June lo, 1880] 



NA TURE 



133 



is then moved to such a position that the observer's eye sees 

 three points in a straight line, viz., the eye of the rod, the centre 

 of the mirror, and the reflected image of a selected point of the 

 clond. The direction of the displacement which the latter under- 

 goes after a time, proportional to the velocity of the cloud and 

 inversely as its distance, is the required direction. 



The Report of the Royal Society of Tasmania for 1878 in- 

 cludes the tri-daily meteorological observations made at Hobart 

 Town by Mr. Fr.incis Abbott, so long an enthusiastic observer 

 there, together with the annual abstract of his observations, 

 and also an annual abstract of observations made by Mr. 

 W. E. Shoobridge at New Norfolk, situated about fifteen 

 miles from Ilobart Town, higlier up the Derwent. Observa- 

 tions were formerly made at Port Arthur, Swansea, Swan 

 Island, and Kent's Group, viz., from 1S61 to 1S66, but at pre- 

 sent Hobart Town and New Norfolk appear to be the only 

 meteorological stations in the colony, the observations at Hobart 

 Town dating from 1S41, and those at New Norfolk from 1S74. 

 Mr. Abbott prints also his daily observations made at 10.33 

 P.M. in connection with Gen. Myer's international synchronous 

 observations, the importance of which we have several times had 

 occasion to refer to in describing the United States weather maps. 

 The regular hours of observation are 7.30 A.M. and 4.30 p.m., 

 these hours having been adopted since 1S76, as stated in the 

 Report, with the view of assimilating the records more closely 

 with those of stations in Europe, America, &c., in order to 

 co-operate in a system of international meteorology. These 

 hours have not been happily chosen for general meteorological 

 purposes, particularly since it is the practice to adopt as the 

 mean temperatures of the separate months simply the mean of 

 the observations at the above hours, which, \vhilst only very 

 slightly below the true mean during the winter months, are from 

 i°'5 to 2°'8 too high for the four warmest months of the year. 



PHYSICAL NOTES 



At the last meeting of the Physical Society of Paris some new 

 and curious experiments upon the so-called magic mirrors of 

 Japan were shown by M. Duboscq and discoursed upon by M. 

 Berlin. Mirrors having a sufficiently true surface to give a fairly 

 good virtual image of an object held near to them may yet be 

 very irregular in the actual curvatm'e of the surface and produce 

 a very irregular real image of a luminous point reflected by the 

 mirror upon a screen. If such a mirror be warmed the thinner 

 portions change their curvature, becoming flatter, and yield 

 dark corresponding patches in the disk of reflected light. A 

 mirror which gives very imperfect effects when cold will give 

 very good ones when heated. If, by means of a condensing 

 pump, a uniform pressure is exerted against the bade of the 

 mirror, the thinner portions are more affected than the thick 

 portions, and therefore, as viewed from the front, become less 

 concave than the rest of the surface, the result upon the reflected 

 beam being that the pattern of the thicker parts comes out bright 

 on the darker ground of the image. Lastly, if a mirror be cast 

 upon the face of the original mirror, and then polished, it will 

 when warmed become a "magic" mirror, though when cold it 

 yields only a uniformly illuminated disk upon the. screen. This 

 last experiment alone suffices to show that the cause of the re- 

 puted magical property is to be sought not in any difference of 

 reflective power in different parts of the surface, but in slight 

 differences of curvature of the surface. 



A NBW zinc-carbon battery, the patent of Mr. R. Anderson, 

 is announced. The exciting liquid is a mixture of hydrochloric 

 acid, bichromate of potash, and of certain other "salts" in a 

 mixture, for the composition of which Mr. Anderson claims the 

 protection of the patent. The battery may be used either with 

 or without a porous cell. It is stated that the E.M.F. of this 

 battery is as high as 2' 15 volts, that it is remarkably fi'ee from 

 local action and internal resistance, and that it is very constant, 

 one cell having twelve square inches of effective surface of the 

 zinc, giving for seventy hours a constant current. 



Mr. a. a. Michelson, of the U.S. Navy, has communi- 

 cated to the New York Academy of Sciences some interest- 

 ing observations upon the diffraction and polarisation effects 

 produced by passing light through a narrow slit. If a fine 

 adjustable slit be narrowed down very greatly, the coloured dif- 

 fraction fringes widen out until when the width of the slit is 

 reduced to less than one-fiftieth of a millimetre, the central space 

 only is seen, and appears of a faint bluish lint. Moreover, the 



light so transmitted exhibits traces of polarisation when regarded 

 through a Nicol prism. If the slit is still further narrowed, 

 the depth of the tint and the amount of polarisation increase, 

 until, when a width of only one-thousandth of a millimetre is 

 reached, the colour becomes a deep violet and is perfectly polar- 

 ised. In this experiment the Nicol prism may be used either as 

 polariser or as analyser. Slits of iron, brass, and obsidian pro- 

 duce identical results, though with the latter material, which 

 can probably be more finely worked, the effects are the most 

 pronounced. The polarisation is in a plane at right angles to 

 the length of the slit. The phenomenon is best observed by 

 using direct sunlight, placing the slit as near the eye as possible, 

 and analysing with a double-image prism, thus enabling the 

 delicate changes of tint to be observed by comparison. The 

 possible explanation that the light which thus comes through the 

 slit is reflected at its edges accords with the direction of the 

 plane of polarisation ; but there remains the difficulty that these 

 effects should take place with all widths of slit and vary with 

 the nature of the materials. One important point is that a slit 

 of this degree of fineness admits the shorter waves of light more 

 freely than the longer waves. 



Lord Rayleigh showed a curious experiment in colour- 

 combinations to the Physical Society, when he produced a yellow 

 liquid by mixing a blue solution of litmus with a red solution of 

 bichromate of potash. We recollect a kindred experiment which 

 is even more curious, namely, the production of white by the 

 mixture of crimson and green. An aqueous solution of cuprous 

 chloride and a solution of rosaniline acetate in amylic alcohol 

 are placed in a bottle in certain relative quantities. The crimson 

 solution floats upon the green solution. But when shaken up 

 together both colours disappear, and the mixture is simply a 

 turbid greyish white. 



Mr. Preece's new microphone or telephone transmitter has at 

 least the merit that it surpasses all others for simplicity. A very 

 thin wire stretched between two points forms part of a circuit 

 containing a Bell telephone and a small battery. When it is set 

 vibrating by sounds, the vibrations, by varying the strain to 

 which it is subjected, alter its conductivity, probably by pro- 

 ducing alterations in its temperature. 



M. Obalski describes a pretty magnetic curiosity to the 

 Academic des Sciences. Two magnetic needles are hung verti- 

 cally by fine threads, their unlike poles being opposite one 

 another. Below them is a vessel containing water, its sm-face 

 not quite touching the needles. They are hung so far apart as not 

 to move towards one another. The level of the water is now 

 quietly raised by letting a further quantity flow in from below. 

 As soon as the water covers the lower ends of the needles they 

 begin to approach one another, and when they are nearly im- 

 mersed they rush together. The effect appears to be due to the 

 fact that when the gravitation force downwards _ is partly 

 counteracted by the upward hydrostatic force due to immersion, 

 the magnetic force, being relatively greater, is able to assert 

 itself. 



The phenomenon of luminosity of a (especially) negative 

 electrode of small surface used in electrolysis of, e.g., acidulated 

 water, has been investigated by Prof. CoUey of Kasan (Jour, de 

 Phys., May). Examining the light (which Slouguinoff foiiud 

 associated with an intermittence of the current) with a rotating 

 mirror, he saw on a weakly luminous ground a multitude of 

 bright star-like points, each appearing only an instant, and dis- 

 tributed without apparent regularity. The spectrum of the 

 negative electrode was found to be composed of bright lines, 

 determined botli by the liquid and the substance of the elec- 

 trode. Some physicists have thought that the electrode is con- 

 siderably heated, and that the liquid round it assumes the 

 spheroidal state, being separated by a layer of vapour. M 

 CoUey finds that with a very strong current the electrode indeed 

 becomes incandescent, and -the liquid ceases to moisten it. He 

 shows, however, that the illumination may be produced on an 

 electrode quite cold, and he seeks the cause of production of 

 vapour (of which he supposes the isolating layer to consist) in 

 the hi"h temperature of the liquid immediately surroundmg the 

 electrode (not in that of the electrode itself), heat being deve- 

 loped by reason of the small surface and small conductivity of a 

 thin sheath of liquid. With a pile of 100 Bunsen couples, 

 waler containing S P^^ cent, of sulphuric acid, and an electrode 

 of 10 sq. mm. surface, 1*3 seconds would suffice to raise the 

 layer next the electrode from 20" to 100° C. The sheath of gas 



