L26 



NATURE 



{March 30, 1876 



"to observing "moraines," "ice-action," " boulders," and "bloc 

 parches " in the same region. 



My object in sending these lines to Nature is to ask for 

 notes of localities where glacial traces may be seen, as an aid to 

 those who hope to examine more closely into the glacial pheno- 

 mena of Central France. W. S. Symonds 

 Pendock Rectory, Tewkesbury, March 25 



Metachromism 

 A lEW words of explanation may seem necessary after Mr. 

 Ackroyd's observations (Nature, vol. xiii. p. 385) on my pre- 

 vious letter regarding the above subject. 

 The question as to whether a change of composition can be 

 . said to produce or to accompany changes of physical properties, is 

 a matter of words which the chromium series does not affect, as 

 the relative number of atoms of the two elements is the test of 

 arrangement followed. 



With regard to the two colour scales — one co- existent vrith 

 alterations of composition, the other with alterations of tempe- 

 rature—I never wished to "criticise" Mr. Ackroyd's results, 

 but solely to point out a resemblance which I had observed a 

 few years ago, and which I was not aware that that gentleman 

 liad noticed. The two series need not necessarily be similar ; 

 and, whatever other reasons may exist for placing white in the 

 ultra-violet, the question in hand is not whether the ultra-violet 

 rays produce the same sensation on our eyes as a mixture of all 

 the colours, but, Do the white compounds in question, whenspec- 

 troscopically examined, only show the ultra-violet, leaving the 

 rest of the range in darkness, or do they show a complete spec- 

 trum ? If the first, then of course their place of classification 

 would be in the ultra-violet ; but if they give a whole spectrum 

 (as the compounds do to which I referred), then they must be 

 classed as having an average refrangibility greater than yellow light 

 (because they have blue in addition to it), and less than blue light 

 (because they have yellow also), for the centre of luminosity (on 

 each side of which the total of light rays is balanced) falls in the 

 green. 



If we had only to deal with monochromatic substances, then 

 of course the \is\xa\ pan-spectral white would not need to be con- 

 sidered, and green (as Mr. Ackroyd says) would be the only 

 appearance to be classed between blue and yellow. 



Thus ' ' the assertion that white comes between yellow and 

 blue "does not "rest upon the colour relation found to obtain 

 between the oxides of the alkali metals," though it is in accord- 

 ance with the rule given on p. 347, in the six sets of the oxides 

 and chlorides there mentioned ; the sole case not agreeing with 

 it being that of the chromium chlorides, wh'ch, however, may 

 be accounted for. 



As to the orange colour of Na^Oa, as Miller does not mention 

 any colour, Turner was referred to ; and if he is in error, that one 

 instance may be laid aside ; in any case it does not affect the 

 relative natural order of blue and white. 

 Bromley, Kent W. M. Flinders Petrie 



Socotra 1 



When I wrote the letter to the Times about Socotra, alluded 

 to in Nature, vol. xiii. p. 414, I was not acquainted vAth. the 

 excellent topographical memoir on this island by Lieut. J. R. 

 Wellsted, published in the Geographical Society's Journal for 

 1835 (Journ. R. Geog. Soc. v., p. 129). After perusing it I am 

 more, than ever of opinion that Socotra is well worthy of the 

 attention of the naturalist, and may probably possess many most 

 interesting indigenous plants and animals. Unless matters are 

 very different from what they were in 1834, there can be little 

 difficulty in exploring the island, and if, as we are told, it has 

 really become British property, I trust we may not have to wait 

 much longer for some information about its zoology and botany. 

 " Socotran Aloes " and " Dragon's Blood " are at present almost 

 its only known natural products, and Lieut. Wellsted mentions 

 but one native animal — a species of Civet. 



P. L. ScL.vrER 



1 1, HanoTcr Square, W., March 27 



Coloured Solar Hales 



Solar Halos such as described by Dr. Frankland (Nature, 

 vol. xiii, p. 404), may be seen on about seventy-five or eighty 

 days in the year, here, and are commonest in the spring, but 

 it is extremely rare for them to be brightly coloured. I speak 



of the ordinary solar halo of about 22° radius, but the great halo 

 of about 46" radius, is always distinctly coloured, though not a 

 common phenomenon. It is not the " murky atmosphere " of 

 London that hides the colours of the ordinary halo ; they 

 usually do not exist, except dull red and orange, and per- 

 haps a faint tinge of blue. This is owing to the great breadth 

 of the halo, which causes the colours to overlap and mix to- 

 gether ; here it is very seldom that the halo is narrow and the 

 colours consequently bright, as they seem to have been when 

 seen by Dr. Schuster (p. 394). I doubt whether the name 

 "parhelia," which he gave them, is correct ; I understand that 

 term to mean mock suns (or a bright small portion of a halo), a 

 phenomenon visible here on thirteen days in a year on the 

 average. 



I may add that though I am rather easily dazzled, I find no 

 difficulty in seeing halos with the naked eye. 



Sunderland, March 28 T. W. Backhouse 



" Euclid Simplified" 



Mr. Morell's defence is a curious one, and amounts to this : 

 " If my book is a bad one I am not to be blamed, because I 

 have copied from Amiot, Legendre, and others. If I have made 

 blunders in derivations, &c., again I am not to blame, but to be 

 pitied, because I could not employ better printers." As in our 

 former notice we limited our remarks to a few only of the objec- 

 tionable features in "Euclid Simplified," so, in our present no- 

 tice, we shall select a few only of the points put forward in Mr. 

 Morell's letter, though we may observe in passing, that we see no 

 reason to retract any of our previous comments. We think that 

 our readers will agree with us when we state our belief that Mr. 

 Morell has utterly failed in most, if not in all cases, to appreciate 

 the force of our objections. Mr. M. correctly quotes Dr. 

 Wormell (pp. 78-81), but fails to see that his own statement is 

 widely different ; had he written " perpendicular to the straight 

 line A A' through its centre" (p. 41), "perpendicular toAB 

 through its middle point" (p. 42), we should not have found 

 fault with him. Again, the reference to Mr. Gerard (p. 310) is 

 not to the point ; we can understand what is meant by a "segment 

 capable of a given angle," but we still object to the term 

 " capable angle. " The revised definition of a. parallelogram is 

 now (see text and letter), "a quadrilateral of which the opposite 

 sides are equal and parallel !" We did not object to the term 

 lozenge, which is a well-known one, but to the way in which it 

 was introduced. 



We turned to Dr. Wormell's definition of circumference with 

 some curiosity, and found that (with the exception of " plain " 

 being printed for "plane") it was perfectly right, and that Mr, 

 Morell had again failed to see the point in our citation of the 

 schoolboy's definition. We contend that Amiot's sentence, as 

 quoted by Mr. Morell, does not mean what Mr, M. makes it to 

 mean. Dr. Wormell's use of G. C. M. is perfectly legitimate, 

 but does not warrant, so far as we can see, the use of R for right 

 angle (seeing it is conventionally applied to another purpose) 

 unless, indeed, it be explicitly stated in the text that R is so 

 used. 



We said (p. 204) that in Theorem VI., p. 148, the reasoning 

 is defective. Mr. Morell replies it " only errs by excess ol 

 proof." We will reproduce the " proof," and leave the decision 

 to our readers, " The area of a trapezium A B C D is equal to 

 the product of its height B E by the half sura of its bases A C 

 and B D. Drop the perpendicular BE on A F, and bisect it by 

 line GH. Produce the base AC to F, making C F =: D B. 

 Then the two triangles D H B and F H C which have for bases 

 the base D B of the trapezium or C F = D B, and which have 

 also the same height, i B E, are equal. The area «! triangle 

 FHC-4DB or FCX^BE; that of triangle D H B = 

 •5 D B X i B E. These triangles, having equal angles, are there- 

 fore equal. But," &c. Upon this we remark, we are not told 

 hozv G H is drawn — the pupil is to infer that it is parallel to B D. 

 Now we must suppose H connected with B and F, and cannot 

 assume that B H F is a straight line, hence, though triangles 

 HFC, B H D are equal, it does not follow that angles F H C, 

 B H D are equal, hence' too we cannot assume A B F to be a tri- 

 angle. But really we must apologise for taking up space with 

 such elementary details. For Mr. Morell's benefit we give the 

 following :— Produce AC to F, making C F = B D, join B F, 

 cutting C D in H, then triangles C H F, B H D are equal, and 

 triangle A B F = A B D C, &c. 



Enough has been written on this, in its present form, objec- 

 tionable book. At any rate we hope that any one who has 



