206 



KNOWLEDGE. 



[September, 1908. 



March 8, 189P, was 261 days. Going back four Saroses 

 we reach 1809, January 12'' 8'' p.m., when the moon's age 

 would have been about the same. This was the date of 

 the death of Sir John Moore, and we thus see that the 

 lines " By the struggling moonbeams' misty light," in 

 Wolfe's beautiful poem, are not strictly in "accord with 

 astronomical facts. 



The Stockwell Ctcle. — This is chiefly interesting 

 for the extraordinary accuracy with which condition B is 

 satisfied ; it is not a good cycle in any other respect. The 

 ninefold cycle is better, as it satisfies condition D 

 accurately and C roughly. A diagram is given showing 

 the past history of two eclipses in this cycle, viz., those of 

 1868, 1927. 



It will be seen that several of the tracks repeat them- 

 selves very well ; thus those of - 167, 364, 885, 1406, 1927, 

 are all total in or near the British Isles. We cannot, 

 however, have great confidence in the longitude of a track 

 X)redicted by this cycle : for example, the tracks of - 1199, 

 -678, also those of —737, —216, show a great shift in 

 longitude. We can, however, predict the latitude very 

 closely, as the diagram shows. 



{To he continued.) 



titers. 



[The Editors do not hold themselves responsible for the opinions 

 or statements of correspondents.] 



THE SURROUNDINGS OF THE 

 NEBULA. 



AMERICA" 



TO THE EDITORS OF KNOWLEDGE. 



Sirs, — In the July number of Knowlrdge Dr. Max 

 Wolf states : " Photographs of this fine nebula have been 

 published not only by myself (Knowledge, 1902), but 

 also within the last few months by Dr. Roberts and by 

 Professor Barnard." I wish this statement, which is 

 inaccurate so far as I am concerned, to be corrected. My 

 photograph of the nebula was taken on October lOtli, 

 1896, and was published in Knowledge, November 

 number, 1898, which chronological computations would 

 sum np to 81 months. This fact is not fairly represented 

 by Dr. Wolf's words " within the last few months." I 

 have also compared the photograph here referred to with 

 mine published in 1898, and find vast differences, in 

 favour of my reflector plate, in the structural details that 

 are delineated. The letterpress which accompanied the 

 photograph explained that the nebula extended beyond 

 the boundaries of the jilate. 



Is it desirable, in these modern days, to attach the 

 names of countries, towns, cities or of individuals to 

 nebulae or other objects in the sky ? Of course we may 

 excuse the harmless lunacy of designating by the names 

 of individuals objects seen on the moon ; " but, if the 

 practice is to be extended to the designation of objects 

 which can be photographed in the sky, there should Ije 

 some method of selection, and I do not see the reason of 

 attaching the name of America more than of England to 

 the nebula here referred to. It was first photographed, 

 on a scale that is suitable for scientific investigation, at 

 Starfield, but I should object to change its designation of 

 y. V. 37 Cygni because Sir William Hersehel was the first 

 to discover it ; and, further, I object to all fanciful desig- 

 nations that may involve controversies. 



Statfield, Crowborough. Ls\ac Robeet.s. 



PROTECTIVE RESEMBLANCE IN BUTTERFLIES. 



TO THE EDITORS OF KNOWLEDGE, 



Sirs, — Some years ago I made a small collection of 

 butterflies in the district of Santos, Brazil, and amongst 

 them are two specimens obtained from the granite quarries, 

 which in colour and markings exactly reproduce the tex- 

 ture of granite. I have no doubt this is an instance of 

 ordinary protective resemblance, for I remember how 

 difficult it was to see these butterflies when once they 

 settled on the granite surface. The point, however, which 

 strikes me as of considerable interest is that the general 

 colouring of the butterfly is a cool blue grey, exactly the 

 shade of the freshly- c^uaiTied stone, on which it invariably 

 settled. The weathered surfaces of the granite were 

 greenish-grey. 



The facts seem to point to a very rapid evolution of the 

 butterfly's ])resent colouring, since the quarries in question 

 have probably only existed for some 200 years, and before 

 that time the butterflies could not have found access to a 

 freshly-cleft granite surface. 



I am not entomologist enough to name the butterfly, 

 but would send a photograph to anyone interested. 



I should be glad if any of your correspondents who have 

 studied the question of protective colouring could say 

 whether there is any warrant for concluding that a change 

 in colour of the kind indicated could be evolved within so 

 limited a period. W. S. Rogers. 



[This observation is of much interest, and there can be 

 little doubt that the colour of the species has changed 

 during the period mentioned under the influence of natural 

 selection. During the last half of the nineteenth century 

 British entomologists have noticed a distinct tendency on 

 the part of certain moths, normally of a pale grey colour, 

 to be replaced by dark varieties, in the manufacturing 

 districts of the north of England, where the habitual 

 restinw-places of the insects — palings, tree-trunks, &c. — 

 tend, through the dejiosition of soot, to be darker than 

 usual. It must be remembered that a century or two is a 

 comparatively long period in the life of a species of insect. 

 — G. H. Carpenter.] 



CURIOUS SUNSET PHENOMENON. 

 to the editors of knowledge. 

 Sirs, — On the 4th July, I observed a curious pheno- 

 menon in conjunction with the sunset of that date. The 

 sun was, roughly, at an altitude of ten degrees above the 

 horizon at the time, and partially obscured by dense dark 





clouds, which, together with a faint haze, hung over the 

 low hills which bounded tlie horizon in that direction. 

 At about 7 p.m. my attention was drawn to an exceedingly 

 bright patch of light, of irregular shape, which exhibited 



