XXI. Intelligence and Miscellaneous Articles. ON THE COMPARATIVE VISIBILITY OF ARAGO'S, BABINET'S, AND BREWSTER'S NEUTRAL POINTS. BY MR. CHASE. IN my communication of January 5, I stated that when Brewster's neutral point is above the horizon, I had frequently determined its position with great ease. My experience was so different from those of Brewster and Babinet, that I commenced on the 6th of March a series of comparative observations upon the three neutral points. The month which has just ended appears to warrant the following conclusions for stations in Philadelphia and its vicinity. The first, second, and sixth seem to be confirmed by observations elsewhere, while the third, fourth, and fifth, which are perhaps dependent partly upon local atmospheric peculiarities, have never, so far as I am aware, hitherto been noticed. 1. Arago's neutral point often assumes a distinctness which is never exhibited by either of the others, merely because the polarized bands in the vicinity of the sun are obscured by the dazzling brilliancy of its rays. 2. For the same reason, Babinet's neutral point is often better defined in the morning and evening than Brewster's during the middle of the day. 3. But when Brewster's and Babinet's neutral points are both above the horizon, if the sky is clear, the former is generally more easily posited than the latter. This is especially the case at midday. 4. On every clear day, and on a large portion of the days which are partially obscured by clouds, the position of each of the neutral points can be determined. Brewster records but two days during five years' observations (Phil. Mag. S. 4. vol. xxx. p. 124), upon which he saw all the points. 5. Arago's neutral point often rises before Brewster's sets. Under favourable atmospheric conditions the three points are, therefore, sometimes simultaneously visible. 6. Halos and clouds are frequently discernible through the polariscope which are invisible to the naked eye. The following abstract embodies some of the results of the month's observations: Satisfactory observations were made on All the neutral points were seen on .... There were no satisfactory observations on.. 39 observations of Arago's neutral point on.. Babinet's and Brewster's the only ones seen on Days. 25 17 6 23 22 20 4 10 11 2 1 1 2 The three points were simultaneously visible on April 5, from Brewster's neutral point was perceptibly more distinct than Babinet's at fifteen observations, and less distinct at two observations. I subjoin a few of my notes, which refer to points of special interest. March 8, 5h 45m P.M.-Near the proper position for Arago's neutral point, the positive and negative polarities coalesce upon clouds, with no intervening space or neutral line. March 9, 6h 25m A.M.-Hazy, and polarization fluctuating. 10h 40m A.M.-The polariscope showed a brilliant halo around the sun, which I had not before noticed, but which was afterwards barely visible to the naked eye. 12h 10m P.M.-Haze continues. Negative polarity remarkably distinct over the face of the sun, and for several degrees north and south. March 11, 3h 50m P.M.-Sky covered with thin clouds. A neutral point in the east, 42° above the horizon, and more than 70° from the antisolar point, with reversed polarization, or positive below and negative above. 5h 25m.—A similar point still observable, but about 5° nearer the horizon. March 12, 6h 30m A.M.-Cloudy. Polarization positive from east and west horizon nearly to zenith. A similar observation was made March 21 at 6h P.M. March 17, 9h 15m and 10h 40m A.M.*, and March 18, 10h 30m A.M.T-Very clear. Sun so bright that I was unable to detect the negative polarity between Babinet's neutral point and Brewster's, even by screening the eye from the direct light of the sun. March 19, 11h 5m A.M.-Halo, visible only through the polariscope. 1 P.M.-Snowing. March 20, 5h 25m P.M.-Cloudy. where positive. Polarization in horizon every March 24 to 28 inclusive.-On each of these five successive days Brewster's neutral point was remarkably distinct and beautiful. April 3, 5h 40m P.M.-Cloudy in west, and polarization positive from zenith to horizon. Strong reflexion sometimes changes the character of a comparatively weak polarization from positive to negative, or vice versa. A fainter reflexion, by showing whether the bands are interrupted or continuous, often aids in determining the character of the polarization. The increased refraction of a piece of glass interposed between the polariscope and the sky will frequently show a neutral point which is otherwise invisible. The normal polarity is often reversed by a stratum of clouds of uniform thickness, especially within the solar primary lemniscate.Proceedings of the American Philosophical Society, vol. x. Feb. 1866. ON OSTROGRADSKY'S HYDROSTATICAL PARADOX. This inference | seems to me to be erroneous, unless we impress an arbitrary constitution on the fluid, and have recourse to the unne*On steamboat in Raritan Bay. † At Eagleswood, near Perth Amboy. In New York. Extract, communicated by the Author, from a Memoir read to the Queensland Philosophical Society on Monday, April 30, 1866. i. e. the inference of Ostrogradsky that the shell will be in equilibrium. His paper in the Petersburg Transactions is reprinted in Taylor's Scientific Memoirs.' cessary hypothesis that a fluid is absolutely continuous. Conceive the contraction to be continued until all the matter of the supposed spherical earth is concentrated at its centre, and we formally as well as substantially have the case discussed by Ostrogradsky. About that centre describe geometrically a sphere passing through one of the points of contact of the particles situate on the inner surface of the liquid vault. Then, from the symmetry of the arrangement, we know that the geometrical sphere will pass through all the points of contact of all the particles situated on that surface, and all the points of contact of any one particle will be in one plane. In a plane, through the points of contact, draw geometrical tangents at all the points of contact of any one particle with all the adjacent particles. Then the symmetry of the supposed arrangement shows that the closed figure so formed will be a regular (equilateral and equiangular) polygon. And that symmetry further indicates that each particle will afford the construction of a similar polygon, that all the polygons so formed are equal, and that each side of each polygon is common to two adjacent particles, and forms the edge of a regular polyhedron. But we know that there are only five regular solids or polyhedra,—namely, the regular pyramid (or tetrahedron), bounded by four equal and equilateral triangles; the cube (or hexahedron), by six squares; the octahedron, by eight equal and equilateral triangles; the dodecahedron, by twelve equal and equilateral pentagons; and the icosahedron, by twenty equal and equilateral triangles. Consequently, however we adjust the magnitude of the spherical balls or particles in reference to that of the geometrical sphere, if we require a system of balls such that each ball shall be capable of being placed in contact with the adjacent balls while each shall be equidistant from the centre of the geometrical sphere, we are restricted to systems of four, six, eight, twelve, and twenty balls, each touching the others of the same system as follows: viz. three others in the system of four, four others in the system of six, three others in that of eight, five others in that of twelve, and three others in that of twenty. A case of fluid equilibrium which can only occur where the particles of the fluid do not exceed twenty in number, can scarcely be held to affect the fundamental principle of hydrostatics. And the fact that while the number of regular polygons is unlimited, that of the regular polyhedra is limited, destroys (except in the particular instances just adverted to) the analogy between a line or circle of particles in equilibrium and a sphere of like particles in equilibrium, und prevents it from being urged in support of the new hydrostatical paradox. I do not at present call to mind any investigations in which a perfect continuity of the fluid is assumed, unless probably in some of those of Professor Challis of Cambridge. But even if I am right in thinking that he has assumed it, all the ends that he had in view would probably be equally well served by changing the assumption to that of particles or distances infinitesimally small in comparison with the particles whose motion is discussed, or the mutual distances of the latter particles. At all events an hypothesis assumed for a special purpose ought not to influence the present discussion, unless it explains phenomena to be explained in no other way.-Queensland CONTRIBUTIONS TOWARDS THE MORE ACCURATE KNOWLEDGE of THE PHENOMENA OF FLUORESCENCE. BY DR. VICTOR PIERRE OF PRAG. The results of this investigation are as follows: (1) That the property of exciting fluorescence is not confined to the most refrangible rays of the spectrum, but that rays of any wavelength can in general excite fluorescence. (2) There is for each substance a definite prismatic colour in which fluorescence first occurs, so that all colours less refrangible than this produce no fluorescence. (3) It is seldom that this colour is the one which produces the most intense fluorescence; generally it is the next more refrangible rays, but always definite rays for a definite substance. (4) If rays of a definite colour, that is, of definite wave-length and time of vibration, evoke fluorescence in a substance, not only are rays produced of greater time of vibration than those of the exciting rays, but the rays produced by fluorescence are, for each substance, always the same, whatever be the duration of vibration of the producing ray. (5) The wave-lengths of the rays produced by fluorescence do not always gradually shade into one another, but there are occasionally jumps, so that rays of a certain length are not developed, in which case the spectrum of the fluorescence-colour is traversed by dark lines; this phenomenon also is independent of the wave-length (direction of vibration) of the exciting rays. (6) Among the new rays resulting from fluorescence, those are always the most intense whose wave-length is either equal, or very nearly equal, to that of the rays in which fluorescence first occurs; in the latter case, however, it is always larger than that corresponding to the beginning of the fluorescence. (7) In substances which fluoresce in solution, in case they are soluble in different agents, the solvent occasionally influences the character of the fluorescence, so that, dissolved in different solvents, the same substance fluoresces differently. In one and the same solvent the concentration of the solution only affects the intensity of the fluorescence, but leaves its character unaltered. Above and below that degree of concentration which makes the phenomenon of fluorescence most intense, the intensity of the fluorescence in all parts of the spectrum in which it is at all developed appears to decrease in almost the same ratio; so that with the feeblest development it is distinctly perceptible only in the position of the maximum, (8) The occurrence of one fluorescing substance with other fluorescing or non-fluorescing substances exercises very different effects on the character of the fluorescence; in many cases it undergoes no change, but in others it is entirely altered. If many fluorescent substances are mixed together, a compound fluorescence is produced, the colour of which, in diffused day- or in direct sunlight, may be very different although the same substances are in both cases mixed together. If the various fluorescent substances do not act on each other so as to alter their fluorescences, such a compound fluorescence may always be resolved into the simple fluorescences of those substances which are contained in the mixture; and so far the presence of certain substances may be detected by fluorescence in a mixture of different substances, but not in the opposite case. (9) There are substances which become strongly fluorescent by the addition of acids, and others by the addition of alkalies: in these cases it is immaterial which acid or which alkali is used; the character of the fluorescence is always the same*. (Hydrochloric and hydriodic acid form an exception, as they destroy fluorescence.) (10) The light of artificial sources, or such as has passed through coloured media, occasionally produces changes in the fluorescence compared with that of the sunlight, inasmuch as the commencement and maximum fluorescence may fall at other parts of the spectrum. Connexion between Fluorescence and Phosphorescence. It was of some interest to include in my investigations the phenomena of phosphorescence which are so markedly developed by the sulphides of the different earth-metals; and I used for this purpose preparations partly obtained from Albert in Frankfort, and partly from Lenoir in Vienna. If the prismatic spectrum is projected on one of these phosphorescent bodies, the phenomena are the same as with fluorescent bodies. Phosphorescence begins sometimes in the visible, sometimes in the ultra-violet rays—in short, in different preparations, in different parts of the same spectrum; it has also a maximum (in some substances I found two maxima), and the colour of the light of phosphorescence is the same in the entire extent of the part of the spectrum which excites phosphorescence. If a linear spectrum projected upon such a body be analyzed by a prism, a derived spectrum is also obtained, which in its principal features has the same appearance as that of a fluorescent substance; but the uneven rough surface of this body permits no pure spectrum; a good deal of light is always irregularly scattered, owing to which it was impossible to ascertain with certainty whether there is not here something analogous with compound fluorescence; I imagine that in some of the substances examined something of that kind is the case. The entire appearance which a phosphorescent body offers in the prismatic spectrum is so completely the same as in a fluorescent one, that from this appearance alone it could not at all be decided whether it was a case of fluorescence or of phosphorescence; the difference of the two phenomena consists only in the fact that fluorescence immediately disappears if the incident light is cut off, while phosphorescence continues in this case, though it quickly diminishes in intensity; the phenomena occur just as with a fluorescent liquid in which the concentration is altered to the disappearance of fluorescence. The phenomena is first imperceptible at the side of the commencement and in the ultra-violet, and is finally only perceptible at the positions of maximum. Thus I agree with the statement of Becquerel, that fluorescence and phosphorescence are only distinguished by their duration, inasmuch as the former is at once extinguished with the cessation of the exciting rays, while the latter continues.-Wiener Berichte, May 11, 1866. *This deportment is, in a certain sense, analogous to that of coloured transparent media, which change their colour by the addition of acids or alkalies In this also the resulting mixture is independent of the nature of |