10 



KNOWLEDGE ♦ 



[Jan. 



1883. 



HAS THE MOON AN ATMOSPHERE ? 



To the Editor of Knowledoe. 



REFERRING to Captain Noble's recent interesting 

 description of " thorns " on the cusps of the partially- 

 eclipsed sun, it may, perhaps, be useful to record what 

 photography has said on the subject. I have in niy 

 possession some nineteen heliographs (prints) of the 

 eclipses of Dec. 22, 1870, and May 26, 1873. These 

 are four and a-half inches in diameter, and when weather 

 has been favourable, the definition is excellent, as shown by 

 the details of sunspots and the indentations of the moon's 

 edge. 



They were taken by Mr. Fitterton, at Ely, and it is to 

 be regretted the work is now discontinued. 



I have also seven gelatine plates, taken by myself, of the 

 eclipse of Dec. 31, 1880, which are fairly sharp, though not 

 equal to Mr. Fitterton's. A careful examination of these 

 prints and plates does not reveal any of the " thorns," but, 

 1 think, does, in one or two cases, show a tendency to 

 blunting of the cusps. 



These instances are, however, among the 1873 and 1880 

 photographs, when clouds and air disturbance have evi- 

 dently somewhat blurred the outline. Those of December, 

 1870, were taken when the sun was high, and, as to six at 

 least, under favourable circumstances. The cusps in these 

 are wonderfully sharp and well-defined, and show neither 

 " blunting" nor " thorns." 



From the photographic evidence it would appear the 

 " thorn " observations are rare ; and, indeed, I have seen 

 no instance myself. I can however confirm the marvellous 

 sharpness of Saturn as it cut the moon's edge on the 

 occasion Captain Noble refers to. So far in favour of the 

 absence of moon's atmosphere ; but one word now on the 

 other side. Previously to the lunar eclipse of Feb. 27, 

 1877, Mr. R. A. Proctor referred in one of the public 

 prints to its being formerly considered that the moon 

 " perhaps was illuminated by auroral light." I did not see 

 this passage till after the eclipse, but I did remark during 

 that event on the " aurora-like " patches of light seen on 

 the moon's disc ; and in the eclipse of Aug. 23-24 of the 

 same year I obtained what I considered strong confirmatory 

 evidence of these patches. When these observations, sup- 

 ported as they were by the views of Professor Alexander, 

 Professor Dorna, and others, were published, they met 

 with less than encouragement ; but now I am rather glad 

 to find that the possibility of local cloud or mist on the 

 moon (which could hardly exist without some form of 

 atmosphere), is not only supported by observations, but 

 accredited by some of our first selenographers. The in- 

 ference from this appears to be that if our satellite has not 

 an atmosphere quite like our own, she may have one, 

 though rare enough to support auroral discharges, under 

 some circumstances sufficiently dense to partly obscure and 

 alter in character portions of her surface as viewed tele- 

 scopically by us. J. Raxd Capron. 



Guildown, Guildford, Drr. 7, 1882. 



[I read over with Mr. Russell, Government Astronomer 

 at Sydney, New South Wales, Mr. Capron's arguments (in 

 his larger work), and listened to Mr. Russell's own ; but I 

 must confess I see nothing in anything yet observed during 

 lunar eclipses which is suggestive of auroras on the moon, 

 or cannot be explained as naturally to be expected during 

 lunar eclipses. — R. P.] 



Now ready. Part XIV. of Knowiedoe (Dec, 1882), price Is. ; 

 post-free, Is. 3d. 



iRebictos. 



LIGHT.* 



THE science of optics is divided into two parts — 

 geometrical and physical optics. They cannot be 

 treated quite independently of each other, any more than 

 the geometrical properties and laws of motion can he com- 

 pletely dissociated from the physical qualities and laws 

 included under the general term Dynamics. But while in 

 books on geometrical optics the propagation, reflection, and 

 refraction of light, according to determinate laws, are 

 chiefly considered, in a treatise on physical optics those 

 effects are considered only as illustrating or establishing 

 the laws according to which light is transmitted, deflected, 

 absorbed, dispersed, and so forth. The book before us 

 deals chiefly with physical optics, yet gives a sufficient 

 explanation of those matters (usually dealt with in books 

 on geometrical optics) which are essential to a right under- 

 standing of the nature and laws of light. 



The work is one of the best of the excellent series of 

 text^books of science published by Messrs. Longmans & 

 Co. There is a careful reference throughout to the re- 

 quirements of students not acquainted with the more 

 advanced departments of mathematics. In fact, only a 

 very small part of the work requires any knowledge of 

 mathematics beyond the elements of plane trigonometry. 

 Of course, the evidence demonstrating the existence of a 

 something which we call the fether of space, but in reality 

 only know of by its effects, could not be given in such a 

 work as the present ; for none but the mathematician can 

 grasp the evidence, any more than non-mathematicians can 

 grasp the reasoning of Newton in establishing the law of 

 gravitation. Here, therefore, only the general evidence is 

 given, just as in our works of astronomy all but the out- 

 line of the simpler parts of Newton's demonstration is 

 omitted. But the undulatory theory of light, as dis- 

 tinguished from the theory of an lether, is fully estaVilished 

 by the evidence considered here — which indeed runs 

 through the whole work. 



Chapters v., VI., VIL, and IX., XIL-XIV., on "In- 

 terference," " Colours of Thin Plates," " Diffraction," 

 " Spectrum Analysis," and " Polarised Light," are especially 

 interesting. We would commend in particular the careful 

 study of the chapter on " Diffraction " to those who wish 

 to follow understandingly the work which is being done, 

 and will hereafter be done in j-et greater degree, with the 

 diff'raction spectroscope. W'e note wit'n some surprise the 

 omission of any explanation of Haloes, for the phenomena 

 spoken of at p. 203 under that name are not Haloes at all. 



There is a very interesting discussion in the last chapter 

 of the various determinations of the velocity of light. 

 We could wish that, besides the metric statements, measure- 

 ments in miles per second had been given. To tell readers 

 that the velocity of light in racuo is 300,.574,000 metres 

 per second is very well ; but some readers, at any rate — 

 say one in a htmdred — would not infer at onee that light 

 travels, according to this estimate, at the rate of 186,771 '4 

 miles per second. 



NAT, THE NATURALIST.! 



We are disposed to envy the boys of the present day — 

 they have such capital books written for them. In our 

 time " Sandford and Merton " was the type of book for boys, 



* "Physical Optics." By R. T. Glazebrook, M.A., Fellow and 

 Lecturer of Trinity College. (Longmans, Green, & Co., London.) 



t " Nat, the Naturalist : or a Boy's Adventures in the Kastem 

 Seas." By Geo. ManvillcFenn. (Blackie iS: Son : Loudon, Glasgow, 

 Edinburgh, and Dublin.) 



