further 

 insight into their nature. In the meantime, I hope the 



