Nov. 9, 1871] 



NATURE 



25 



22° 30' radius marked DCE, together with a detached portion of 

 F, had the mcon for lis centre ; at the apex of this circle was the 

 apex of another of simihr dimension-, HCG, whose centre was 

 about 45° a! ove the mocn. On the horizontal level of the moon 

 on either side were mock mo ins, Al!, and immediately above 

 the moon with'n the same circle was an o^al mock moon, C. 



^y0:'mmmii,^ 



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Both A and C, though very apparent, were nevertheless not 

 brilliant, the cTandeur of ihe phenomenon ceniermg m the double 

 mock moon B ; this was so brilliant that it attracted immediate 

 attention, and that portion nearest the moon was sensibly orange- 

 red At first it apneared as one large nock moon (twice as 

 bro'ad as it was long), and at 7>' 19'" divided into two with a thin 

 dark band between. Whil-t the two moons (touching each other) 

 were visible, each had a la 1 of 10° or more in length, and these 

 were included within a gigantic tail of 25° long, considerably 

 more brilliant, but colourless, and contrastmg much with the 

 orange-red of the mock moons. At the lime of the phenomenon 

 a fog spread over the valley, and overhead were strong cirn m 

 parallel bands The temperature was if -2, and on the grass 



The moon shone brightly and the sky was cloudless near her 

 throughout the whole time. At 7" 33'" a cloud of considerable 

 density obscured both the moon and the phenomenon. 



E. J. Lowe 



Structure of Lepidodendron 

 I MAY, perhaps, notwithstanding the editorial injunction to the 

 contrary,' be permitted to make one remark by way of addition 

 to what I said in my last letter on this subject. I have been 

 favoured by Mr. J. T. Voung with the inspection of some Lepi- 

 dodendroid stems from the Lancashire coal-fields. Tiiese are 

 somewhat different from any others which I have seen, and are 

 probably similar to those Prof Williamson is working wi'h. At 

 any rate they enable me to understand, what otherwise I have 

 failed to comprehend, namely, the three structures which Prof. 

 Williamson sees in the vascular axis of these plants. In Mr. 

 Young's specimens a transverse section of the vascular axis ex- 

 hibits (I) the investing cylinder, (2) a zone of larger scalaiiform 

 vessels, (3) a cen'ral irregular mass of vertically disposed rows of 

 scalariform cells with transversely truncate ends. Suppose the 

 transverse septa separating these cells absorbed, as probably 

 eventually they would have been, and the rows of cells become 

 scalariform vessels. I see no reason therefore to lead me to 

 alter my views upon this matter, or to look upon 2 and 3 as 

 forming more than one central structure distinct from i, the 

 investing cylinder. W. T. Thiselton Dyer 



Is Blue a Primary Colour? 



No exception can be taken against Dr. Aitken's argument in 

 your number for Oct. 12. The ddours of the substances he ex- 

 perimen ed on could not be regarded as simple. But he does 

 not consider how loosely nil names of colours must be applied 

 in common language. The colours of most blue pigm-nts, 

 especially in thin washes, no doubt contain a large proportion of 

 green. But let the colour of the blue salvia, or that of the pig- 

 ment called French blue or ultramarine (often given as the best 

 example of true blue) be tested in conjunction with the purest 

 yellows (even with the almost greenish yeUow of the pigment 

 called lemon-yellow) and the two will be found perfectly com- 

 plementary. This is the colour of Nesvton's indigo rays, which 

 he himself in his colour circle put opposite to his yellow. In 

 fact, in good English, not only sea-greenish blues, like the 

 colour of Newton's blue part of the spectrum, or that of the 

 pigment called azure or ca;ruleum, but even the colour of the 

 violet itself, is properly called blue. Witness Milton's " beds of 

 violets blue." The violet of the spectrum is in truth little more 

 than a pure blue diluted with white by reason of the fluor- 

 escence of the retina, as recent researches have shown. (See 

 J. J. MiiUer's paper in Poggaidorffs AnnaLii, March and April 

 last.) I must, therefore, protest against substituting a fanciful 

 term like violet for the good English blue, as the desienation of 

 a simple colour- sensation. It is hard enough to make artists 

 believe that yellow is not a simple colour. To tell them the 

 same of its complementary blue would add to their disgust, and 

 not unreasonably. William Benson 



Mr. Aitken in his letter in Nature, Oct. 12, seems to con- 

 found primary with pure colours ; it is true they are pure in a 

 certain sense, but in what sense is fully explained in Prof. J. C. 

 Maxwell's lecture, given in NATiruE, vol. iv. p. 13. AH the 

 experiments mentioned by Mr. Aitken merely prove that the 

 blue colours we commonly see are mixed ones ; but the same is 

 the case with almost all the colours we see, while any tint of the 

 spectrum, whether primary or not, may be had pure, i.e., con- 

 sisting of homogeneous light. Likewise colours which appear 

 just the same to the eye may be made of very different 



ponents. 



T. W. Backhouse 



A Shadow on the Sky 



O.v the 2ist of last August, being at Zermatt, Switzerland, I 

 witnessed from the balcony of l\\e salk-d-maiigi-y of the Hotel du 

 Mont Cervin a very remarkable appearance. Ttie sun had recently 

 set, and, as I was intensely enjoying the view of that extra- 

 ordinary moun'ain, the Matterhorn, I saw its shadow thrown 

 upon the clear sky in the most distinct manner. It was the 

 exact fi<»ure of a cone lying obliquely, with its apex somewhat in 

 an upward direction, and its base taking its origin from the 

 S.S. E. side of the mountain. The cone was well defined, the 

 edges of the shadow being sharp and regular. The moon was, 

 from our point of view, at this time behind the Malterhorn. I 

 immediately acquainted some gentlemen, who were at supper in 

 the salh--a-inaiigcr, with this interesting appearance, and all were 

 much struck with it. My son, Marshall Hall,' had just retired 

 to rest, having to be up at two the next morning, in order to 

 make a new ascent in this locality ; but I called him out into the 

 garden to'enjoy with me this striking scene. The deep, distinct 

 shadow added to the weird effect always produced by this extra- 

 ordinary moun'ain, and it so impressed me that I thought the 

 phenomenon might be worth recording in your journal. 



Brighton, Oct. 23 Charlotte Hall 



A Plane's Position 



This question is becoming one de giis/il'iis, and its further dis- 

 cussion will probably be profitless. I retain my opinion, .-ind 

 am content wi'h the few who side with me. In the two finest 

 treatises on astronomy published during the present century, Her- 

 schel's "Outlines of Astronomy," and Grant's "History of 

 Physical Astronomy," the word position is used as I use it. Not 

 systematically, I admit ; for Herschel sometimes wrote "situa- 

 tion" where I should write "position." Grant in one place 

 deals somewhat definitively with the word, for at p. 258 he 

 writes, " The position of Saturn's ring is usually determined by 

 the inclination of its plane to the ecliptic and the longitude of 

 its ascending node," the longitude of this node being defined, 



