August 1, 1898.] 



KNOWLEDGE 



181 



and finance, outfitting and expenses, and so on, all well authentieated 

 in every pivrticulai- — explicit details, definite conditions, actual results. 



Life Histories of American Insects. By Clarence Moores Weed. 

 (Macmillan.) Illustrated. 6s. net. Consists of a series of able 

 sketches of a few of the principal American insects, profusely illus- 

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 booli will, we think, form a useful aciiuisition, and general readers who 

 occasionally indulge in light scieni-'e may liope for both information 

 and amusement by a perusal of these bright and stimulating pages. 



Xatiire Studies in Ulemenfari/ Schools. By Mrs. Lucy Wilson, 

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 and for each month full particulars are set down as to the subjects 

 of instruction; the curriculum including weather, plants, animals, 

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BOOKS RECEIVED. 



Practical Ori/anic Chemistry. Bv Samuel Rideal, D.sc. Second 

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The Birds of Montreal. Bv Ernest D. Wintle. (John Wheldon 

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A Text-Booh of Zoology. By H. G. Wells and A. M. Davies. 

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Financial SietcJtes. By Helene Gingold and Dudley Hardy. 

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Stepping/ Stones to Literature. By Surah Louise Arnold and Chas. 

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IHreclorii (revised to June, 189SJ, irilh Segulations for establishing 

 and conducting Science and Art Schools and Classes. (Spottis- 

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On Sea Beaches and Sandbanks. By Vaughan Cornish, si.sc. 

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Archives of the SOntgen Sag. Edited by W. S. Hcdley, M.D., and 

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Erttfrs. 



[The Editors do not hold themselves responsible for the opinions or 

 statements of correspondents.] 



SUNSPOTS. 

 To the Editors of Knowledge. 



Sirs, — In a letter of Mr. Shackleton'a in the May 

 issue of Kno\vlei>i;e (p. 113), he quotes the late E. A. 

 Proctor as confirming a theory of refraction lately 

 advanced by Mr. East, and illustrateil by him by the 

 familiar old experiment of a bowl of water with a penny 

 at the bottom of it ; and in the concluding paragraph of 

 his letter he seems to consider that this theory is therefore 

 a settled matter, which ought to have found its way into 

 recent books treating on the subject as an accepted view 

 of one of the phenomena of sunspots. 



I cannot help thinking that Mr. Shackleton attributes 

 rather too much importance to the few words said by 

 Proctor in the passage alluded to in " Old and New 

 Astronomy." He is not discussing the visibihty of the 

 umbra in a foreshortened spot ; he does not mention it or 

 allude to it in any way, here or anywhere else, except by 

 a few words in a short note, after he had made his draw- 

 ing for a totally different purpose. He is describing the 

 manner in which he imagines a sunspot to be formed, and 

 he gives a sketch of what he conceives would be the section 

 of a spot according to this view. It must have struck him 

 that, if this section was a true one, it would be impossible 

 to see the umbra at all when a spot was foreshortened ; 

 and he then suggested the idea of refraction as a means of 

 bringing the umbra into view. Surely this theory requires 

 a little more discussion and explanation than a few words 

 added casually in a note. It is hardly one to be accepted 

 as if it was the most obvious thmg in the world, which 

 only had to be stated to be received without a shadow of 

 doubt as to its truth. 



I do not therefore think that Proctor intended to lay 

 down this theory for general acceptance upon bis authority, 

 as one thoroughly examined and deliberately adopted by 

 him. However that may be, it is clear that this theory is 

 only needed if the section of a spot drawn by Proctor is a 

 true one. Js the umbra something at the bottom of a 

 cavity, which requires, in a foreshortened spot, to be 

 brought into view by refraction like the penny in the bowl 

 of water ? I maintain that it is not, but that, on the con- 

 trary, the umbra ia more or less on a level with the outer 



v_ ,^/ 



FlQ. 1. — Ideal Section of a Sunspot. 



_J 



edge of the penumbra. I did not adopt this view hastily, 

 or merely as a way out of a difficulty, but only after having 

 bestowed a good deal of attention to the matter. It was 

 only after having spent some time on every available day 

 for some months, carefully observing and drawing every 

 spot that occurred, that the conclusion forced itself slowly 

 and irresistibly upon me, in opposition to what I had 

 before imagined to be the generally accepted view — that 

 the umbra, instead of being a hole at the bottom of a 



Fig. 2. — Changes in the Appearance of a Sinnmetrical 

 Spot due to foreshortening. 



depression, was in reality on a general level with the top of 

 it. My idea is that at the edge of the penumbra there is at 

 first a considerable depression, which continues for a certain 

 distance towards the centre, and that the surface then rises 

 gradually into a cone, the open top of which is the umbra. 



Fig. 3. — A Group of Sunspots on the First Day after 

 passing the East Limb. May 6th. 



I take it to be impossible to draw any true section of a 

 sunspot, and the outline I have drawn is only intended to 

 show my meaning in the roughest and most general 

 manner. If, then, the umbra is, as I imagine, the open 

 stummit of a cone, its circumference would form a ring 

 with a dark centre (I am taking the case, of course, of an 

 ordinary symmetrical spot), while the edge of the penumbra 



