Sept. 8, 1882.; 



KNOW^LEDGE 



247 



then on the side towards A, of a series of double lines 

 very regularly spaced. In this second part of the group 

 it is always the least refrangible of the two rays which is 

 the darkest ; and it was the least refrangible ray of the 

 pairs \\hich was the most affected at the moon's limb. I 

 tried to estimate to what distance from the limb of the 

 moon the reinforcement extended. The part of each line 

 which was darkened appeared to me to correspond to 

 aliout l-40th of the height of the spectrum. Since the 

 length of the slit is 7 millimetres, and corresponds to an 

 eighth part of the diameter of the solar image, it is easy to 

 conclude that the absorbing layer, if it exists, which is 

 capable of producing the observed reinforcement, does not 

 extend to a height above 5" above the moon's limb. The 

 phenomenon was much less ^^sible in the apparatus of M. 

 Tliollon, as Mr. Ranyard and M. Andre Puiseux bore 

 witness. On the hypothesis that there is an absorbing 

 layer about the moon, this difference of effect may be easily 

 e.xplained by the difference of the orientations ; it will even 

 be a necessary consequence. I ought to add that even in 

 my apparatus the increase of intensity of the lines was 

 incomparably more feeble during the last phases of the 

 eclipse — very doubtful for Mr. Ranyard and nothing for 

 MM. Thollon and Andre Puiseux." 



I have given these observations at length, so that your 

 readers may judge of the comparative certainty with which 

 the observers principally concerned speak of the evidence 

 they obtained as to the existence of absorption in the lunar 

 atmosphere. My own examination of the spectrum before 

 totality was very short, and in order to speak with certainty, 

 I should have been glad of an opportunity of further ex- 

 amining the adjustments of M. Trepied's instrument. I 

 can only speak with certainty as to the non-existence of 

 absorption after totality when I examined the spectrum at 

 greater leisure. Before totality, I did not notice the 

 difference in the thickness of the double lines of the B 

 group, tliough M. Trepied called my attention to the 

 matter while I was looking through his instrument, and 

 said that he had noticed such a difference. All the lines 

 seemed to me to be slightly intensified near to the edge 

 of the spectrum, where they were sharply cut off by the 

 moon's limb. 



I did not at the time make an estimate of the distance 

 from the edge of the spectrum to which the slight thicken- 

 ing of the lines I thought I could detect extended, but the 

 phenomena I observed (which may have been a more 

 physiological effect of contrast, or may have been due to a 

 want of accurate focussing of the image of the solar crescent 

 on the slit), extended to a distance of certainly more than 

 l-40th of the breadth of the spectrum. The next day, on 

 talking the matter over, I thought that it must have ex- 

 tended to at least 110th of the breadth of the spectrum. 

 A height of 5" to which the darkening of the lines observed 

 by M. Trepied extended would correspond to a height of 

 nearly six miles above the moon's limb. In the terrestrial 

 atmosphere absorption increases very rapidly as we ap- 

 proach the earth, and making all allowance for the difference 

 between terrestrial and lunar gravity, it seems difficult 

 to believe that phenomena of absorption could be traced to 

 a height of twenty four miles above the lunar limb. 



It is quite possiVile that there might be a difference in 

 the condition of the lunar atmosphere at the two limbs, for 

 the limb of the moon which was examined before totality 

 had just been exposed to the intense heat of the sun for a 

 fortnight, while the limb which was seen upon the sun's 

 disc after totality had been cooling during the long lunar 

 night, and it is possible that absorbing vapours miglit 

 arise under the solar heat which would again be deposited 

 during the lunar night Fortunately, it will not be 



necessary to wait for another total eclipse to repeat the 

 observation. A partial solar eclipse is all that is required, 

 and large spectroscopes in all parts of the world from 

 which such an eclipse is visible may be simultaneously 

 directed to the lunar limb. 



In connection with this observation, I should like to 

 direct the attention of naked-eye observers to the line of 

 faint illumination which, at the time of new moon, is 

 visible along the outer edge of the dark moon; that is, 

 along the limb which has recently been exposed to the 

 solar rays. Is this line of faint illumination due to 

 dispersion of the sun's rays within an atmosphere 

 which has been raised by the sun's heat? I think that 

 it is pretty certain that the phenomenon is not due 

 to an effect of contrast between the linnicre cendre on 

 the body of the dark moon and the background of sky ; 

 for I have held a circular diaphragm at a distance 

 from the eye, so as just to exclude the light of the sky, 

 and the band of faint illumination along the moon's 

 limb was still visible ; and my old friend, Jlr. Webb, of 

 Hardwick, has kindly repeated this observation for me, 

 and confirms me as to the objective character of the 

 phenomenon. It is difKcult to see why, if the light is due 

 to earthshine, the limb of the moon should be the brightest 

 part. Such a band might possibly be caused by a phos- 

 phorescent light from the lunar surface, which slowly dies 

 away as the lunar night advances. But if this were the 

 case, there should not be a similar band of brightness seen 

 along the dark limb of the morning moon, which has been 

 in darkness for half a lunation. I should be glad to learn 

 whether other observers see the band extending with equal 

 brightness along the whole limb, or does it seem to them 

 to be decidedly brighter in the region of the lunar equator i 

 And do they see a similar band of brightness along the 

 limb of the morning moon ? 



SCIEXCE IN CANADA. 



By the Editor. 



A PROPOSAL has been made that a meeting of the 

 British Association should be held in Canada, and 

 in fact it has been decided, at least for awhile, that the 

 meeting of 1 884 should be held at I^Iontreal. For my own 

 part., I think the decision a mistake, and rather a serious 

 one. Apart from objections raised with good reason by mem- 

 bers of the British Association, the American Association 

 for the Advancement of Science, which has much more life 

 in it than tlie British, has been held this year at Montreal ; 

 and I rather imagine that if the British Association should 

 meet there two years hence, there would be some degree of 

 disappointment. I know wliat American tastes are in 

 matters scientific, how much they pi-cfer freslx to dried 



