Decemuek I, 1898J 



NATURE- 



117 



had been observed between the vapour-pressure and the surface 

 I tension of the mixtures. Mr. Lehfeldt was not sure whether 

 the surface-tensions of the components pass into one another at 

 ' the critical point of mixture. — Mr. L. N. G. Filon then gave an 

 account of his paper on certain diffraction fringes as apphed to 

 fc micrometric observations ; it is to a great extent a critical 

 [ investigation of a paper by A. A. Michelson on the same subject 

 ,1 {Phil. Mag.,vo\. xxx. pp. :-2i, July 1890). Michelson there 

 [ describes a method for measuring the angular distance between 

 L the components of a double star, or the angular dimensions of 

 [ very small celestial bodies, by means of interference-fringes, 

 [ using two adjustable slits in front of the objective of a telescope. 

 [ If the star is double, or if it has an appreciable disc, then by 

 widening the distance between the slits, the fringes become 

 , fainter, and in some cases almost vanish. But, by still further 

 widening the slit, the fringes reappear, disappear, and so on. 

 In the paper (/.<'.) Michelson develops the law of these appear- 

 ,, ances and disappearances, and gives an expression for the ratio 

 1 of the angular distance between the components of the double 

 • star, or the angular radius of the single source, to the distance 

 , between the slits, on the assumption that the slits arc infinitely 

 I long and infinitely thin. Mr. Filon considers that this assump- 

 tion is unjustified by the conditions of measurement ; he reviews 

 the original investigation, and modifies the results. He then 

 proceeds to find equations to represent the intensity of light in 

 the focal plane, for a point source, and for a two-point source. 

 These fringes are only visible over a certain rectangle, called 

 , " the rectangle of illumination" of the source. In the case of 

 a two-point source, if the distance perpendicular to the slits, 

 between the geometrical images of the two points, is an integer- 

 multiple of the distance between two fringes, the maxima of one 

 system correspond with the maxima of the other, the fringes 

 overlap and their intensity is augmented. If, however, this 

 distance should be an odd multiple of the half-distance 

 between the fringes, the maxima of one system correspond 

 to the minima of the other, and if the fringes that are 

 superposed are of similar intensity, the fringing is nearly 

 obliterated, a result that agrees with Michelson's law. But it 

 is now shown that for this phenomenon to occur (i) the rect- 

 angles of illumination of the two sources must overlap to a very 

 large extent, this consideration was neglected by Michelson, and 

 {2) the angular distance between the two stars measured parallel 

 and perpendicularly to the slits, must be less than a definite 

 amount, depending upon the wave-length, and the length and 

 breadth of the slits. In astronomical cases, the second condi- 

 tion is generally satisfied. If the rectangles of illumina- 

 tion do not overlap they can be respectively distinguished, 

 and thus the star can be resolved by direct observation. 

 If, however, an accurate measurement of the distance between 

 the components is required, by Michelson's method, the rect- 

 angles must be made to overlap. The paper includes an 

 investigation of a refractometer that Michelson (I.e.) pro- 

 posed to use for increasing the effective aperture of a tele- 

 scope ; it is shown that Michelson's law is generally true for 

 that instrument, but certain limitations are pointed out. Ex- 

 tended sources are next considered, and also the shape and size 

 of the object. The paper concludes with the description of a 

 method, by means of which the ellipticity of a very small disc 

 may be measured by these diffraction fringes in the special case 

 where Michelson's law holds good. In reply to a question from 

 Prof. S. P. Thompson, Mr. Eilon said that the minimum breadth 

 of slit with which he had found it practicable to work, using 

 monochromatic light with his telescope, was about half a milli- 

 metre. — The President proposed votes of thanks, and the 

 meeting adjourned until December 9. 



Chemical Society, November 17. — Prof. Dewar, President, 

 in the chair. — The following papers were read : — Determination 

 of the constitution of fatty acids, Part i. , by A. W. Crossley and 

 H. R. Le Sueur. The authors have devised a general method 

 for determining the constitution of a fatty acid of the type 

 CH2X . CH„. COOH ; the method has been proved upon valeric, 

 isovaleric and isobutylacetic acid. — The crystalline form of iodo- 

 form, by W. J. Pope. lodolorm is deposited from acetone 

 solution in magnificent hexagonal crystals, of which measure- 

 ments are given. — The characterisation of racemic compounds, 

 by F. S. Kipping and W. J. Pope. Ladenburg states that if a 

 mixture of an externally compensated substance with one of its 

 active components, deposits on fractional crystallisation fractions 

 of dift'erent specific rotations, the compensated substance is 

 racemic ; if it is not racemic, the various fractions have the 



same specific rotation. The authors show that this rule does 

 not hold. — The occurrence of orthohydroxyacetophenone in 

 Chicne glabra, by W. R. Dun.stan and T. A. Henry. The wood 

 of Chione glabra has a strong somewhat foecal odour mving 

 to its containing orthohydroxyacetophenone. — Preparati"n of 

 hyponitrite from nitrite through oxyamidosulphonate, by E. 

 Divers and T. Haga. Sodium carbonate and sulphur dioxide 

 convert sodium nitrite into oximidosulphonate, which on 

 hydrolysis yields sodium oxyamidosulphonate ; the latter is 

 decomposed into hyponitrite and sulphite by potash. These 

 facts lead to a good method of preparing hyponitrites. — Absorp- 

 tion of nitric oxide in gas analysis, by E. Divers. A concentrated 

 alkaline solution of sodium or potassium sulphite rapidly absorbs 

 nitric oxide, and may be used for this purpose in gas analysis. — 

 Interaction of nitric oxide with silver nitrate, by E. Divers. 

 Silver nitrate decomposes in a current of nitric oxide at lower 

 temperatures than in air or carbon dioxide ; the products are the 

 same in the two cases, namely, silver, silver nitrite and nitrogen 

 peroxide. — Preparation of pure alkali nitrites, by E. Divers. 

 Nitrous gases containing excess of nitric oxide convert potassium 

 or sodium hydroxide or carbonate into nitrite in absence of air ; 

 no nitrate is formed. — The reduction of an alkali nitrite by an 

 alkali metal, by E. Divers. — Hyponitrites : their preparation by 

 sodium or potassium, by E. Divers. — Paranitro-orthanisidine, 

 by R. Meldola. A number of derivatives of /-nitro-iJ-anisidine 

 are described. 



Mathematical Society, November 10. — Prof. Elliott, 

 F.R.S., President, in the chair. — The President feelingly 

 alluded to the losses to the Society occasioned by the recent 

 deaths of Mr. Walter Wren and Dr. J. Hopkinson, F. R.S. — The 

 Treasurer read his report, which was a favourable one. Its re- 

 ception was moved by Mr. A. B. Kempe, F.R.S., seconded by 

 Mr. S. Roberts, F.R.S., and carried unanimously. The ballot 

 was then taken, with the result that Lord Kelvin, G.C.V.O , 

 was elected President, and Messrs. Elliott, F. R. 8. , H. Lamb, 

 F. R.S. , and Lieut. -Colonel Cunninghani,R.E., Vice-Presidents. 

 The other members of Council of the last session remain ia 

 office, with the exceptions noted on p. 602 (Nature, vol. Iviii. ). 

 The retiring President having vacated the chair, his place was- 

 taken by Lieut. -Colonel Cunningham, who called upon Prof. 

 Elliott to read his address, entitled " Some Secondary Needs 

 and Opportunities of English Mathematicians," of which the 

 following is a brief abstract. The address congratulated the 

 Society on the work it had done during the last two years, and 

 in particular on the printing in its Proceedings of Sylvester's. 

 "Outlines of Seven Lectures on the Partitions of Numbers." 

 It referred to some of the losses by death which had occurred 

 during the two years among mathematicians and members of 

 the Society. It expressed gratification at the holding at Zurich 

 in 1S97 of an international Congress of mathematicians, and at 

 other signs of growing co-operation among mathematicians of 

 different nationalities. This co-operation, which the history of 

 the Society had proved to be so valuable at home, should in. 

 every way be encouraged on the widest possible scale. Reference 

 was made to some advantages which had accrued from co- 

 operation and mutual encouragement in the history of the 

 Society, and it was in particular enforced that much stimulatioti 

 had once been exercised by the actual meetings of the Society, 

 in ways for which opportunity still presented itself. The in- 

 fluence which the Society had exercised in widening the scope 

 of British enterprise in pure mathematics was dwelt upon at 

 some length, and illustrated by reference to a former need for 

 advanced and comprehensive treatises on modern subjects, w hich 

 had been inspiringly exposed by one of the Society's earlier 

 Presidents, Prof. Henry Smith, and since his time largely satis- 

 fied. Secondary work was necessary that the transition from 

 narrow to widened views of mathematical opportunity be efiectcd 

 surely and without discouragement. The passion among us for 

 examination into elegant incidentals, which shows itself in the 

 fascination exercised by problem making and solving, must be 

 reckoned with and, in the speaker's opinion, not discouraged^ 

 The eflbrt must be to increase the range of interest among, 

 students without weakening thefacilityof acquiring that interest. 

 Unambitious work of definitely educational intention, in subjects 

 now made known to the select few by ambitious treatises, is 

 needed. Instructors are required where leaders have been 

 found. The logical improvement of elementary teaching is 

 proceeding. Unassuming, partial and introductory books of 

 didactic character on modern subjects are wanted. Opportun- 

 ities for didactic work, and for the utilisation of our love of 



NO. I 5 18, VOL. 59] 



