and the Method of determining their Depth. 499 



Observations of the spots with powerful instruments have esta- 

 blished beyond a doubt, that on the ijlack of the nucleus there 

 are occasionally spread out semilucid veiis of the form of frizzly 

 curls, and usually the heralds of some* irruption that the matter 

 itself of the photosphere is about ta^ rbake into the nucleus. 

 This phseuomenon, winch for brevity, a'n-d that we may agree upon 

 our terras, I have distinguished by the name of curls or clouds, 

 T have observed most distinctly, very- 'frequently, indeed almost 

 always, but especially in a splendia.".wiir.led spot obsei'ved on 

 the 7th of Slay, 1857, and likewise in the magnificent spot 

 visible to the naked eye the day of the eclipse in March 1858, 

 in which these clouds presented the aspect of a kind of semilucid 

 promontoiy, and were of a decided reddish colour. And it was 

 moreover curious to see how at a certain point the filaments were 

 united into an aggregated mass, forming, as it were, a whii-lpool, 

 or boiling in a state of the greatest agitation*. 



These details demand, indispensably for their being visible, two 

 circumstances — a quiet state of the air, and a magnifying power 

 of at least 300. The aperture employed by me was commonly 

 that of the entire 9 French inches, or at least 6| inches, of the 

 great equatorial, according to the state of the air. The draw- 

 ings made under these circumstances are of a nature that proves 

 highly instructive, and they have been published on another 

 occasion f. 



Now these clouds or semitransparent veils frequently confuse 

 the filamentous aspect of the spots ; and occupying, it woidd seem, 

 the lower portion of the solar atmosphere, they conceal to a cer- 

 tain extent the structure of those spots, as a cloud in our atmo- 

 sphere would render indistinct terrestrial objects to a person look- 

 ing down on them from above. Moreover the solar atmosphere 

 in its lower regions cannot but be excessively dense and absorb- 

 ent ; and consequently the depth even of only a third of the 

 semidlameter of the earth must absorb a great portion of the 

 rays ; and therefore the cavities of the penumbras will, from that 

 reason alone, appear more obscure. 



When, then, the difference of the inclination of the radiating 

 surface pointed out by "Wilson is combined with the two causes 

 referred to (above that is to say, the presence of numerous indi- 

 stinct and reddish clouds and the occun-ence of a deep bed of 

 dense atmosphere), it will be more readily understood why the 

 interior of the cavities forming the spots should be fonsiderably 



* These clmtds were observed by the late Mr. Lawson of Bath, some 

 fifteen years ago, with a 7-inch refractor of DoUond. The full aperture of 

 the instrument was used. — Tkanslator. 



t For one of these sketches, sec the Astrouomisclic Nachrichtm, No. 

 1089. — Tkanslatok. 



2K2 



