August 25, 1892] 



NATURE 



191 



OUR BOOK SHELF. 



Ostwald's K/asstker der Exacten Wissenschaften. (Leip- 

 zig: Verlag von Wilhelm Engelmann.) 

 It is extremely important that every student of science 

 should as far as possible make himself familiar with the 

 history of discovery in the various subjects in which he 

 is interested. He can hope to understand thoroughly the 

 present position of any department of science only if he 

 understands the stages of development through which it 

 has passed. And by far the most effective way in which 

 this knowledge can be attained is by the study of the 

 memoirs in which the great masters of research have 

 recorded their discoveries and described the methods by 

 which their results have been reached. These documents 

 bring the student into contact with the finest intellects 

 which have been devoted to original inquiry ; and he will 

 be surprised to find how much freshness is often given to 

 an old doctrine when it is apprehended precisely in the 

 way in which it presented itself to the investigator by 

 whom it was first brought to light. Judged from the 

 point of view of later thinkers, the achievements of even 

 the most illustrious workers belonging to past times may 

 be in some ways found wanting ; but the mistakes of 

 great men, when properly understood, may sometimes be 

 almost as instructive as those of their conclusions which 

 have stood the test of the closest and most prolonged 

 examination. 



Important as it is that the classics of science should be 

 widely and carefully studied, they have hitherto, un- 

 fortunately, been accessible only to a comparatively small 

 class. It was therefore an excellent idea to issue a series 

 of them in a convenient form and at a moderate price, 

 so that they might be brought within easy reach of all 

 to whom the study of science is either a duty or a source 

 of interest and pleasure. Upon the whole, those who 

 planned the present series may be congratulated upon the 

 manner in which their scheme is being executed. Dr. 

 W. Ostwald is acting as general editor, while particular 

 departments have been entrusted to specialists — astro- 

 nomy to Dr. Bruns, mathematics to Dr. Wangerin, 

 crystallography to Dr. Groth, physiology to Dr. G. Bunge, 

 the physiology of plants to Dr. W. Pfeffer, physics to 

 Dr. A. von Oettingen. The only serious fault we have to 

 find is that memoirs in foreign languages have not been 

 printed in their original form, but have been translated 

 into German. This cannot but diminish the usefulness 

 of the series from an international or cosmopolitan point 

 of view ; and we may doubt whether it is really the best 

 plan even for German students. So far, at least, as 

 English and P>ench memoirs are concerned, there are 

 probably few serious students in Germany who would not 

 have preferred to have before them the actual words used 

 by the authors themselves. 



The memoirs are not being issued in chronological 

 order. The series opens with Helmholtz's paper on the 

 conservation of energy (1847). This is followed by 

 papers by Gauss, Dalton, Wollaston, Gay-Lussac, Galileo, 

 Kant, T. de Saussure, Laplace, Huyghens, Woehler, 

 Liebig, Bunsen, Pasteur, and many other famous men of 

 scientific light and leading. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part oj Nature. 

 No notice is taken of anonymous communications,] 



Aurora Borealis. 

 The auroral display of Friday, the 12th inst., referred to in 

 last week's issue of Nature, would seem to have been visible 

 over a wide area. Between 9.30 and 10 p.m. I observed it at 



NO. I 191, VOL. 46] 



Boppard, on the Rhine, a few miles above Coblence. The 

 streamers were clearly defined, but presented no unusual features, 

 being merely rays of whitish light which slowly dissolved as the 

 moon rose above ihe crest of the range of hills running along the 

 right bank of the river. On the previous evening I was at 

 Strassburg where, owing, I suppose, to the gas and electric 

 lights, I took the greyish appearance of the northern horizon to 

 be nothing more than the usual light in that quarter at this 

 season. Further south, in Switzerland and Austria, aurorse 

 were seen on both nights. As to " the unusual time of year for 

 such a display," I may mention that on Sunday, August 2, 1891, 

 I witnessed a brilliant aurora from the Deck of the R.ALS. 

 Teutonic, in latitude 48^° N., longitude 30° W. It varied con- 

 siderably in intensity, and continued to do so for half an hour up 

 to 10 p.m. Hv. Harries. 



Bayswater, August 20. 



An Unusual Sunset. 



This evening (July 29th) we were treated to a sunset of rare 

 type, one which is unique at least in the experience of the 

 writer. 



The fog was apparently forming round about the outer range 

 of the mountains which lies between Mount Hamilton and the 

 coast of the Pacific. Ordinarily, about this time of day, one 

 can see the fog drifting over the tops of these mountains, and 

 pouring into the valley on this side. 



To-day, however, the crest of this range was barely visible 

 above a sea of fog, which was unusually level and flat, as seen 

 from above. Just over and along the crest was stretched a 

 slender, thin, cloud which obscured the lower half of the sun's 

 disc. Suddenly there formed underneath this semi-disc another 

 of the same shape and size, and similarly placed, but not quite 

 so bright as the true solar disc. 



The accompanying figure shows essentially what was seen. 

 The lower image I take to be that of the lower limb of the sun, 

 shining down (from behind the upper strip of cloud) upon 

 this quiet lake of fog and there reflected. But this amount of 

 reflecting power in a fog if that be the true explanation, is very 

 surprising, the image formed being not only bright and sharp, 

 but very free from the usual glare of what are known as 

 " brilliant " sunsets. 



Another phenomenon, certainly not frequent in this country, 

 showed itself on the limb of the sun at the point indicated by the 

 dotted line A. 



Here twice, just before the disc disappeared, the deep red 

 colour of the solar surface turned to a bright blue, the change in 



colour being just about what one would experience in examining 

 a prominence first through the C line and then through the F. 

 Then again at the last moment, when all had disappeared but a 

 narrow strip at the eastern limb, this flashed out into the same 

 light blue, an effect apparently due to the greater refrangibility 

 of the blue rays, combined with a very steady atmosphere. 



Mr. Barnard says that for half an hour after sunset he 

 observed "a small bright spot of light" at the point where the 

 sun had disappeared, Henry Crewt. 



Lick Observatory, July 29. 



The Red Spot on Jupiter. 

 On August 19, at I4h. 40m., I began observing Jupiter with 

 my ID-inch reflector, power 252. The red spot was seen slightly 

 east of the central meridian, and it looked decidedly fainter and 



