TRANSACTIONS OF THE SECTIONS. SECTION A.-MATHEMATICAL AND PHYSICAL SCIENCE. PRESIDENT OF THE SECTION-Professor OLIVER J. LODGE., D.Sc., LL.D., F.R.S. THURSDAY, AUGUST 20. The PRESIDENT delivered the following Address: DURING the past year three or four events call for special mention in an annual deliverance of this kind by a physicist. One is the Faraday centenary, which was kept in a happy and simple manner by a cosmopolitan gathering in the place so long associated with his work, and by discourses calling attention to the modern development of discoveries made by him. Another is the decease of the veteran Wilhelm Weber, one of the originators of that absolute system of measurement which, though still hardly grasped in its simplicity and completeness by the majority of men engaged in practice, nor even, I fear, wholly understood by some of those engaged in University teaching, has yet done so much, and is destined to do still more, for the unification of physical science, and for a thorough comprehension of its range and its limitations. A third event of importance during the year is the discovery in America of a binary system of stars, revolving round each other with grotesque haste, and with a proximity to each other such as to render their ordinary optical separation quite impossible. Ideas concerning the future of such systems, if, as seems probable, their revolution period is shorter than their axial period, will readily suggest themselves, in accordance with the principles elaborated by Prof. George Darwin. The subject more properly belongs to our President, but I may parenthetically exclaim at the singular absurdity of the notion which was once propounded by a philosopher, that motion of stars in our line of sight must for ever remain unknown to us; whereas the mere time of revolution of a satellite, compared with its distance from its central body, is theoretically sufficient to give us information on this head. As a matter of pedagogy it is convenient to observe that the principle called Doppler's, which is generally known to apply to the periodic disturbances called Light and Sound, applies equally to all periodic occurrences; and that the explanation of anomalies of Jupiter's first satellite by Roemer may be regarded as an instance of Doppler's principle. Any discrepancy between the observed and the calculated times of revolution of stars round each other can possibly be explained by a relative motion between us and the pair of bodies along the line of sight. If our text-books clearly recognised this, we should not so often find examination candidates asserting that the apparent time of revolution of a satellite of Jupiter depends on the distance of the earth from that planet, instead of on the speed. 1 Dr. Huggins has just pointed out to me a perfectly clear statement to the above effect in Professor Tait's little book on Light. I should indeed be sorry to be judged by the performance of my own students, but I fear that many of the less obvious mistakes made by reasonably trained examination candidates are more directly traceable to their teachers than some of us as teachers would like to admit. The change in the refrangibility of light by reason of the motion of its source, though familiar enough now, was at first regarded as too small to be observed, and one or two attempts directed to detecting the effect of this principle on the spectra of the stars, or sometimes on sunlight reflected by a 45° mirror into the line of the earth's motion (which is not a possible method), wholly failed. I take pleasure in remembering that this effect was clearly observed for the first time by the gentleman we this year honour as our President; and that it is by this very means that the latest sensational discovery in astronomy of the rapidly revolving twin star B-Auriga, by Prof. Pickering and the staff connected with the Draper Memorial, was made. The funds for the investigation that led to this result were provided by Mrs. Draper, as a memorial to her late husband; and if B-Aurige does not constitute a satisfactory memorial, I am at a loss to conceive the kind of tombstone which the relations of a man of science would prefer. The fourth event to which it behoves me to refer is the practical discovery of a physical method for colour photography. When I say practical I do not mean commercial, nor do I know that it will ever become applicable to the ordinary business of the photographer. Whether it does or not, it is a sound achievement by physical means of a result which the chemical means hitherto tried failed, some think necessarily failed, to produce. I say practical, because already it had been suggested as possible theoretically; and a step toward it, indeed very near it, had been actually made. The first suggestion of the method, so far as I know, was made by Lord Rayleigh in the course of a mathematical paper on the reflection of light, and with reference to some results of Becquerel obtained on a totally different plan. He said in a note that if by normal reflection waves of light were converted into stationary waves, they could shake out silver in strata half a wave-length apart, and that such strata would give selective reflection and show iridescence. The colour of certain crystals of chlorate of potash, described in a precise manner by Sir George Stokes, and also the colours of opal and ancient glass, bad been elaborately and completely explained by Lord Rayleigh on this theory of a periodic structure (the laminated structure in the case of chlorate of potash being caused by twinning 2); and he subsequently illustrated it with sound and a series of muslin discs one behind the other on a set of lazy-tongs. Each membrane reflected an inappreciable amount, but successive equidistant membranes reinforced each other's action, and the entire set reflected distinctly one definite note, of wavelength twice the distance between adjacent muslins. So also with any series of equidistant strata each very slightly reflecting. They should give selective reflection, and the spectrum of their reflected beam should show a single line or narrow band, corresponding to a wave-length twice the distance of the strata apart.3 2 Phil. Mag. Sept. 1888, pp. 256 and 241. The Proc. Roy. Soc. Feb. 1885. The footnote of Lord Rayleigh on page 158, Phil. Mag. 1887, vol. xxiv., is brief and forcible enough to quote in full:- A detailed experimental examination of the various cases in which a laminated structure leads to a powerful but highly selected reflection would be of value. The most frequent examples are met with in the organic world. It has occurred to me that Becquerel's reproduction of the spectrum in natural colours upon silver plates may perhaps be explicable in this manner. various parts of the film of subchloride of silver with which the metal is coated may be conceived to be subjected during exposure to stationary luminous waves of nearly definite wave-length, the effect of which might be to impress upon the substance a periodic structure occurring at intervals equal to half the wave-length of light; just as a sensitive flame exposed to stationary sonorous waves is influenced at the loops, but not at the nodes (Phil. Mag. March 1879, p. 153). In this way the operation of any kind of light would be to produce just such a modification of the film as would cause it to reflect copiously that particular kind of light. I abstain at present from developing this suggestion, in the hope of soon finding an opportunity of making myself experimentally acquainted with the subject.' |