49° 



SCIENCE. 



The second, more portable form, does not allow such an 

 accurate quantitative air analysis. The instrument is 

 attached to a weathercock, and thus is always directed 

 against the wind, which traverses it, and deposits, as in 

 the other permanent form, its solid matter on a glycerine 

 plate. An anemometer placed in the vicinity serves to 

 give an approximate idea of the quantity of air which has 

 passed through the apparatus. These instruments have 

 been called aeroscopes by their inventor. It is likely 

 that the second form given to the apparatus will be best 

 fitted for the purpose which the Committee has in view. 



THE NEW ASTRONOMER ROYAL. 



Mr. William Henry Mahony Christie, who has suc- 

 ceeded Sir George Airy in the office of Astronomer Royal 

 at the Royal Observatory, Greenwich Park, was born on 

 October i, 1845, at Woolwich. He is a younger son of 

 the late Professor S. H. Christie, of the Royal Military 

 Academy, Woolwich, and formerly Secretary to the 

 Royal Society. Mr. W. H. M. Christie was educated at 

 Kings College School, London ; and at Trinity College, 

 Cambridge, which he entered in 1864, having won a 

 Minor's Scholarship of that 

 College ; he subsequently gain- 

 ed a Foundation Scholarship 

 and was afterwards elected a 

 Fellow of Trinity College. He 

 took his degree of B.A. in 1868, 

 as fourth wrangler in the Ma- 

 thematical Tripos, and in 1871 

 proceeded to the M. A. degree. 

 In 1870, Mr. Christie was ap- 

 pointed Chief Assistant at the 

 Royal Observatory ; and he 

 has, during the past ten years, 

 done special good service by 

 contriving and introducing 

 several valuable improvements 

 in the scientific apparatus there 

 in use ; a new form of spec- 

 troscope, an instrument for 

 determining the colors and 

 brightness of the stars, a re- 

 cording micrometer, and a pol- 

 arising solar eye-piece, are to 

 be mentioned as his inventions. 

 In the recent address of the 

 President of the British Asso- 

 ciation, at York, a passing re- 

 ference was made to Mr. Chris- 

 tie's work in verifying the results obtained by Dr. Huggins, 

 with regard to the motions of stars, as inferred from 

 spectroscopic observations. The new Astronomer Royal 

 has directed particular attention, at the Royal Observa- 

 tory, both to spectroscopy and to photography, as a means 

 of recording the observations. He is a fellow of the 

 Royal Society, and was elected Secretary of the Royal 

 Astronomical Society last year. He contributed to the 

 proceedings of the Royal Society, in March, 1877, a paper 

 "on the magnifying power of the half-prism, as a means 

 of obtaining great dispersion, and on the general theory 

 of the half-prism spectroscope." To the monthly notices 

 of the Royal Astronomical Society, he has furnished 

 these : in June, 1873, a paper on the recording micro- 

 meter ; in January, 1874, on the color and brightness of 

 stars, as measured with a new photometer; in May, 1875, 

 on ihe determination of the scale in photographs of the 

 Transit of Venus; in 1876, (January) on a new form of 

 solar eye-piece ; (May) on the displacement of lines in the 

 spectra of stars; (November) on the effect of wear in the 

 micrometer screws of the Greenwich Transit Circle; same 

 year (December) on the gradation of light on the disk of 

 Venus; in 1878 (January) on specular reflection from 

 Venus; (June) on the existence of bright lines in the 

 solar spectrum ; in 1879 (January) on a phenomeno 



seen in the occultation of a star by the moon's bright 

 limb; in 1880, November, on the spectrum of Hartwig's 

 comet of that year ; in 1881 (January) on Mr. Stone's al- 

 terations of Bessel's refractions; (May) on the flexure of 

 the Greenwich transit circle, and some further remarks 

 on Mr. Stone's alterations of Bessel's refractions; besides 

 various papers on the Greenwich spectroscopic and pho- 

 tographic observations, communicated by the late As- 

 tronomer Royal ; and a paper which will be found in the 

 Memoirs of the Royal Astronomical Society, published 

 in January, 1880, on the systematic errors of the Green- 

 wich North Polar distances. Mr. Christie is also the 

 founder and editor of a journal entitled " The Observa- 

 tory, a Monthly Review of Astronomy," which has been 

 published during the past four years ; and he is author 

 of the "Manual of Elementary Astronomy," published 

 in 1875 by the Society for Promoting Christian Knowl- 

 edge. 



William H. M. Christie. 



ON THE ELECTRIC CONDUCTIVITY AND 

 DICHROIC ABSORPTION OF TOURMALINE* 

 By Prof. Silvanus P. Thompson. 



The electric conductivity of 

 tourmaline differs in different 

 directions ; being, according to 

 the author's experiments, a 

 minimum along the optic axis. 

 Tourmaline also possesses the 

 optical property of dichroism, 

 its absorption being a maxi- 

 mum for rays parallel to the 

 axis, and greater for blue rays 

 than tor red, equal thicknesses 

 of crystal being considered. 

 According to the electro-mag- 

 netic theory of light, bodies 

 which are good conductors of 

 electricity should be opaque 

 to light. The author has in 

 the August number of the 

 Philosophical Magazine re- 

 written the equations of Max- 

 well's electro-magnetic theory 

 for the case of crystalline 

 media possessing different con- 

 ductivities in different direc- 

 tions. From these equations 

 it appears that in tourmaline 

 and negative uniaxial crystals 

 electric displacements at right angles to the axis will be 

 more absorbed than electric displacements parallel to the 

 axis. This accounts for the well-known greater absorption 

 of the ordinary ray, provided the views of Stokes and Fres- 

 nel are correct, that these displacements are at right angles 

 to the so-called plane of polarization. The difference 

 of velocity between the rays of different color ac- 

 counts for the difference of absorption being greater 

 in that direction in which the conductivity is a min- 

 imum. It was also pointed out that in positive un- 

 iaxial crystals, in which the electric conductivity is a 

 maximum along the axis, there will be maximum absorp- 

 tion of the extraordinary ray, and there will be least 

 opacity along the axis. Smoky quartz and magnesic pla- 

 tinocyanide fulfil the latter condition. Specimens ot 

 tourmaline cut into cubes to show the colors in different 

 directions were shown, and also specimens of magnesic pla- 

 tinocyanide and of herapathite. Mechanico-optical models 

 were also shown illustrating the theory ; a tourmaline 

 being represented by a cube built up of layers of glass 

 and wire-gauze. In conclusion it was shown that crystals 

 in which the electric conductivity differs in three different 

 directions will exhibit trichroism; and that di- or t ri- 

 ch roic absorption is a general property of all coloied 

 crystals other than those of the cubical system. 



* British Association, 1881. 



