LETTERS TO THE EDITOR

[The Editor does not hold himself responsible for opinions expressed by his Correspondents. No notice is taken of anonymous communications.]

Thickness of the Earth's Crust

UPON my return to London yesterday, I received the two last numbers of NATURE (May 11 and 18), in both of which I find communications on this subject. In the first of these, by Archdeacon Pratt, that gentleman inserts a quotation from a lecture delivered by me, on January 29, this year, "On the Nature of the Earth's Interior" (vide NATURE, February 9, 1871), to the effect that the recent experimental researches of the eminent astronomer and mathematician, M. Delaunay, had destroyed the basis upon which the late Mr. Hopkins's reasonings, as to the solidity of the earth's interior, were founded, and asks the lecturer, i. c., me, "I wonder why he has taken no notice of my letter in reply to M. Delaunay, which was printed in your journal for July 1870, six months before the lecture was delivered, and which also appeared about the same time in the Philosophical Magazine and the Geological Magazine. In this I showed that M. Delaunay had evidently misconceived the problem, and that Mr. Hopkins's method is altogether unaffected by his remarks." As Archdeacon Pratt has the candour to admit that "any one with an ordinary degree of knowledge of popular astronomy and of mechanical action is quite competent to form a good opinion on the point in dispute," I would, in answer to the question he puts to me, simply state that, after a careful study of the letter be refers to, upon its first appearance in the Philosophical Magazine, I purposely avoided referring to it in my lecture, since I failed to discover that the author had in it "showed that M. Delaunay had evidently misconceived the problem," or any reasons whatsoever which could shake my faith in the conclusions of M. Delaunay, subsequently confirmed experimentally by M. Champagneur. I would also mention that, previous to this lecture, I attended the meeting of the Royal Society on the 22nd December, 1870, expressly to hear a subsequent paper by Archdeacon Pratt "On the Constitution of the Solid Crust of the Earth," on which occasion the opinions of Professor Stokes and the experimenial demonstration of Mr. Siemens, as to the untenable nature of the author's conclusions, still further confirmed me in the views I put forth subsequently in my lecture.

It is now superfluous to specify in detail the precise reasons for my rejecting the arguments of Archdeacon Pratt, as I have, in a great measure at least, been anticipated in so doing by the substance of two letters, signed respectively "A. J. M." and "A. H. Green," which appeared in my absence in the last number of NATURE; to these I may refer in support of my view, in which I may also add one of our first English mathematicians has concurred; that M. Delaunay has not changed his will be seen from the Proceedings of the Academy of Sciences at Paris, March 6, 1871.

Having always entertained the highest opinion of the scientific labours of the late Mr. Hopkins, I have taken pains to make myself acquainted with his writings as far as possible for me; but when Archdeacon Pratt states "what Mr. Hopkins did may be divided into two parts-he first conceived an idea, which was to be the basis of his calculation; and then he made the calculation," I regard the whole pith of the question as embodied in these words, which admit that Mr. Hopkins based his elaborate calculations upon an idea, now shown by M. Delaunay to be incorrect, whilst the latter gentleman, on the contrary, founds his deductions upon premises which he first proves to have stood the test of experiment. Where eminent scientific men are arrayed on each side of a question of this nature, the remarks made in the last paragraph of the archdeacon's communication seem rather out of place, and might be applied with equal force in an entirely opposite sense to that intended by their author. May 20 DAVID FORBES

The Geographical Distribution of Insects IN NATURE (No. 74, p. 435) was a very interesting article on geographical distribution by Mr. Wallace, combating some recently urged views of Mr. Murray's. Mr. Wallace took, as an example, the Madeira Islands, and sustained his position upon the numerical statistics furnished by Mr. Wollaston in his books. That these conclusions are very different from those arrived at by Mr. Wollaston is evident and as a six months' residence in

the more remote group of the Canary Islands confirmed to my mind Mr. Wollaston's position, while bringing into relief facts utterly incompatible with Mr. Wallace's, I have ventured to publish a few remarks on the question.

Mr. Murray's views of the distribution of beetles seem to me resolvable into saying that there are two faunas, a tropical (Brazilian and Africo-Indian) and an extra-tropical one. My own slight researches in exotic coleoptera (confined hitherto to the Coccinellidae) strongly confirm this; and a curious instance of the connection between the northern and southern extra-tropical faunas occurred to me the other day. Eriopis connexa, a rather pretty little ladybird, occurs from Hudson's Bay and Vancouver's Island all the way to the Straits of Magellan; following, of course, the line of the Andes. But my object was principally to question some of Mr. Wallace's conclusions with regard to the Madeiran fauna. First of all, I was struck by the absence of any hypothesis for the origin of the very curious endemic forms which form the most important part of the fauna, and which most closely unite it to that of the Canaries and Azores. These Mr. Wollaston, myself, and apparently Mr. Murray regard as affording proof that these islands, or rather groups of islands, were once parts of a considerable continent, and I certainly am at a loss to see how else they are to be explained; for though Mr. Wallace regards the Madeira islets as possibly formerly connected, he would, I suppose, be unwilling to extend this to the other groups. Mr. Wallace appears to regard Mr. Murray's hypothesis to be that the Atlantic continent, of which Madeira is a remnant, derived its fauna from Europe; but it seems rather to be that in the Miocene period (or earlier) there was a similar continent, connected indeed with Europe, not deriving its fauna from Europe any more than Europe from it. Perhaps the best way of answering Mr. Wallace's view will be to take the case of the Canary Islands, whose fauna, resembling the Madeira as it does so closely, must have had a similar origin. Here the argument from apterous genera fails to a very great extent. Thus Carabus is represented by three species, while in S. Spain there is one, and in N. Africa only six or seven. Thorictus has three representatives, and here it may be noticed that ants'-nest beetles are decidedly not numerous in the islands, so that the "unusual means of distribution" fail on the whole to get them across the water. Rhizotrogus is represented by the closely allied also N. African genus Pachydema. Of the very numerous European Rhizotrogi only two Sicilian ones are apterous, so that its absence in Madeira tells either way. Otiorhynchus is no doubt absent, but its place is more than supplied by Atlantis (20 sp.) and Laparocerus (30 sp.). Pimelia again is represented in the Canaries by twelve species, and the apterous genera of Heteromera by more than fifty species, which almost demonstrates the necessity of looking for Tenebrionidae in localities where they are likely to occur.

Then

Tarphius it certainly is difficult to conceive carried across by winds or waves, seeing that its habits are so retired that it has escaped notice till very recently in Europe. Now, however, it is beginning to turn up in suitable mountain localities of Andalusia, Portugal, the Apennines, Sicily, and Algeria; four species are described, and I have seen two others, all agreeing inter se and differing in structure from any Atlantic species. Moreover, it must have been carried apparently to the Azores as well. of the peculiar apterous genera quoted, Thalassophilus, Torneuma, Scoliocerus, Xenomma, and Mecognathus occur now also in Europe, requiring only a collecting-power equal to that of Mr. Wollaston for their discovery. There remain as puzzles upon the hurricane theory twenty-two blind species in the Madeira and the Canaries, and the whole series of Euphorbiainfesting species, fifty in number, all winged, and forming for the most part special genera. Finally, with regard to the fauna of the Azores, the condition of the islands must be taken into account; if the species found round Santa Cruz, Oratava, and Funchal were enumerated, about this proportion of European species would be found. The best island, Pico, has not been worked, and in the others almost all the original vegetation has disappeared. The fact that in the scraps (as they literally were) of Euphorbias, Tarphius and Acalles occurred, shows that if any of the pristineflora could be found a fair number of species might be expected. Elastrus dolosus may certainly have come from Madagascar by the very ingenious route sketched out by Mr. Wallace; but the occurrence of Urania in Madagascar, Brazil, and the West Indies suggests a possibly shorter route, even though no Elastrus be known as yet to occur in America.

In conclusion I may state that I am going to spend a year or

perhaps two in the West Indian Islands, and hope there still further to investigate the theories of geographical distribution, especially endeavouring to see if they can in any way be regarded as having been connected with this submerged continent of Atlantis. G. R. CROTCH

The Coronal Rifts

WILL Captain Tupman kindly explain what he means by "actinic rifts"? I should have supposed that the rifts are evidence of the absense of actinism at the places where they occur.

I am not at all surprised that anyone observing through a telescope should fail to notice the rifts. The eye would naturally be attracted by the bright light of the corona and the red prominences. It may be observed, also, that there is in the photographs a considerable amount of bright corona at the places where the rifts occur, so that Captain Tupman might have had the telescope pointed at the very places where the rifts were, and yet they would escape his notice. The rifts are there, unquestionably, in the two photographs taken (after the lapse of nearly an hour) at Cadiz and Syracuse, and the sketches taken in Spain also show the gaps. The evidence appears to me to be conclusive against Captain Tupman.

Spectrum of the Aurora

A. BROTHERS

THERE is one point in Mr. H. R. Procter's letter in NATURE, vol. iii. p. 468, which I do not agree with He says the bands of the auroral spectrum are seldom visible, with the exception of that whose wave length is 557; whereas I have found two bands, doubtless Winlock's 464 and 431, to be invariably visible when the aurora is bright enough to show them. Also, I suspect the red line is always present when there is any red colour in an aurora, although our instruments do not show it unless the luminosity is considerable. Of the thirty-four auroras in whose s ectra I have seen the line 557, fourteen showed the bands 46 and 431, and three others at least one of these, while eight showed the red line. In five auroras, all more less red, I have seen a faint band, whose wave-length, I believe, is 500 or 510. I have never seen the line 532 (the coronal line), unless it be once; probably from want of instrumental power. As regards the zodiacal light, I have looked at its spectrum several times when it has been at its brightest, but have never seen anything but a continuous spectrum. I am satisfied therefore that if the line 557 exists in it, it must be much fainter in proportion to the rest of its light than is the case with the T. W. BACKHOUSE Sunderland, May 16

aurora.

Science for Farmers

or

As the independent and powerful advocate of scientific education, will you allow me to draw your attention to the object of the enclosed letter?

Within a short period I have seen such remarkable results attended with such an enormous saving of money arising from a limited knowledge of science amongst a committee of farmers, that I am desirous the future generation should have at least a common sense idea of some of the laws of nature which more immediately concern their business and pecuniary interests.

I have the more faith in the success of what I am advocating, because the kind and amount of scientific instruction I propose is really a business necessity. I have not forgotten the results of the Great Exhibition of 1851, how the members of each particular profession or trade were interested, especially in such stalls or departments that concerned this main object of their lives, how to make their own calling more successful or profitable.

I believe therefore in the teaching of science a much greater prospect of success exists when it can be combined with a practical business pursuit. I have read with much interest your article "The Hope of France" on the paper read by M. Deville before the Academy of Science.

The advantages arising from scientific culture, in other words, the study of nature and her laws, are beyond appreciation, and for this reason a student of science must reason and think for himself; he must do his own thinking, and not allow any other person to do this duty for him; and it is my conviction that the real power of any State is exactly in proportion to the number of independent reasoners and thinkers that go to constitute it; and I

"To the Members of the Lincolnshire Farmers Association, and other Agricultural Associations in Great Britain "Gentlemen,-Will you allow me to ask your earnest com. sideration and reflection on a subject which I believe is of vital importance to the future generation of farmers? The question I would put to you is this: Do you wish your sons whom you destine to the pursuit of agriculture to be entirely ignorant of such of the simple elements of chemistry as would give them a complete knowledge of the application and properties of the various materials used in the manufacture of artificial manures, when such knowledge may be acquired with little trouble, in a short time, and at small expense?

"I cannot for a moment believe that any intelligent farmer, with the costly and bitter experience of the past few years in rela tion to the tricks and impostures of the artifical manure makers, can be so indifferent to the future success of his child as not to give him, by a brief course of practical scientific education, not only the power of protecting himself against fraud, but also the knowledge that will enable him to apply the gifts of science to the greatest possible advantage, and at the same time liberate himself from the large and plausible army of manure compounders

"Why should the business and pursuit of agriculture be an exception in the rules of guidance for the successful pursuit of any other business or profession? For the practice of medicine, law, engineering, architecture, &c., a special course of study is re quired, and is really necessary. Agriculture as a business pur suit offers abundant occupation for the highest order of intelli gence, and stands second to none in its claim to scientific skill and sound practical sense, and has therefore an equal claim with other professions, that those engaged in it should be properly qualified by a special form of education.

"What can be more embarrassing to the present generation of farmers than the reading of the reports of chemical analysts on the composition of soils and manures? What can they understand by water of combination' other than that it may have too near a relation to a pump; or the term organic matter,' which may mean flesh or bread, woody fibre, peat, sawdust, or coal dust, most likely a large proportion of the latter elements; of that very intelligible term 'soluble phosphate equal to hone earth or tribasic phosphate of lime made soluble; or alumina, silica, alkaline, salts, &c., which generally mean clay, sand, and common salt, concluding with earthy matter as the dirty founda tion upon which all the other perplexities stand.

"Is it not worth while, by a brief course of practical study, to rid one's self of the influence of all this chemical necromancy? The days of alchemy, witchcraft, and astrology have passed away, and so must the charlatanism and quackery of the inferior order of manure compounders.

"What would be the history of many of these occult persons if it could be traced? Should we find that they have at any time been diligent students under such guides as Liebig, Ville, Voelcker, and other honourable and distinguished chemists? No! I do not hesitate to say that many of them have been mere wandering vagabonds having no disposition or ability to get an honest living by ordinary industry, and as a last resource trade on the credulity of farmers as artificial manure makers. A case in point was recently reported to me. Two discharged lackeys, a butler and footman, embarked, for want of honest employment, in this trade. They are now millionaires; one is au M.P. and the other has received the honour of knighthood. Recently I was over the works of a large and respectable manufacturer of phosphatic manures, who was also a maker of sulphate of ammonia. He informed me that he mixed these two ingredients in such proportion that he could well afford to sell it for 67. per ton. The mixture went in immense quantity to Liverpool, where it was christened under the name of phospho-guano, and wa actually returned, more than a hundred miles, near to the original works, and sold for 127. per ton. Are not such cases, and a thousand others, sufficient to make every farmer ask himself f one object of his being born into this world was to feed and fattea knaves?

"A first-class tailor, hatter, shoemaker, butcher, or baker, desires before all things that his customer should thoroughly understand the composition and quality of the goods he has for sale. Can the same be said of the manure compounders? The

remedy we have in our own hands: either mix for ourselves, or buy subject to analysis; but to properly understand the several terms of an analysis, a course of practical instruction in a laboratory is necessary.

"It is indeed surprising that in a country where the practice of agriculture is carried to such a high degree of perfection, and where it is one of the chief sources of wealth, and is besides the means of employing such an immense amount of labour and capital, so little should be done towards the scientific education of those engaged in its pursuit. As a rule, agriculture is practised almost exclusively under the guidance of a slowly-earned experience, and mere traditional principles and habitual routine, without those engaged in it having any appreciation of the phenomena and natural laws which govern the growth and production of animal and vegetable food, the first necessaries of man. "The success we have already arrived to in agriculture is, I believe, more mechanical than scientific. Drainage, steam culture, and a liberal use of capital and labour, are amongst the chief causes; but now that chemistry in its relation to artificial manures is taking such a prominent position, it is of the first importance that the future generation of farmers should have such à general knowledge of science as will enable them to correctly appreciate the value and properties of the various compounds offered by the too numerous chemical manure makers.

"I cannot imagine a more dangerous, unfortunate, and lamentable position for any person to be in, whilst in the practice of his business, by which he hopes to gain his bread for himself and family, than being entirely dependent on the scientific skill and integrity of another man; or that his capital, time, labour, care, and hopes should be in many cases completely out of his own control. Such a state of things cannot-must not-last.

"What I propose to do to correct this state of things is this at a very moderate cost to give to an agricultural pupil a six or twelve months' course of scientific education and practical laboratory teaching after he has left his regular school; and I will engage that any boy of average ability, at the expiration of this time, shall have such a knowledge of all the materials employed in the compounding of chemical manures as will enable him to dictate what should be used without the interested interference of the manure maker. He shall besides have such an insight into the science and laws of chemistry as to make the reading and future study of scientific agriculture not only perfectly easy, but a delightful and intellectual employment.

"To carry out this important object three conditions are necessary a good qualified teacher, a laboratory, and pupils. The first could be had for a very moderate salary; the laboratory, with the necessary instruments and materials, would involve no serious outlay; the third condition, the pupils, gives rise to this question-Would farmers send their sons to supplement their previous education by a six or twelve months' practical scientific instruction? The want of such knowledge amongst the present generation must be so strongly felt that I believe they would be too glad to have the opportunity, especially if they knew that probably a moderate fee would more than pay the costs.

of phosphate manure-of course, I buy no mixtures or nostrums— is about twenty tons a year; and previous to the formation of our Association I paid to the most respectable makers 67. 10s. per ton for manure, containing 25 per cent. of water. The percentage of soluble phosphate was entirely a matter of speculation. According to an elaborate report of Anderson and Way, from an analysis of 171 samples, the average per-centage of soluble phosphate at that time was 15 per cent., and a ton of this manure was valued by these chemists at 77. 55. per ton, Therefore, if a 15 per cent. manure was worth 77. 5s. a 26 per cent, manure would be worth 12. 10s.; but our Association price for the 26 per cent. manure is now 37. 185. per ton in bulk, delivered free at any station in Lincolnshire, with a further advantage of a watchful system of analysis, free of any cost, to ensure quality and dry condition.

My saving on these calculations would be 8/. 12s. per ton, or a total of 1727. on a consumption of twenty tons yearly. I state facts just as they are recorded by the most eminent chemists, and every farmer will believe me when I say that a very large proportion of the manure sold at that time had little or no value whatever, consisting as it did of dried mud and road scrapings, flavoured with a little gas water just to flatter the olfactory nerves of the wise and cautious farmer of that period. A tenth of 172/. would be 177. 45. for my contribution, but my requirements are much more modest. I think, therefore, I had better leave every member of our Association to form his own estimate of what he should contribute, suffice it to say that in the first instance the only contribution I ask for is a free and unprejudiced opinion as to the necessity and desirability of what I have in view, viz., the formation of a laboratory, in which agricultural chemistry shall be taught at small cost, in a short time, and in a practical way, to pupils who have received an ordinary education. "I have only alluded thus far to the material advantage to be derived from a brief course of scientific instruction. Allow me, in conclusion, to quote the language of one of the best and most highly gifted of our chemical philosophers as to its moral influence. The late Dr. Faraday says:-'I do think that the study of natural science is so glorious a school that with the laws impressed on all created things by the Creator, and the wonderful unity and stability of matter, there cannot be a better school for the mind.' Vain and foolish ideas, the fruit of ignorance, cannot be uprooted and destroyed by violence, the natural and more gentle method must be adopted, what in chemistry is called the law of substitution; the mind must be fertilised by knowledge, then truth and useful ideas will take the place of error and ignorant conceits. It is the absence of the exercise of the higher and intellectual faculties that leads so often to vacuity of the human mind, and the consequent indulgence in grosser and more material excitements, injurious alike to body and mind. A better form of education would eradicate the greatest of all human enemies-Ignorance and Intemperance.

"The Hall, Heckington, Lincolnshire"

of London

66

'W. LITTLE

"I am aware that certain schools exist where agricultural science is professedly taught, but I consider such a combination as almost waste time, the two kinds of teaching cannot be well carried on together, and what is most important, the mind must arrive to a certain maturity before it can grasp with sufficient reasoning power the beautiful and wonderful phenomena arising out of laboratory practice.

"If the foregoing observations should be thought worthy of the serious consideration of persons interested in agriculture, I shall indeed be glad to receive any communications or suggestions in promotion of the object of this letter; and if our members will support this object by the subscription of a sum equal to only a tenth of one year's savings effected through the agency of our association, I will devote myself most earnestly to establish a school laboratory in this village that I trust may serve as a model and example to be followed by many other localities in Great

Britain.

"I shall certainly not ask a single member of our Association to do that which I am not prepared to do myself. My consumption

IN one of your Notes for May 11 you refer to the failure of a motion which I brought before Convocation of the University of London at its recent meeting, to the effect that it was desirable that Greek should cease to be a compulsory subject at the matriculation examination. I have no wish to trouble your readers with a discussion of this subject, because it has been already so well ventilated in various quarters, and general opinion with regard to it has so nearly crystallised into form in other convocations than that of the University of London, that I have no doubt that the Senate of the University will ere long see the absolute necessity, if the University is to be kept en rapport with the scientific culture of the country as it has been hitherto, of adopting the course which I have urged, and the expediency of which has been endorsed by some of the highest educational authorities. rejection of my motion is not the first illustration that Convocation has given of the highly conservative tendencies of many of its members, and of their incapacity to appreciate the liberal spirit in which the University was founded; and I am quite content to leave the case as it stands, with the remark made by one of old under similar circumstances

Victrix causa Diis placuit, sed victa Catoni.

The

I shall, however, be glad if you can favour me with a little

space in your columns to air another subject which I also brought before Convocation, with, I am sorry to say, equal want of success, and that is the desirability of modifying the examinations for the degree of Bachelor of Science by omitting the biological subjects, so as to induce engineering students to take it. At present the biological subjects required for the degree, viz., Zoology, Botany, Physiology, and Organic Chemistry, are so entirely foreign to the studies and requirements of such students that in most cases it is scarcely practicable, even if it were desirable, for them to travel so far out of their regular line of work, for the purpose of getting them up for the Bachelor of Science examination. Such a course would be precisely analogous to that which is now prescribed for medical students proceeding to their M. B. degree, who are required to take up those subjects of the B. Sc. examination which are cognate to their routine of study, and who then branch off to those of a purely professional character.

Only two objections were urged in Convocation against this scheme which are worth consideration. The first was that it would tend to lower the standard of the degree by diminishing the comprehensiveness of the examination. In order to meet this objection I suggested that candidates not wishing to take up the biological subjects should be required to substitute for them others of a mechanical nature, such as Applied Mechanics, Engineering and Architectural Construction, and Geometric Drawing, which, as all who have had any experience in teaching them know, are quite as capable of being made efficient educational tests as those which they would replace. The second objection was, that to make such a change would be equivalent to instituting a degree in engineering. That this would be the practical result of the suggested alteration I am prepared to admit, and it is the object which I had distinctly in view in proposing it. What there was in the suggestion to provoke the unconcealed opposition of so many of the members of Convocation, I am a loss to imagine, unless it was the illusion that the profession of engineering is a less scientific one, and the education of its members less worthy of being encouraged, than that of the professions of law and medicine, to which so large a proportion of the London graduates belong.

For my own part, it seems to me a scandal of no mean gravity that, whilst the practice of that profession requires intellectual qualifications of the highest order, and a scientific training of the widest kind, no means should exist in this country whereby either the public should be provided with any guarantee that those who practise it possess either of these qualifications, or its practitioners themselves should be enabled to give evidence of the fact of their own accord. I do not know of any department of education in which the University of London could, at the present time, do more service than in this, and, I trust, there are men in its Senate, who, with more breadth of appreciation than the majority of Convocation, will give the matter their earnest FRANCIS T. BOND

attention.

Hartley Institution, Southampton

Mechanical Equivalent of Heat

I AM afraid your publication, without adding the date, of my letter last week (which I only saw this morning) puts me in a false position in regard to Dr. Joule, inasmuch as it appears to ignore a correspondence of mine with him, which took place between the time that letter was written (now a long time since) and the time of your publishing it.

In that correspondence I allowed that Dr. Joule's theory remained the same in its main features, though I thought he virtually retracted one statement which I had particularly argued against. Dr. Joule, however, did not allow he had made any alteration.

He also informed me that a paper of mine had been read at the meeting of the Manchester Literary and Philosophical Society, in which I showed (as I believe) in a detailed examination that his theory was inconsistent with the results, both of his own and of M. Favre's experiments. Dr. Joule also kindly communicated to me the substance of the reply which he had made, but I have not seen either in print. Of course the question is one of facts; are facts consistent with the new laws of thermodynamics as supposed to have been established during the last twenty years? Tait, in his preface to his Thermodynamics, says: "The subject s one of vast importance, but very few indeed are yet acquainted with even its most elementary facts; and by many of these it is not yet accepted as true." These laws, therefore, can scarcely

yet be put on a level with Newton's laws, even if they should be shown to be consistent with facts, which, at least in their present H. HIGHTON form, I believe to be impossible. May 18

MR. HIGHTON's letter in NATURE is almost identical with his communication to the Chemical News. My answer is similar to that which I sent to the latter publication, viz., that the object of my paper in the Proceedings of the Literary and Philosophical Society was simply to place the theory of the electro-magnetic engine in a form which might prove useful to those who had not worked on the subject, and not in any respect to withdraw the reasonings in what Mr. Highton is good enough to term my "famous paper." Mr. Highton handsomely acknowledged the justice of my note to the Chemical News in a letter addressed to me on the 28th ult. JAMES P. JOULE

Optical Phenomenon

IN reading over Prof. Clerk Maxwell's paper on Colour in NATURE (Vol. iv. p. 13), I was reminded of the following, to me, curious phenomenon which was seen by me on several occa sions in the summer and autumn of 1859.

Whilst standing before a black board making geome'rical figures in white chalk, I was struck by one side of each chalk line appearing blue, the remaining half retaining its proper white. The cause was at once evident to me, for I found that the sun shone fully upon one eye, but not upon the other. By appeared wholly white. Opening the eye again, the half blue, closing the eye upon which the sun shone, the chalk marks half white marks appeared; then closing the eye upon which the sun did not shine, the whole of the marks appeared a pale blue, scarcely so deep in colour as when in contrast with the white By squinting, or forcing the eyes to see double, two sets of marks appeared, the one set all blue, the other wholly white.

Subsequently, with the sun upon both eyes, the whole of the marks were blue; whilst upon another occasion, when the sun shone very fully upon both eyes, only the white marks were evident; but shading the eyes by the hand, and allowing a ray to fall upon one eye, the usual half blue half white lines appeared. On every occasion that I tried the experiment I met with the same results, and when I looked away a beautiful orange. coloured spot-the complementary colour of the blue, I suppose -appeared for some time wherever I looked. What is the cause why only the blue rays were visible? and why blue rather than red or yellow? THOS. WARD

Yellow Rain

THE following notice will perhaps be of some interest to the readers of NATURE. In December 1870, after a heavy rain at Rosario de Cucuta (New Granada), a great many small round specks of a yellow clayish substance were found on the leaves of plants that had been exposed to the rain. A sample of this substance was sent to Dr. A. Rojas, of this town, who forwarded it to me in order to examine it under the microscope. It proved to be composed almost entirely of a species of Triceratium, and another of Cosmarium, which must have been carried away by

a violent storm from their lacustrian abodes. Caracas (Venezuela), April 1871

The Irish Fern in Cornwall

A. ERNST

My first impulse, on reading the note on this subject in NATURE for the 4th of May, was to apologise to Mr. Dymond for having caused him so much regret by making known a Cornist station for this fern. This first impulse was however checked by the reflection that something is due to the advancement of the study of distributive botany; and I could scarcely have expected even Mr. Dymond to place any very great degree of confidence in my bare assertion, unaided by any reference to localities.

Now that I have done the mischief and made known that the Trichomanes is a Cornish plant, and have been corroborated by Mr. Dymond, it would be interesting to know whether the writer of the note on this fern in the Cheltenham Natural History Society's report found his specimen at the same place at St. Knighton's, EVERARD F. IM THURN