November 3, 1892 J 



NATURE 



proposes to bring out triennially. The present two volumes 

 form a continuation, and extend as far as § 5 of the second 

 chapter in the second book. The author proceeds on the 

 same lines as formerly, and places before the reader 

 in a concise way all the new methods of development, 

 measuring lenses, apparatus, &c., from the particulars 

 of constitution which characterize developers down to 

 the latest form of kodak or tie camera. Not only is 

 each subject treated with the greatest care, but 

 illustrations are numerously distributed. That which 

 will add great value to the work as a whole is the inser- 

 tion of references, for what, after all, is more annoying 

 than having to wade through a great quantity of literature 

 when the presence of one or two words would have 

 eliminated all trouble .^ W. 



Tlic Reliquary : Quarterly At chaological Journal and 

 Rtviciv. Vol. VI. (New Series). (London : Bemrose 

 and Sons, 1892.) 



This volume consists of the four numbers of The Reli- 

 quary which have appeared during the present year. 

 The contents include many things which do not quite 

 come within the scope of Nature ; but it is satisfactory 

 to be able to note that the writers, speaking generally, 

 have done their work in a thoroughly scientific spirit. 

 Mr. J. Lewis Andre contributes an interesting and well- 

 illustrated paper on leather in the useful and ornamental 

 arts, and a clear account is given by the editor of a part 

 of an early dial, bearing runes, which he was lucky enough 

 to find some months ago in the churchyard of Skelton, 

 Cleveland. An illustration gives a good impression of 

 the general character of the stone, the runes on which, 

 according to Canon Browne, are " Danish." Among the 

 other papers are two articles, by Mr. D. A. Walter, on 

 ancient woodwork, and a discussion, by the Rev. A. 

 Donovan, of some of the problems connected with the 

 career of Columbus. 



LETTERS TO THE EDITOR. ' 



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

 pressed by his correspondents. Neithei' can he undertake 



J ■ to return, or to correspond with the writers of, refected 

 manuscripts intended for this or any other part o/ Nature. 

 No notice is taken of anonymous communications.^ 



Nova Aurigae, 

 , On October 5 the Nova Auriga: was again observed under 

 favourable circumstances, and the observation as to precautions 

 in focussing necessary on account of chromatic aberration of 

 the refractor was amply verified. [Nature, September 22, p. 489, 

 in which note two corrections should be made : eighth line, lor 

 "varying" read "ranging," and fourteenth line, for "(?F)" 

 ■read "(?G)"] The line near C was distinctly seen at times; 

 but the blue and violet lines observed on September li. were 

 cot seen ; the three green lines were very distinct. 



On October 14 the red line was much fainter, but there was 

 an obvious bright line in the yellow, which may be the line 

 which Dr. Copeland estimated as 5801 onA"gust 28 (NATURE, 

 -September 15), or may be lliat " hich has been measured several 

 times at the Lick Observatory [Astrophysics, October, p. 717), 

 and appears to have a wave length ol about 575. It had escaped 

 my notice before, but I was induced to look most carefully in 

 the yellow by considerations arising out of an attempt to recon- 

 cile Mr. Barnard's observations of apparent nebulosity surround- 

 ing the Nova, as seen in the 36inch refractor at Mount 

 Hamilton, with -my own observations of September 14. Mr. 

 Barnard's "stellar nucje.u-^ " vvas the difficuUy. 'Ihere appears 

 to be no doubt that the Nova is emitting a spectrum similar to 

 that of a planetary nebula, biit it seems to me necessary to have 

 further spectroscopic evidence before it is established that 

 nebulous extension can be seen ; if it is to be seen with a simple 

 eyepiece, it must be looked for in a reflecting telescope, as the 

 following considerations will show. 



Prof. Keeler's study of the chromatic correction of the Lick 



NO. I 20 I, VOL. 47] 



Refractor shows ("Pub. Ast. Soc. Pacific," Vol. IL p. 164) 

 that the circle of aberration of F light on the focal plane for the 

 D line has a diameter which is in teims of the focal length 

 •COC0349. We may take this diameter as very nearly that of the 

 circle ol aberration of D light on the focal plane for the F line. 

 Thus if a star emits only D and F light, and the F light is 

 focussed, then the D light will fill a circle nearly 7" in diameter, 

 and the star will look like a planetary nebula with a stellar 

 nucleus. If the star tmhs light of wave lengths 500 and 575, 

 then interpolation based on Keeler's measurements shows that 

 round a stellar nucleus in the focus for wave length 500 there 

 must be a circle of aberration of nearly 4 " diameter. 



Mr. Campbell found lines ol wave lengths 500 and 575 in the 

 spectrum of Nova Aurigse with respective intensities 10 and r. 

 Mr. Barnard describes the appearance of nebulosity as "pretty 

 bright and dense," and as measuring 3" diameter. My own 

 inabihty to see either the circle of aberration for the yellow line 

 when the green was focussed, or the alleged nebulosity, may be 

 explained in several ways {eg. smaller aperture of object glass, 

 climatic conditions, &c.). The spectroscope could probably 

 decide the question at Mount Hamilton by showing whether 

 the minimum length of any of the lines is that corresponding 

 with 3" diameter on the slit. I have not been able to do more 

 than observe that the yellow line is not visible when the 5CO 

 line is focussed on the slit of a spectroscope having an effective 

 dispersion of two 60° prisms. H. F. Newall. 



Observatory, Cambridge, October 24. 



Formation of Lunar Vokanoes. 



While we have, on the lunar surface, a series of markings 

 so evidently volcanic that no one thinks of applying any other 

 term to them, we have on the other hand no explanation of 

 their mode of formation which will stand examination. The 

 explanation given by Messrs. Nasmyth and Carpenter in their 

 splendid work on the moon, founded upon explosive expulsion 

 of lava, fails to satisfy the mind when applied to wide craters 

 with a low wall such as Shickard or Grimaldi, of which there 

 are so many on the moon, and which look more like some dis- 

 turbance in a semi- liquid surface than an accumulation of 

 volcanic debris. 



The umbrella-like eruption figured in Messrs. Nasmyth and 

 Carpenter's book does not represent any phenomenon within our 

 experience, as the erupted material (unless light enough to be 

 driven by wind) invariably falls back int o the neighbourhood of 



the vent, and we could not conceive of its being shot neatly 

 out twenty-five miles on every side to form the familiar ring. 



An explanation of the mode of formation founded upon lunar 

 tidal motion occurred to me about seventeen years ago, from 

 observations on a cooling slag ; but until the recent publication 

 of Mr. Darwin's work on the history of the tides I vvas doubtful 

 if that force were sufficient to account for observed results. 



I had noticed that the rise and fall of a fused slag through 

 holes in its solidifying crust, formed craters exactly like those in 

 the moon ; and I enclose a photograph of a piece of that slag 

 in which is reproduced all the salient features of the lunar 

 surface. 



The mode of formation was as follows : — 



The fused liquid (which was potash "black ash " containing 

 a mixture of substances of very varied melting point) was still 

 giving off some gas, which escaped as at a in Fig. I, building up 



