470 



NATURE 



{March 13, 1884 



cafletannic acid, and which he proposes to call labacotannic 

 acid. 



Geological Society, Febraary 20. — Prof. T. G. Bonney, 

 F.R.S., president, in the chair. — Thomas Lionel Bates, G. J. 

 Williams, and Alfred Prentice Young were elected Fellows of 

 the Society. — The following communications were read ; — On a 

 recent exposure of the shelly patches in the Boulder-clay at 

 Bridlington, by G. W. Lamplugh, communicated by Ur. J. 

 Gwyn Jeffreys, F.R.S. — On the so-called Spoi!gia paradoxica, S. 

 Woodward, from the Red and White Chalk of Hunstanton, 

 by Prof. T. McKcnny Hughes, F.G.S. — Further notes on rock- 

 fragments from the South of Scotland embedded in the low- 

 level Boulder-clay of Laucashire, by T. Mellard Reade, C.E., 

 F.G.S.— Ripple-marks in drift, by T. Mellard Reade, C.E., 

 F.G.S. 



Cambridge 



Philosophical Society, February 25.— The following were 

 elected Fellows of the Society : — Mr. A. R. Forsyth, B.A., 

 Trinity College, Mr. W. J. Ibbetson, B.A., Clare College.— 

 The following communications were made to the Society : — On 

 the sums of the divisors of a number, by Mr. J. W. L. Glaisher. 

 — On primitive roots of prime numbers and their residues, by 

 Mr. A. R. Forsyth. — A comparison of Maxwell's equations of 

 the electromagnetic field with those of Helmholtz and Lorentz, 

 by Mr. R. T. Glazebrook. The author pointed out that the 

 main difference between the two theories turned on the fact that 

 while Maxwell considers the electric displacement throughout 

 the field, Helmholtz deals v\'ith the electric moment of each 

 element of volume supposing that by the action of the inducing 

 force opposite electricities are driven to opposite ends of each 

 element. Maxwell's displacement corresponds to the induction 

 in the magnetic field, Helmholtz's polari-ation to the induced 

 magneti-ation. Tlie existence of a normal wave was discussed, 

 and it w as shown that Maxwell's equations without the solenoidal 



condition -i -f _s -(-__ = o, lead to the same result as those 



ax ay dz 

 of Helmholtz, at any rate in the case in which a plane wave is 

 traversing the medium. It was further pointed out that in the 

 case in « hich the induction is due to the presence of electricity 

 at rest outside the portion of the field considered, tlte above 

 solenoidal condition must hold. 



Edinburgh 



Royal Society, February 4. — The Right Hon. Lord Mon- 

 creiff, president, in the chair. — The President gave a review of 

 the hundred years' history of the Society, a full report of which 

 appeared in our issue of February 14 (p. 368). — The Abbe 

 Renard and Mr. John Murray communicated notes on the micro- 

 scopical characters, the chemical composition, and distribution of 

 volcanic and cosmic dust ; and also a paper on the nomenclature, 

 origin, and distribution of deep-sea deposits. Dust obtained by 

 melting snow fiom Ben Nevis was not volcanic in character. 

 — The Abb^ Renard gave a note on a large cry^tal of calc-spar 

 found by I'rof Tait in Lough Corrib. 



Dublin 

 Royal Society, January 21. — Physical and Experimental 

 Science Section. — G. F. Fitzgerald, F.R.S. , in the chair. — Prof. 

 W. N. Hartley, F. R.S.E., read a paper on a simple method of 

 observing faint lines with diffraction spectroscopes. The author 

 states that he works in a darkened room, the goniometer of the 

 spectroscope being illuminated by a shaded lamp which stands to 

 right of the telescope. The grating is movable, while the colli- 

 mator and telescope are fixed in such a position as to include as 

 small an angle between them as possible. The telescope being 

 to the right of the collimator, a small gas jet is placed upon the 

 left, the rays of which proceed to the grating and are reflected 

 into the field of the telescope. By the adjustment of this light 

 the field may lie illuminated in any colour of the spectrum, and 

 by selecting that tint which is complementary to the colour of 

 the lines to be measured, they are sure to stand out apparently 

 in rehef on a bright ground.— Howard Grubb, M.E., F.R.S., 

 read a paper on a new form of equatorial telescope. The author 

 referred to an instrument of his construction which has been 

 at work in Cork Observatoiy for the last two years, in which 

 the eyepiece is placed in a fixed position in the interior of a 

 building. The success of this instrument induced the author to 

 attempt to carry out the same principle on a larger scale, the 

 difficulty to be overcome being that ot producing a perfect plane 



of sufficient size. The author described a form of instrument 

 which, by a combination of a dialytic telescope and his sidero- 

 static form of mounting, would admit of its being of the largest 

 dimensions without the necessity for employing very large re- 

 flectors, as in the case of the new French instrument described 

 inNA'iURE, Novembers, 1883 (p. 36). Mr. Grubb claimed that 

 the form of instrument now described pos.sesses all the chief ad- 

 vantages of the French form, while the difficulties of manufacture 

 would be one-ninlh, and the cost about the same as the ordinary 

 construction, including dome. Another im|)ortant advantage 

 claimed is that the difficulty of construction is not iucrea-ed in 

 the same proportion as in the French form, and therefore Mr. 

 Gtubb's arrangement w ould be applicable to instruments of the 

 largest size. — Greenwood Pim, F.L.S., communicated a paper 

 on the rendering by photography of light and dark colours in 

 their natural values, in the cou' se of which he pointed out that 

 while the ordinary bromide gelatine plates at present so exten- 

 sively employed rendered a blue of low illuminating power 

 almost white and a yellow of high illumination very dark, by 

 using the isoehromatic plates patented by Messrs. Attout-Tailfer 

 and John Clayton of Paris these colours were reproduced in 

 shades corresponding to the illuminating power. Numerous 

 prints from ordinary and from isoehromatic plates of ribljons, 

 coloured fabrics, coloured drawings of flowers, &c., were ex- 

 hibited, clearly showing the superiority of the latter plates when 

 blue and yellow colours had to be photographed ; thus avoiding 

 over-exposing the blue in order to bring out the detail of the 

 yellow porti m. These isoehromatic plates are prepared with 

 cosine in presence of an alkali, usually ammonia, and appear to 

 owe their property more to the chemical action than to the 

 physical action of its red colour ; for a screen of eosined collodion 

 interposed betw een a band of coloured ribbons and the .sensitive 

 plate, so as to cover part and leave part uncovered, had but 

 little effect, all that cou'd be noticed being a general slowing 

 action, and not more in the blue than in the yellow. 



Natural Seieuce Section. — Rev. Maxwell Close, M.A., in 

 the chair. — Rev. S. Haughton, F.R.S., read a paper entitled 

 "Remarks on the unusual sunrises and sunsets that characterised 

 the close of the year 18S3." The older writers on astronomy, such 

 as Brinkley and Maddy, state that on the average twilight lasts 

 until the sun is 18° below the horizon. From this it has been 

 computed that the height of the twilight' producing atmosphere 

 is — 



40 miles on hypothesis of one reflection, 

 12 ,, ,, two reflections, 



5 ,, ,, three ,, 



3 I, II four ,, 



Herschel and Newcomb make no statement whatever as to the 

 duration of twilight. Chambers (in his compilation) says that 

 the average depression of the sun is 18°, which is reduced to 16° 

 or 17° in the tropics, but in England a depression ranging from 

 17° to 21° is required to put an end to the twilight phenomena. 

 Dr. Ball informs me that Prof Schmidt, of Athens, gives for that 

 place 15° 51', and also that Liais (Paris) fixes the first twilight 

 arc to set at 10° 41', and the second at 18° 18'. In the follow- 

 ing observations I calculate the zenith distance of the sun at the 

 close of the phenomena by the well-known formula — 



cos 2 = a -I- (8 cos h, 

 w here 



2 = sun's zenith distance, 



h = sun's hour angle, 



a = sin \ sin 8, 



j8 = cos \ cos S, 



A = latitude of place of observation, 



8 = declination of sun. 



Observation I. — Mr. Bishop, observing at Honolulu, found the 

 phenomenal sunsets to commence on September 5, 1883, and to 

 last up to 7.25 p.m. 



Here \ = 22°, 



8=6° 16'. 



This gives the sun's place 18° 22' below the horizon. This indi- 

 cates tw ilight phenomena intensified by some unusual cause, but 

 does not denote an extension of twilight reflection into regions 

 of the air higher than the time-honoured traditional 40 miles. 

 The epoch of the main eruption of Krakatoa has been fixed by 

 Gen. Strachey at August 27 9.32 a.m. If the explosion of 

 Krakatoa on August 27 was the cause of the Ijrilliant sunset at 

 Honolulu on September 5, the result is nothing short of miracu- 



