154 



NATURE 



[Dec. 25, 1873 



between this cylinder and the sides of the box. Polarised light 

 was thrown up through the fluid parallel to the axis, and the 

 inner cylinder was then made to rotate. I was unable to obtain 

 any result with solution of gum or syrup of sugar, though I ob- 

 served an. effect on polarised light when I compressed some 

 Canada balsam which had become very thick and almost solid in 

 a bottle. 



It is easy, however, to observe the effect in Canada balsam, 

 which is so fluid that it very rapidly assumes a level surface after 

 being disturbed. Put some Canada balsam in a wide-mouthed 

 square battle ; let light, polarised in a vertical plane, be trans- 

 mitted through the fluid ; observe the light tlirough a Nlcol's 

 prism, and turn the prism so as to cut off the liglit ; insert a 

 .spatula into the Canada balsam in a vertical plane passing 

 through the eye. Whenever the spatula is moved up or down 

 in the°fluid, the light reappears on both sides of the spatula ; 

 this continues only so long as the spatula is in motion. As soon 

 as the motion stops, tlie light disappears, and that so quickly 

 that I have hitherto been unable to determine the rate of relaxa- 

 tion of that state of strain which the light indicates. 



If the motion of the spatula in its own plane, instead of being 

 in the plane of polarisation, is inclined 45° to it, no effect is ob- 

 served, showing that the axes of strain are inclined 45° to the 

 plane of shearing, as indicated by the theory. 



I am not aware that this method of rendering visible the state 

 of strain of a viscous fluid has been hitherto employed ; but it 

 appears capable of furnishing important information as to the 

 nature of viscosity in different substances. 



Among transparent solids there is considerable diversity in 

 their action on polarised light. If a small portion is cut from a 

 piece of unannealed glass at a place where the strain is uniform, 

 the effect on polarised light vanishes as soon as the glass is re- 

 lieved from the stress caused by the unequal contraction of the 

 parts surrounding it. 



But if a plate of gelatine is allowed to dry under longitudinal 

 tension, a small piece cut out of it exhibits the same effect on 

 light as it did before, showing that a state of strain can exist 

 without the action of stress. A film of gutta percha which has 

 been stretched in one direction has a similar action on light. If 

 a circular piece is cut out of such a stretched film and warmed, 

 it contracts in the direction in which the sti-etching took place. 



The body of a sea-nettle has all the appearance of a trans- 

 parent jelly, and at one time I thought that the spontaneous 

 contractions of the living animal might be rendered visible by 

 means of polarised light transmitted through its body. But I 

 found that even a very considerable pressure applied to the sides 

 of the sea-nettle produced no effect on polarised light, and I 

 thus found, what I might have learned by dissection, that the 

 sea-nettle is not a true jelly, but consists of cells filled with 

 fluid. 



On the other hand, the crystalline lens of the eye, as Brewster 

 observed, has a strong action on polarised light when strained, 

 either by external pressure, or by the unequal contraction of its 

 parts as it becomes dry. 



I have enumerated these instances of the application of polar- 

 ised light to the study of the structure of solid bodies as 

 suggestions with respect to the application of the same method 

 to liquids so as to determine whether a given liquid differs from 

 a solid in having a very small " rigidity," or in having a small 

 "time of relaxation,"* or in both ways. Those which, like 

 Canada balsam, act strongly on polarised light, have probably a 

 small "rigidity," but a sensible "time of relaxation." Those 

 which do not show this action are probably much more " rigid," 

 and owe their fluidity to the smallness of their " time of relax- 

 ation." 



" On the Period of Hemispherical Excess of Sun-spots, and 

 the 26-day Period of Terrestrial Magnetism." By J. A. Broun, 

 F.R.S. 



It appears from the interesting communication to the Royal 

 Society, June 19, by Messrs. De La Rue, Stewart, and Loewy,+ 

 that the difference of the area of spots on the visible northern 

 and southern quarter-spheres of the sun seems, during periods of 

 considerable solar disturbance, to obey a law such that the 

 difference is a maximum in tlie same quarter-sphere during 

 several successive rotations of the sun, the difference being a 

 maximum alternately in the northern and southern hemispheres ; 



* The "time of relaxation '' of .1 substance strained in a gii 

 tlie time required for the complete relaxation of the strain, supposing the 

 rate of relaxation to remain the same as at the beginning of this time. 



t Proc. Royal Soc. vol. xxi. p. 399. 



the time from maximum to maximum, for the same hemisphere, 

 being variable between 18 and 32 days, but having a mean 

 value of about 25 '2 days. 



It occurs at once that if the variations of the mean terrestrial 

 magnetic force are connected in any way with the solar spots, or 

 the causes which produce them, we might here find some expla- 

 nation of the magnetic period of 26 days, the difference of spot 

 area in one hemisphere from that in the other being related to 

 a difference of the solar magnetic action. 



In order to determine whether such a connection existed, I 

 projected first the curves of excess of spot-area given in the 

 paper of Messrs. De La Rue, Stewart, and Loewy, and below 

 them the daily mean horizontal force of the earth's magnetism 

 during the same periods. The conclusion from these projec- 

 tions is, that tliere is no relation w/ialcz'er hd'Micii the two classes 

 of eurves. The maxima and minima of the one agree in no ways 

 with those of tlie other ; the greatest excesses of sun-spot area 

 in the one hemispliere over those in the other occur when the 

 earth's magnetic force is the most constant ; the greatest varia- 

 tions of the earth's magnetic force from the mean occur in 

 several instances when the sun-spot area is equal in the two 

 visible quarter-spheres. 



It should be remembered, in considering the curves of sun- 

 spot excess, that the minima and maxima are in some cases only 

 relative ; sometimes the one, sometimes the other being really 

 cases in which there is neither maximum nor minimum ; that is 

 to say, cases in which the sun-spot area is equal, or nearly so in 

 the two visible quarter-spheres. 



It would be hasty to conclude from this comparison that the 

 variations of the mean magnetic force are really unconnected 

 with the mode of distribution of the sun-spots. Other methods 

 of grouping the spots may perhaps be employed with advantage 

 relatively to this and other questions, for example, were the 

 position of the centre of gravity of the sun-spots determined for 

 the visible quarter-spheres and hemisphere, giving each spot a 

 spot-weight in proportion to its area, tlie variation of these 

 positions in latitude and longitude and their weights, might give 

 a more satisfactory base for this comparison and for other de- 

 ductions. 



It will be obvious also that this investigation refers only to 

 one -'M?7'//f hemisphere of the sun ; an approximation to the spot- 

 distribution on the other hemisphere will, however, be frequently 

 possible. 



"On the Nervous System of Actinia" Part I., by Prof. P. 

 Martin Duncan, F.R.S. 



" On certain Discrepancies in the published numerical value 

 of TT," by WilHam Shanks. 



Mathematical Society, Dec. 11. — Prof. Cayley, F.R.S., 

 V. P. , in the chair. — Prof. Clifford gave an account of his paper 

 on the graphic representation of the harmonic components of a 

 periodic motion. The paper was an application of a theorem of 

 Fourier's, which asserts that any motion having the period P 

 may be decomposed into simple harmonic motions having 

 periods P, 4 P, \ P, &c., and assigns the amplitudes and phases 

 of these motions by means of definite integi'als. — Prof. Cayley 

 next spoke on the subject of Steiner's surface. The author stated 

 that he had constructed a model and drawings of the symmetri- 

 cal form of Steiner's surface, viz. that wherein the four singular 

 tangent planes form a regular tetrahedron, and consequently the 

 three nodal lines (being the lines joining tlie middle points of 

 opposite edges) a system of rectangular axes at the centre of the 

 tetrahedron. He then described the general form of the surface, 

 and finally discussed its analytical theory. — Lord Rayleigh, Mr. 

 Roberts, Prof Clifford, and Prof. Cayley made further extempore 

 communications to the .Society. 



Linnean Society, Dec. iS. — G. Bentham, F. R. S., president, 

 in the chair. — Dr. Hooker exhibited a magnificent zoophyte from 

 Bermuda, sent liy General Lefroy ; also a six-lobed Seychelles 

 cocoa-nut {Lodoicea Scycluilarum) and two tazzas made from the 

 shell of a Seychelles cocoa-nut sent from the Seychelles by Mr. 

 Swinburne Ward to the Kew Museum ; also some small boxes 

 from Mauritius and Madagascar made from some grass-haulm ; 

 and two walking-sticks from Bermuda made of the " cedar- 

 wood" of commerce (Jiinitci-iis /hrmiidia)ia). — Mr. Bowring 

 exhibited an inflorescence of an orchid with a remarkable smell, 

 probably a Ihilhophyllum. — The following papers were then 

 read, viz. : — "Contributions to the Botany of the Challenger Ex- 

 pedition," No. 2, by II. N. Moseley, M.A. On the \'egetation of 

 Bermuda and the surrounding sea. About 160 species of flower- 



