90 



kNOWLEDGfi. 



[Apbu, 1, 18d8. 



spot is the larger grows the vent, and the more the crater 

 form tends to disappear. And these forms may be modified 

 at any stage of development — with this exception, that the 

 form in Fig. 4 always comes last. 



Now, if these pulp spots were self-luminous, and seen 

 from above and not in section as the diagrams are drawn, 

 and if the usual terms used to describe sunspots may be 

 used, it is evident that they would, when filled with light- 

 absorbing vapours, appear as follows : — 



Fig. 1 would show as a black umbra surrounded by a 

 lighter border, this latter being due to the light of the 

 photosphere having to travel through a comparatively 

 shallow stratum of absorbing vapour ; the black vent or 

 nucleus at the bottom of the crater-like spot might or 

 might not appear, according to its position in the bottom 

 and the clearness of the " seeing." 



Pig. 2 would show as a black umbra surrounded by a 

 lighter penumbra, with an overhanging "thatch "at its 

 outer edge. 



M t PI 



Fl8. 3. — Unsymmetrical Spot : Penumbra wanting on one side. 



Fig. 3 would appear as an unsymmetrical spot, i.e., with 

 the penumbra wider on one side of the umbra than the 

 other. 



Fig. 4 would appear as a spot consisting of an umbra 

 alone, not surrounded by any penumbra. 



It is easy to see how the Figs. 1, 2, and 3 come to be as 

 they are : in the lowest part or vent, the hot vapours are 

 confined by the weight of the photospheric matter ; as 

 they approach the surface the weight is less, and they are 

 able to thrust the granules aside into the crater-like 

 form. When the surface is reached they expand more 

 suddenly, sometimes making a salver-shaped orifice as 

 Fig. 1, and sometimes scouring out the sides into the cup- 

 shaped form of Fig. 2. This latter is often beautifully 

 shown in the artificial spots, the stray granules playing 

 within the hollow in a most realistic manner. 



An objection to these diagrams as truly representing 

 actual sunspots will no doubt be made that the umbra is 

 often seen when the spot is close to the limb, and that 

 therefore a spot must be nearly always relatively shalloir, 

 otherwise the umbra would be hidden ; and herein, as in 

 the general discussion of the appearance of spots seen 

 obliquely, I venture to suggest that there occurs occasion- 

 ally a very great fallacy. The text-books say : " Imagine a 



Fig. 4. — Spot without Penumbra, and level with Photosphere. 



saucer with a blackened middle slowly turned edgeways to 

 the observer, and see the black part gradually disappear." 

 This is quite true of an empty saucer, but a full saucer will 

 behave differently, and the black middle in the /'nil saucer 

 will apparently keep on rising long after it should have 



been hidden. It is, of course, as everyone knows, refracted 

 upwards, owing to the difference in density between the 

 water in the saucer and the air through which the observer 

 views it ; and a spot is not an nnpty saucer but a full saucer, 



K,G. .-,, — Siin-iN.t-. 'Fn.ii, Sir K E:,ll- '-i,,,-; ut ' ■ - , y 

 kind permissiou of Messrs. Cassell £ Co.) 



filled with dense vapour, and doubtless the bottom of the spot 

 is refracted upwards more and more as the spot approaches 

 the limb, and making it visible long after it apparently 

 should have disappeared. Not only will the umbra be 

 affected in this way, but the whole of the farther side of 

 the spot, causing the curious optical effect of the edge of 

 the spot appearing to rise up, and tend to face the observer, 

 when, in reality, it of course lies flat on the solar surface. 

 To show that these analogies between the form of 

 artificial " spots " and the real solar spots are true ones — 

 at least, if not wholly, yet in part — I would refer to the 

 very beautiful photograph of a sunspot taken by M. 

 Janssen, and reproduced by kind permission of the pub- 

 lishers of Sir Kobert Ball's " Story of the Sun " (Fig. 5); 



Fig. 6. — Empty Vessel, with black bottom just in view. 



and I would ask the reader to compare one feature in this 

 photograph with Fig. 3 of the plate in Knowledge of 

 December, 1897. 



There appears in this photograph of M. Janssen's the 

 black umbra with a few wandering granules within : the 

 lighter penumbra with sides vertical apparently in one 

 part, steeply inclined inwards elsewhere ; the brilliant 

 bridge extending across the chasm, and the granulated 

 surface of the photosphere beyond : but the brightest part 

 of the whole plate, except the bridge, is the portion next 

 to the penumbra. Looking at it, it is almost impossible 

 to doubt that we are looking down upon a vast mnund or 

 tiimuh(.^ with a yawning opening and steeply shelving sides 



