Oct. 6, 1887] 



NATURE 



531 



Prof. Flower and the able heads of departments, for all of whom 

 I have the greatest respect ; and I am further convinced that 

 much credit is due to them for doing the very utmost that is 

 possible under the circumstances of the case. My strictures on 

 the Museum were intended to apply solely and exclusively to the 

 fundamental /r/V/fi^/^ underlying its arrangement, which principle 

 is embodied in the new building as in the old one. I contrasted 

 strongly the principle of moderate-sized rooms as compared with 

 large galleries, — the principle of exhibiting, to the public, on the 

 one hand, strictly limited typical collections ; on the other, 

 almost complete series of species, — the principle of making a 

 geographical arrangement the main feature of a museum, as 

 compared with that in which almost no provision at all is made 

 for such an arrangement. 



I had always understood that for this fundamental system of 

 arrangement neither the present Director nor the heads of depart- 

 ments of tlie Museum were in any way responsible, and that in 

 criticising it frankly I should not be considered to reflect on 

 them. So clear was I in my own mind that I was discussing 

 this general system only, that I used some expressions which I 

 now see, with much regret, were capable of being misunderstood. 

 After referring to some of the improvements in the New British 

 Museum, I say, " but the great bulk of the collection still 

 consists of the old specimens exhibited in the old way in an 

 interminable series of overcrowded wall-cases, while all attempt 

 at any effective presentation of the various aspects and problems 

 of natural history as now understood is as far off as ever." To 

 the latter part of this sentence. Prof. Flov er objects, as not 

 recognizing the many improvements recently made and still 

 making ; but I intended it to apply, as I think the whole context 

 of my article shows, to the system and the btiilding, which them- 

 selves, from the point of view I have taken throughout the article, 

 render any attempt at an " effective " presentation of these aspects 

 and problems impossible. Again, at the end of my article I 

 speak of Prof Agassiz having said that he intended his museum 

 "to illustrate the history of creation as far as the present state of 

 scientific knowledge reveals that history," and then go on : 

 " It is surely an anomaly that the naturalist who was most 

 opposed to the theory of evolution should be the first to arrange 

 iiis museum in such a way as best to illustrate that theory, while 

 in the land of Darwin no step has been taken to escape from 

 the monotonous routine of one great systematic series of crowded 

 specimens arranged in lofty halls and palatial galleries, which 

 may excite wonder, but which are calculated to teach no definite 

 lesson." Here I was referring to the fact that the new Museum 

 at South Kensington was constructed and arranged substantially 

 on the same lines as the old one at Bloomsbury, and regretting 

 that the only effective step towards inaugurating a new system 

 of arrangement was not then taken. Prof. Flower, I find, thinlss 

 that I imply that no steps are being taken now to render the 

 Museum more instructive and generally interesting. This was 

 very far from my meaning, and I am exceedingly sorry that 

 such an interpretation of my words should have been possible. 

 I visited the Museum several times last summer before leaving 

 for America, and I noted many improvements that were being 

 introduced in all departments ; but I could not fail to see that 

 the main principle of the arrangement, both of the building 

 itself and of the collections in it, had not been changed, and it 

 was to this that all my criticisms were directed. 



Godalming, September 22. Alfred R. Wallace. 



The Law of Error. 



Mr. F. Y. Edgeworth has, in Nature of September 22 

 (p. 482), replied to Dr. Venn's letter from the mathematical 

 standpoint ; perhaps a few words from the meteorological side 

 may not be out of place. The gist of Dr. Venn's remarks lies 

 in his statement that the law of error applies to cases where 

 there are " equal and opposite independent disturbing causes" 

 (September i, p. 412). Now, the excess and defect of baro- 

 metrical pressure from the average, depend mainly on anti- 

 cyclones and cyclones respectively, which though in many 

 respects opposite in character are by no means equal, the latter 

 being much more intense than the former ; and there is no reason 

 in the nature of the case why they should be equal, as many of 

 their characteristics are so dissimilar. 



As regards the second instance given by Dr. Venn, the chief 

 factor in the variations of temperature at different times of the 

 jFear is the varying declination of the sun, the rate of change of 

 declination passing through two minima yearly — namely, at the 



solstices, so named for this verj' reason. One would naturally 

 expect that about these times the temperature should remain 

 more nearly the same than about the equinoxes ; Dr. Venn's 

 curve would consequently give two maxima. The deviations 

 of the temperature of each day from the average would not be 

 unlikely to conform to the law of error, but it is evident that a 

 curve formed from the temperatures for the whole year would be 

 of a totally different kind. T. W, BACKHOUSE. 



Sunderland, September 26. 



Lunar Rainbov^rs. 



On Sunday night, August 28, a lunar rainbow was visible 

 here. As the occurrence seems to be uncommon,some particulars 

 may interest your readers. 



We had a very heavy shower before 1 1 o'clock, with a south- 

 west wind. The rain left off suddenly, as it began, a few 

 minutes past 1 1 ; and as the heavy cloud moved away to the 

 north-east it left a gloriously clear sky behind, with the moon, 

 then a little past its first quarter, shining brightly a few degrees 

 above a heavy bank of cloud which lay on the horizon. Looking 

 out of a window on the opposite side of the house, I had the satis- 

 faction of seeing a complete pale white bow in the black cloud to 

 the north-east, which lasted very clear and distinct for about five 

 minutes, when it quickly grew faint as the bank of clouds on the 

 horizon began to rise and obscure the falling moon. The outer 

 edge of the bow was well defined against the intense black 

 of the cloud beyond ; the inner edge was much less distirct, 

 and the area within was covered with a slight suffused light, 

 which, however, appeared to diminish as the distance from the 

 bow increased. 



The drops of rain were unusually large, and the downpour, 

 while it lasted, was extraordinarily heavy. 



A. F. Griffith. 



15 Buckingham Place, Brighton, September 22. 



A lunar rainbow was visible here shortly after li o'clock 

 last night. It extended without break through three-quarters of 

 a semicircle, the top of the arch being about 60° high. In 

 colour the bow resembled a moonbeam shining between two 

 clouds, and its brightness was sufficient to cause it to be im- 

 mediately detected by a casual glance, in spite of the presence 

 of numerous white clouds occupying its centre. The sky just 

 outside the bow appeared darkest, probably by contrast with 

 these clouds. Ten minutes elapsed before the rainbow faded. 



Rock Ferry, September 27. S. J. H. 



The Perception of Colour. 



Is Mr. Stromeyer sure that the observations he made (see 

 Nature, July 14, p. 246) prove any difl'erence in the rapidity 

 of perception of colour, and that they do not rather show a 

 difference in perception of brightness ? It is well known that 

 faint objects are not so quickly perceived as bright ones (see 

 Webb's "Celestial Objects," p. 368 of the 4th edition, under e 

 Pegasi) ; and as the violet end of the spectrum is much fainter 

 than the rest, the effect described would be produced by the 

 difference in brightness apart from the difference in colour. I 

 have tried Mr. Stromeyer's experiment of rotating the spectrum, 

 and it appears to me that the red as well as the violet end lags 

 behind the middle ; though as the red is so much shorter, this 

 is more difficult to see. T. W. BACKirouSE. 



Sunderland, September 15. 



Tertiary Outliers on the North Downs. 



In August of last year (Nature, vol. xxxiv. p. 341), I 

 ventured to draw a distinction between the unfossilifcrous sands 

 found at certain places on the North Downs and the fossiliferous 

 deposits at Lenham. For reasons assigned, I suggested a certain 

 degree of probability of their being of Bagshot age, aad in- 

 dicating a former extension. by overlap of the higher beds of that 

 important Eocene formatioi.. This summer I have had oppor- 

 tunities of examining all the principal outliers referred to ; and I 

 must say that I am strongly impressed with the Bagshot character 

 of these unfossilifcrous sands, and of the well-rolled flint pebbles 

 associated with them, in some cases (as at Headley) in great 

 quantity. I speak only of those which can be identified with 



