288 



KNOWLEDGE. 



[December 1, 1897. 



propose to use the H and K lines, on which lines M. 

 Deslandres photographed in 1893, but a bright line near 

 G, of whose coronal nature there can be no doubt. In the 

 same neighbourhood Captain Hills, E.E., will use two slit 

 spectroscopes on achromatic Cooke lenses of four and a 

 half inches aperture and with prisms of different materials 

 and angles. 



At or near Sohagpur, on the railway line from Jub- 

 bulpur to Bilaspur, will probably be located the third 

 official expedition sent out by the Joint Permanent 

 Eclipse Committee of the Eoyal Society and Royal 

 Astronomical Society, and consisting of the Astronomer 

 Eoyal and Prof. H. H. Turner. The Astronomer Royal 

 will take the Thompson photoheliograph of nine-inch 

 aperture and eight and a half feet focal length, with 

 a secondary magnifier placed a short distance within the 

 focus giving an image of the sun four inches in diameter. 

 This will be fed by a ccelostat with a sisteen-inch plane 

 mirror. Prof. Turner's instrument will also be fed by 

 a similar ccelostat, and consists of a double camera, one 

 of whose tubes contains the "Abney" lens of four-inch 

 aperture and five and one-sixth feet focus, giving an image 

 of the sun of 0-57 inch in diameter ; the other is the 

 photoheliograph objective No. 2, used in the Transit 

 of Venus Expedition of 1874, and is of four-inch aperture 

 and five feet focal length, giving an image one and a half 

 inches in diameter after enlargement. 



From Nagpur there is a good road stretching in the 

 north-easterly direction to meet the centre of the shadow 

 track at a distance of about fifty miles. At this point the 

 Satpura Hills terminate, and this is the station which Dr. 

 Copeland, the Astronomer Eoyal for Scotland — also sent 

 out by the Joint EcHpse Committee — proposes to occupy. 

 It may be remembered that on the rising moorland above 

 the town of Vadso, in the unsuccessful eclipse expedition 

 of 1896, the " hundred-ton gun " in the Scottish camp of 

 " Muckle Dreep " was a most conspicuous object. As it 

 was manifestly impossible to mount the monstrous forty- 

 foot tube in equatorial fashion in the short time available 

 for the preparation of an eclipse expedition, it was fixed 

 rigidly at its proper altitude and azimuth for the sun in 

 mid-totality, and the photographic plate was made to 

 follow the sun instead of the telescope. Let us hope that 

 Dr. Copeland's great telescope will meet with better success 

 on the sunny plains of India than in the mists of Fin- 

 marken, and that his notable success will be rivalled by that 

 of all the other expeditions stationed along the central line. 



Dr. Copeland will also use an object-glass prism during 

 part of the eclipse in order to procure the spectrum of the 

 prominences. 



The American expeditions have probably suffered from 

 the Yerkes dedication. Prof. Campbell, of the Lick 

 Observatory, left California for India in the first week of 

 October. 



[November 20th.— The expedition of the British Astro- 

 nomical Association will probably break up into two, 

 possibly three, parties ; one at Buxar on the Ganges, and 

 one near Pulgaon.] 



ARTIFICIAL SUNSPOTS. 



By the Rev. A. East. 



SO much is being written on the subject of the elevation 

 nrsiis the depression theory of the constitution of 

 sunspots, that it may not be amiss to draw atten- 

 tion to one point which, apparently, is occasionally 

 overlooked. It is assumed sometimes that a spot, if 

 sufficiently large in point of area, and relatively deep, 



ought to show on the extreme edge of the sun as a notch 

 out of the edge, if it is a cavity at all ; and the fact that 

 spots are but seldom detected showing any such appearance 

 is put down on the one hand as some proof that the spots 

 cannot ba cavities, and on the other that they must be 

 relatively shallow. 



To show that this is not necessarily the form which 

 every large and deep cavity would take when seen on the 

 edge of the sun, the two first photographs in the accom- 

 panying plate are given. As may be seen, they are taken 

 from a wooden ball in which three holes have been made, 

 and the ball is shown in two positions, representing the 

 manner in which the spots on the sun are carried by 

 rotation from one position to another. The great majority 

 of persons, if asked, would probably say that a round hole 

 in the sun would appear as a hitf out of it when it got to 

 the edge ; but it will be seen that the appearance of a 

 round or symmetrical spot seen on the edge is nothing of 

 the kind, but is a fttiaii/lit lim. It will be seen that, as the 

 cavity reaches the edge, the limb becomes more and more 

 flattened until it becomes at that place entirely flat, but 

 never shows any indentation. These examples have, it is 

 true, vertical sides, but a moment's consideration will show 

 that a saucer-shaped cavity would behave in exactly the 

 same manner so long as it remained circular. 



And that this must be the cise with every circular spot 

 a simple illustration may make evident. The cavity of an 

 empty barrel seen from above is circular ; as the eye 

 observes the top of the barrel more and more in profile, 

 the cavity appears more and more elliptical, until, when 

 the eye sees along the top of the barrel, the top is flat. 

 Now take your barrel and plunge it up to the top rim in 

 an imaginary sun, and turn the sun round until the barrel 

 is on the extreme edge, and you get the appearance which a 

 circular spot must present on the limb; and this, be it 

 observed, is quite independent of the depth or shallowness 

 of the cavity, but depends only on its regularity — that is 

 to say, upon the edge being at every point equidistant 

 from the centre of the sphere. 



It follows from this that there is only one form of cavity 

 (any form, that is, at all likely to occur) which can 

 show us a notch in the limb, and that is a spot of valley- 

 like form, whose length very greatly exceeds its width, 

 and then only under the conditions that it is seen length- 

 ways or in the direction of its major axis. 



This consideration reduces the number of spots which 

 could possibly be seen as a notch in the limb to an 

 exceedingly small number ; and when there is borne in 

 mind the difficulty of seeing on the limb where there is 

 any tendency to " boil," and the ease with which the 

 exact moment may be missed, it will not appear wonderful 

 that any appreciable notching of the limb so seldom 

 comes to support the depression theory. 



I may, perhaps, be permitted to add to the foregoing 

 remarks an account of some experiments which I have 

 lately made in order to elucidate some of the phenomena 

 of sunspot formation. The flocculent appearance of the 

 solar envelope is of course well known and often referred 

 to, but the attempt to experiment with flocculent matter 

 does not seem to have been made — owing, possibly, to its 

 extreme simplicity. 



First, then, as to the apparatus. A tin pan some twelve 

 inches wide by three inches deep, a spirit lamp giving a 

 small flame, and some flocculent material such as the curd 

 of skim milk, or, by preference, some pulp of white paper 

 softened by boiling, and rubbed fine in the hands. 



The pulp is now mixed with clear water, and will repre- 

 sent for us the solar granules floating in the sun's atmo- 

 sphere. The amount of flocculent matter must be consider- 



