584 



NATURE 



{Oct. lo, 1889 



and the nucleus when at an infinite distance apart. Tha col- 

 lisions will increase enormously when the nebulae coma nearer 

 the sun, and will take place about every second at points, in 

 round numbers, 100 miles apart. 



Prof. C. Piazzi Smyth, late Astronomer-Royal for Sotland, 

 read a paper entitled " Re-examination of the Spectra of 

 Twenty-three Gas-vacuum, End-on Tubes, after six to ten 

 years of existence and use " This inquiry bejan in an attempt 

 to ascertain, by refined mensurations, whether there was any 

 sensible difference of spectral place for hydrogen lines, when 

 they appeared adventitiously and scantily in tubes of other and 

 very different gases, or in tubes of nothing but pure and abund- 

 ant hydrogen by original intention. Bat, afcer having obtained 

 a negative in every case, the inquirer became more taken up 

 with the changes that had occurred in certain of the tubes sub- 

 sequent to 1880, when he published upon them in the Trans- 

 actions of the Royal Society of Edinburgh. Thus, a chlorine 

 tube, of which it was printed in 1880 that it was then still 

 showing its chlorine lines, though fainter, after two years' use ; 

 while carbon bands and hydrogen lines had begun to appear ; 

 yet now, in 1889, it has nothing but hydrogen lines, ani in 

 great brilliance, to show. Again, an iodine tube which had a 

 comparatively large quantity of solid iodine granules introduced 

 into, and sealed up in, its interior eleven years ago ; and showed 

 then a splendid spectrum of 148 measured iodine lines, extend- 

 ing discontinously from red to violet, ani had nothing else, sav.'. 

 three very faint puny imiges of the three principal lines of 

 hydrogen — this tube, in 18S9, has not a single iodine line now 

 left ; but its spectrum, which is brighter than ever, is composed 

 of nothing but hydrogen lines, so that the once solid iodine 

 granules would seem to be partly changed into hydrogen, and 

 partly deposited on the inside of the tube as yellow haze, besides 

 leaving a trifle of loose dust. The author also mentions with 

 much satisfaction that a London mxker, Mr. Charles Ca?ella, 

 transcended all others by supplying him with one tube of CO, 

 two of N, and two of O, which have, through six years of ex- 

 istence and work, shown their respective spectra without a trace 

 of hydrogen. 



Lord Rayleigh, Sec. R.S. , in a paper on the t me; of bells, said 

 that observations were mide on a considerable number of bells 

 of the usual church pattern. Tne pitch of the varioiu tones, 

 usually five for each bell, was fixed by comparison with a har- 

 monium and with the aid of resonators. The nominal pitch of 

 the bells appears to depend on the kighcst of the tones re- 

 corded. From a musical point of view it would seem that all 

 the bells are far removed from perfection ; while the differences 

 of relative pitch in the varioui cases recorded indicates that it 

 may be possible to effect an improvement even to the extent of 

 bringing all the five tones into harmonious relations. The 

 quality of the sound is, however, very difficult to estimate, and 

 even the imperfect octaves, of which several examples occur, 

 give much less of a dissonant effect than might have been 

 expected. 



Captain Abney, F. R.S., read a paper on the quantity of 

 deposit of silver produced by the development on a photographic 

 plate in terms of the intensity of light acting. The author con- 

 cludes from his experiments that the deposit of silver made by 

 different intensities of light varies directly as the intensity of light 

 acting — this, of course, within such limits that i-eversal of the 

 image is not commenced, and that the film is njt at any part 

 exhausted of the silver salt which can be reduced. 



Lord Rayleigh, Sec. R. S., read a paper on pin-hole photo- 

 graphy. It was shown, in the Philosophical Magazine for 1880, 

 that a simple aperture was as effective as the best possible lens in 

 forming an image, provided only that the focal length (/) was 

 sufficiently great. Conversely, if /be given, the aperture maybe 

 made so small that the use of a lens will give no advantage ; but 

 if the focal length was such as was usually afforded by a camera, 

 the admissible aperture, being very much less than that of the 

 pupil of the eye, is insufficient for reasonably good definition. 

 In some recent experiments the focal length was about 9 feet, 

 and the aperture xV i°ch. The specimens exhibited were taken 

 upon gelatine plates, and represented a weathercock seen against 

 the sky. The amount of detail was not mxterially less than th^t 

 observable by direct vision in the case of ordinary eyes, but 

 modern plates are so sensitive that there would be no difficulty 

 in working with an aperture equal to that of the papil, other 

 than that incurred in p.-oviding a focal length of 65 feet, with 

 the necessary exclusion of foreign light. 



Prof. O. J. Lodge, F.R.S.,and Mr. R. T. Glazebrook, F.R.S., 



read a paper on the determination of v by means of electric 

 oscillations. The authors have recently made a determination 

 of V, using the oscillatory discharge of a condenser. The period 

 of the discharge, which passed between two terminals connected 

 through a circuit of measured self-induction to a condenser of 

 known capacity, was determined by forming an image of the 

 spark on the edge of a rapidly revolving circular photographic 

 plate, the rate of revolution being accurately ascertained. 



Prof. A. W. Riicker, F.R. S., read a paper on the instruments 

 used in the recent magnetic survey of France. A magnetic sur- 

 vey of France has recently been completed by M. Mourcaux, 

 who has determined the magnetic elements at some seventy 

 stations. Prof. Riicker exhibited a set of instruments recently 

 made under the supervision of M, Moureaux for the Science 

 Museum at South Kensington. The point aimed at in their 

 construction was to secure accuracy combined with dimensions 

 and weight much less than those of the Kew pattern instruments. 

 The main points in the construction are (i) that the needles 

 used are much smaller than those used in the Kew pattern instru- 

 ments ; (2) the end of the declination needle forms a concave 

 mirror, and a reading is taken when the image of a linear mark 

 formed by the mirror is in the prolongation of another line which 

 is exactly opposite to the first on a thin piece of metal ; (3) the 

 geographical meridian is determined by a theodolite, which 

 forms part of the apparatus, instead of by using — as in the Kew 

 — a plane mirror to reflect the image of the sun into a horizontal 

 telescope ; (4) extremely fine silk threads are able to support the 

 small magnets used ; (5) in the dip circle the graduated arc 

 rotates in its own plane about a horizontal axis, and a reading is 

 taken when the end of the needle and its image, formed by a 

 concave mirror attached to the graduated circle, coincide when 

 viewed through a microscope. 



Prof. J. A. Ewing, F. R.S., in a paper on the magnetic 

 viscosity of iron, described experiments showing that in certain 

 circumstances the process is gradual by which iron assumes mag- 

 netization after the imposition of magnetizing force. The 

 experiments are given in detail in Proc. Roy. Soc, June 20, 

 1889. 



Prof. Everett, F.R.S., read a paper on the relation between 

 brachistochrones and ray-paths, in which he pointed out an 

 application of well-known laws of optics to the comparison of 

 the law of force for a free path with the law of force for a path 

 of least time, the path itself being supposed given ; and to the 

 closely allied problems of deducing the form of the path of least 

 time from the form of the free path when the law of force is 

 given. Most of his results, he stated, have previously been ob- 

 tained by means of the principle of least action, but the optical 

 method will be available for many students who have not 

 mastered that principle. 



Prof. Cayley, F.R.S., addressed the Section on curves in 

 space. 



Prof. J- A. Ewing, F. R.S., read a paper on hysteresis in the 

 relation "of strain to stress. It is now well known that when 

 an iron wire is subjected to the alternate application and removal 

 of tensile stress, many times repeated, certain of its qualities 

 which are affected by the changes of stress exhibit hysteresis 

 with regard to the changes of stress. If the load is cyclically 

 varied between definite limits, these qualities do not have the 

 same values at corresponding intermediate points during the 

 application and removal of load ; there is hysteresis or lagging 

 in the change of quality, and in some cases this appears to be of 

 a static character — that is to say, independent of the time-rate 

 of variation of stress. Conspicuous instances of this action are 

 seen in the change of magnetic and thermo-electric qualities 

 under change of stress, some of which have been described by 

 the author in former papers. It is natural to look for an effect 

 of the same kind in the extension and retraction which the wire 

 undergoes. We should expect that, after the change of loads 

 has been frequently repeated so that a cyclic rSgiine is established, 

 the wire will, for any value of load intermediate between the 

 two extremes, be longer during unloading than during loading. 

 Evidently, if such an effect exist, it must be small , as it is well known ■ 

 that the proportionality of strain to stress which is expressed by 1 

 Hooke's law is at least approximately exact. Sir W. Thomson's > 

 experiments on the damping of torsional vibrators have long ago 

 shown that an action of the kind spoken of occurs in quickly- 

 performed cycles of torsional strain. But it does not appear to 

 have been looked for in slow cycles of longitudinal pull. The 

 author has, with the assistance of one of his students, Mr, D, 

 Low, looked for the effect in question, and has found it, not only 



