XXIV. On the Structure of the Luminous Envelope of the Sun. By JAMES NASMYTH, ESQ., C.E. In a Letter to JOSEPH SIDEBOTHAM, Esq. Read March 5th, 1861. THINKING that it might interest some of your scientific friends to be informed about the remarkable details which I have discovered in the general luminous surface of the sun, as also in the structure of the solar spots, I send you for that purpose a rough but faithful drawing, which I hope may serve to convey a pretty clear idea of the details I refer to. In order to obtain a satisfactory view of these remarkable objects, it is not only requisite to employ a telescope of very considerable power and perfection of defining capability, but also to make the observation at a time when the atmosphere is nearly quite tranquil and free from those vibrations which so frequently interpose most provoking interruptions to the efforts of the observer; without such conditions as I allude to, it is hopeless to catch even a glimpse of these remarkable and delicate details of the solar surface. The drawing I send you represents a spot on the sun, which I had a most favourable opportunity of observing on the 20th July 1860; although it is in some respects a rather remarkable spot, yet it may be taken as a fair average type of those of the larger class in their general aspect. The chief object which 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 I have as yet had an opportunity of estimating their actual magnitude, their average length appears to be about 1000 miles, the width about 100. Diagram No. 2 conveys a pretty clear idea of the manner 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 rendered visible. It is the occurrence of the infinite number of these interstices, and the consequent visibility of a corresponding portion of the dark or penumbral stratum, that gives to the general solar surface that peculiar and well known mottled appearance which has for a long time been familiar to the observers of the sun. You will note that I consider the penumbra to be a true secondary stratum of the sun's luminous envelope, and that what is termed the penumbra of a spot is simply a portion of it, revealed to us by the removal so far of the external and most fuminous envelope. A slight approach to symmetrical arrangement of the details may be observed at the edges of the exterior luminous envelope as it appears surrounding the edge of the spot, and the same may be seen at the edges of the penumbra; the tendency to symmetrical arrangement being a slight approach to a radial formation; the filaments tending in their general position, at the parts in question, to the average centre of the spot. As I have before said, nothing like a tendency to symmetrical arrangement is observed in the filaments that form the entire luminous surface of the sun. Diagram No. 2 conveys in this respect a very faithful representation. I may also here note that, although I have most carefully watched for it, I have never seen any indication of a vortical or spiral arrangement of the filaments within or about any of the solar spots; this observation appears to set aside all likelihood of any whirlwind-like action being an agent in the formation of the spots, as has been conjectured was the case. When a solar spot is mending up, as was the case with the one represented in the drawing, these luminous filaments, or willow-leaf shaped objects (as I term them), are scen to pass from the edges and extend across the spots, thus forming what I term "the bridges" or bright streaks across the spots; if these are carefully observed under favourable conditions, the actual form of these remark SER. III. VOL. I. 3 G 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 luminosity. 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 subject 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 |