March i8, 1886] 



NATURE 



469 



THE SUN AND STARS'" 

 III. 

 The Spots 

 TN the large photographs now secured at Meudonand in India, 

 ■'■ and smaller ones now received from India, the Mauritius, 

 and Austraha, showing the spots as they are photographed 

 there, I am glad to say almost every day now, on a scale of 8 

 inches to the sun's diameter, we get wonderful records of what 

 a spot really is, and how it changes. 



In a nonnal spot the exterior shade is called the penumbra. 

 The inner darker one is called the umbra, and very often 

 there is a deeper shade still, which is called the nucleus. 

 In some spots there are many umbrse for one penumbra ; 

 and very likely, if one had examined them carefully with 

 the telescope at the time, one would have found that each 

 had its interior nucleus. The idea is that we have at the edge 

 of the penumbra, where the penumbra joins the photosphere, 

 the greatest height of the spot ; that the penumbra is an incline 

 going down as gradually as you like, but still down, so that the 

 level of the photospheric stuff, whatever it is, at the edge of the 

 umbra, is below what it is at the edge of the penumbra. 



In the penumbra the domes seen on the general surface are 

 drawn into elongated shapes, hence we speak of the " thatch " 

 on the penumbra. Visually the part of the penumbra adjoining 

 the umbra seems brighter than that adjoining the photosphere. 

 In photographs this is not so. 



Now, if the view that the spots are cavities be correct, and 

 the appearances they put on in travelling over the sun are suf- 



ficient to prove it, it will be clear that there ought to be occasions 

 when a spot going over the limb should show as a depression. 

 The idea that sunspots are cavities is a very old one. It was 

 first put forward by Wilson of Glasgow in the last century ; but 

 it was not so easy to demonstrate it to a large audience in the 

 days when one had no photographs. 



Here is a photograph showing the retreat of a sunspot over 

 the edge of the sun in 18S4. We see that it writes its record 

 in an unmistakable notch at the limb of the sun (Fig. 5). 



In other photographs we can conveniently study the connec- 

 tion between the faculse and the spots, especially if the spots be 

 near the limb ; the neighbourhood of spots in this position is 

 very rich in faculae. 



When we come to examine these spots carefully, we find 

 that there are apparently in the main — (I want to speak as 

 guardedly as I can) — two different kinds. Some spots seem to 

 be pretty regular, and to undergo no very violent commotion. 

 I mean that the penumbra and the umbra are not so tremen- 

 dously contorted and mixed up as sometimes happens ; and, 

 again, the ridge of facula round the spot is not so honeycombed 

 ^ A Course of Lectures to Working Men delivered by J. Norman Lockyer. 

 F.R.S., at the Museum of Practical Geology. Revised from shorthand 

 notes. Continued from p. 429. 



by convection-currents and the results of convection-currents. 

 On the other hand, as representing the other class where we 

 get violent action, there seems to be no limit to the enormous 

 energies indicated, and the areas over which these energies hold 

 their sway. I believe that one spot, or at least a spot system, 

 was observed in 1858, of 140,000 miles or eighteen earth-diameters 

 in length. Telescopic examination of each minute part of these 

 enormous disturbances indicates that the most violent changes 



i photograph taken at Dehra Du 

 ispot passing over the Sun's edge. 



are going on — changes which the eye can detect, after a few 

 minutes' interval, in different parts of the spot (see Fig. 6). 



The Hhlory of a Spot 



A spot seems to be the first disturbance of the photosphere 



in the region where it is formed. I mean the facula follows. 



Fig. 6. — Sunspot showing details of the penumbra- the dark portion in the 

 centre is the umbra, the surrounding half-tone is the penumbra, a, a 

 " bridge " or tongue of facula being carried over the umbra; B, clouds 

 forming at the end ; c, part of the penumbra being driven over the spot 

 (the domes are drawn out); D, domes on photosphere ; F, " thatch " on 

 penumbra. 



and does not precede, the spot. On this point I first quote Dr. 

 Peters,' one of our highest authorities : — 



" The spots arise froin insensible points, so that the exact 

 moment of their origin cannot be stated ; but they grow very 



