and the predictions of a theory in which it is assumed that the office of the opake body is merely to stop a portion of the incident light. But when diffraction is produced by a fine grating, the angle of diffraction is no longer restricted to be small; and it becomes an open question whether the precise circumstances of the diffraction may not have to be taken into account, and not merely the form and dimensions of the apertures through which the light passes. If so, the problem becomes one of extreme complexity. In my memoir on the Dynamical Theory of Diffraction, published in the ninth volume of the Cambridge Philosophical Transactions, I investigated the problem on the hypothesis that in diffraction at a large angle, as we know to be the case in diffraction at a small one, the office of the opake body is merely to stop a portion of the incident light. I distinctly stated this as a hypothesis, and I always regarded it as rather precarious. I was guided by the following consideration. Let AB be the section of a transparent interval by the plane of diffraction, supposing for simplicity the diffraction to take place in air or in a homogeneous medium, and not at the confines of two different media; let AB=b; let B be the angle of diffraction, and λ the wave-length in the medium. Supposing the light to be incident perpendicularly on the grating, the difference of phase of the secondary waves which started from A, B, respectively, will be determined by the length of path b sin ẞ within the medium. In experiment this will usually be a considerable multiple of λ. In the line AB take two points, A', B', equidistant from A, B, respectively, and comprising between them as large a multiple as possible of λ cosec B. If we suppose the influence of the opake body insensible at the distance AA' or BB' from A or B, the secondary waves which start from all points in the interval A'B' will neutralize each other by interference, so that the whole effect will be due to the secondary waves which start from AA' and BB'. Suppose the angle ẞ to belong to the brightest part of a "spectrum of the first class" (Fraunhofer); then AA'+BB'λ cosec B, λ referring to mean rays, so that AA' or BB' is only equal to λ cosec B. If, for example, ẞ=30°, AA' is only equal to λ. At such very small distances it may well be doubted whether the influence of the opake body may not have to be taken into account.

When diffraction takes place at the confines of two different media, suppose air and glass, the problem is still further complicated. We may, however, apply the theory to which reference has been made on the two extreme suppositions, first, that the diffraction takes place wholly in the first, secondly, that it takes place wholly in the second medium. The results of my own experiments were very fairly represented by theory, the vibrations

being supposed perpendicular to the plane of polarization, provided the diffraction be conceived to take place in the first medium, or in other words, just before the light reaches the grating; but they would not at all fit the hypothesis of vibrations parallel to the plane of polarization. I put forth some considerations, founded on probable reasoning, to show that the supposition of diffraction taking place in the first medium was in accordance with the physical circumstances of the case. So decided was the result obtained, that it seemed to me a strong argument in favour of the hypothesis that the vibrations are perpendicular to the plane of polarization, though I still felt the necessity of repeating the experiments under varied circum

stances.

But since the appearance of M. Holtzmann's researches the state of the question is changed. I have no reason to doubt the correctness of his results, while on the other hand the result I myself obtained was far too decided to be passed by. The conclusion which, in the present state of the question, seems to me most probable is, that the polarization of light diffracted at a large angle is, in fact, influenced by the nature of the diffracting body. The subject demands a much more extensive experimental investigation, in which the circumstances of diffraction shall be varied as much as possible. I hope to have leisure to undertake such an investigation: meanwhile it would be premature to offer any decided opinion. It seems to me, however, worthy of attentive consideration, whether a glass grating may not offer a fairer experiment for the decision of the question as to the direction of vibration in polarized light than a smoke grating, inasmuch as in the former we have to do with an uninterrupted medium, glass, the surface of which is merely rendered irregular, whereas in the latter the problem is complicated by the existence of two distinct media, glass and soot, placed alternately. I call the layer of soot a medium, for though no light can pass through any sensible thickness of it, we must not conclude from that that it is without influence on the light which passes excessively close to it.

I have not mentioned the effect of oblique refraction in the experiments of M. Holtzmann, because if it were allowed for, the character of the results obtained would remain unchanged, the magnitude of the observed effect would only be somewhat diminished.

Pembroke College, Cambridge,
February 20, 1857.

XXVI. On M. J. Nicklès' claim to be the discoverer of Fluorine in the Blood. By GEORGE WILSON, M.D., F.R.S.E., Regius Professor of Technology, University of Edinburgh*.

I

AM very reluctant to occupy the time of this meeting with a personal matter, but as I am necessitated to defend my priority in reference to certain researches which in greater part were first communicated to this Society, and first made public through its 'Transactions,' it seems the proper tribunal, at least in this country, to adjudicate on a question liable to dispute.

A communication was made to the French Academy, at its meeting on the 3rd of November, 1856, by M. J. Nicklès, entitled "Presence du Fluor dans le Sang." From the tenour of M. Nicklès' remarks, it would seem that he is not aware that the existence of fluorine in the blood was announced by me in 1846, and specially demonstrated in 1850; nor is he acquainted with the researches which others besides myself have made in this country and in America, into the distribution of fluorine throughout the different kingdoms of nature. In justice accordingly to all parties, I seek to recall the following facts, which may save M. Nicklès needless labour and prevent future disputes, His announcement is as follows: it is reported in the Comptes Rendus for November 6, 1856, and in the Journal de Pharmacie et de Chimie, December 1856, p. 406, from which I take it :"Présence du Fluor dans le Sang. Par M. J. NICKLES. (Communiqué à l'Académie des Sciences, dans la séance du 3 Novembre 1856.) "Par suite de considérations que j'aurai prochainement l'honneur de soumettre à l'Académie, j'ai été conduit à vérifier cette assertion tant contestée, de la présence de fluor dans les os. Mes expériences ayant été affirmatives, j'ai recherché le fluor dans le sang, seule voie par où il ait pu arriver jusqu'au tissu osseux. J'y en ai trouvé de notables proportions, non pas seulement dans le sang humain, mais encore dans celui de plusieurs mammifères (porc, mouton, bœuf, chien), et de plusieurs oiseaux (dindon, oie, canard, poulet),

"Des résultats si concordants me semblent donner au fluor une importance qu'il n'a pas eue jusqu'à ce jour en médecine ou en physiologie; ils infirment évidemment cette opinion de Berzélius, suivant laquelle la présence du fluor dans les os est purement accidentelle, et qu'en tout cas elle n'est pas nécessaire.

"S'il fallait d'autres preuves en faveur de la nécessité de reviser le jugement de l'illustre chimiste, on le trouverait dans les faits suivants: il y a du fluor dans le bile, il y en a dans l'albumine de l'œuf, il y a dans la gélatine, il y en a dans la salive, dans l'urine, dans les cheveux; il y en a dans les poils d'animaux (boeuf, vache et veau); en un mot, l'organisme est pénétré de fluor; on peut s'attendre à en trouver dans tous les liquides qui l'imprègnent.

"Dans un prochain travail je ferai connaître les procédés très simples à l'aide desquels j'ai pu reconnaître la présence du fluor dans toutes ces

*Communicated by the Author; having been read to the Royal Society of Edinburgh, February 16, 1857.

matières. Pour le moment, je dois me borner à prendre date et à prier l'Académie de me donner acte de cette communication.'

I subjoin for convenience of reference an English translation:"From considerations which I shall shortly have the honour to submit to the Academy, I have been led to verify the much-disputed assertion of the presence of fluorine in the bones. My experiments having been affirmative, I sought for fluorine in the blood, the only channel by which it could have reached the osseous tissue; and I found notable quantities of it, not only in human blood, but also in that of several of the Mammalia (pig, sheep, ox, dog), of several birds (turkey, goose, duck, fowl).

"Results so uniform appear to me to give to fluorine an importance which it has not yet obtained in medicine or physiology; they manifestly contradict the opinion of Berzelius, that the presence of fluorine in the bones is purely accidental, and that it is at any rate non-essential.

"If other proofs were needed to show the necessity of revising the judges ment of the illustrious chemist, they would be found in the following facts; there is fluorine in the bile, in the albumen of eggs, in the saliva, in the urine, in the hair; in the hairs of animals (ox, cow, calf); in a word, the organism is penetrated by fluorine; and we may expect to find some in all the liquids with which it is impregnated.

In an early work I shall make known the very simple processes by means of which I have recognized the presence of fluorine in all those substances. For the present I limit myself to noting the date, and asking the Academy to give me formal acknowledgement of this communication."

From the statement of Nicklès, which I have quoted in full, it will be seen that its author was led by his verification of the conclusion, first announced at Rome by Morichini and GayLussac in 1802, that fluorine occurs in the bones of animals, to infer that it must be conveyed to these organs by the blood, and to seek for it in that fluid.

The majority of analysts, however, have long ago justified the early Roman observations, In particular the question of the presence of fluorine in bones was keenly contested in London in 1843, and analyses confirmatory of its occurrence in them were published by Professor Daubeny and Mr. Middleton; to which in 1846 I added, in a communication made to this Society, the accordant results obtained by Professor Gregory and myself, and drew attention to the suggestion of Professor Graham of London, and of Dana, the American geologist, that animals possibly derived the fluorine found in their tissues from fluoride of calcium held in solution by water containing carbonic acid. In the same paper I adverted to the conclusion of Mr. Middleton, founded on his detection of fluorine in a multitude of aqueous deposits, that "beyond a doubt it is present in water, though perhaps in very minute quantity: . . . .The simple fact that the blood con veys it to the bones would, I apprehend, sufficiently confute any scepticism on the subject."

At this point I took up the inquiry in January 1846, and on April 6 of that year communicated a paper to this Society, in