496 



NATURE 



\_Sept. 



1880 



1567, which varies from 5J to 64, is excessively red ; ir' Gruis is 

 of .1 deep crimson, while its neighbour ir" is conspicuously white ; 

 magnitudes respectively 67 and 5-9. The star No. 10163 of 

 Oeltzen's Argelander is remarkable for its superb crimson colour ; 

 its place for 1S75 is in R.A. gh. 45m. iSs., N.P.D. 112° 25'.' 

 /u Musc:e (5'3m.) is intense orange-red. 



7 Centauri, varying between 4-5 and 5-1, appears also to vaiy 

 in colour, having been repeatedly noted as reddish, while at 

 other times it was found without any marked tinge. N Velorum 

 which has a peculiar yellow colour, contrasting markedly with 

 that of the numerous red stars in its vicinity, is suspected to vary 

 in colour as well as in brightness, the period of the variations 

 appearing to be not far from 4J days, though the number of 

 observations is not sufficient to give the law of the fluctuations. 

 The positions of these stars for 1875-0 are: 7 Centauri, R.A. 

 I3h. 42m. :3s., N.P.D. 123" 49'-5; N Velorum, r!a'. 

 9h. 27m. 25s., N.P.D. 146' 29' -o. 



Dr. Gould says that there is " a decidedly greenish tinge to 

 the light of ;8 Libra;, although its colour cannot properly be 

 called conspicuous ; " this confirms Smyth's judgment on its 

 tinge — pale emerald. 



Telegraphic Determinations of Longitude. — The 

 Hydrographic Office at Washington has published a number 

 of geographical positions determined in 1S7S and 1879 liy 

 parties under the direction of Lieut. -Commanders F. M. Greeii 

 and C. LI. Davis of the United .States Navy. The longitudes 

 were fixed by telegraphic exchanges of time-signals, the initial 

 point of measurement being the meridian of the Royal Observa- 

 tory at Greenwich. The latitudes (with the exception of Lislnn, 

 determined by the Director of the Royal Observatory, Capt. 

 F. A. Oom) result from numerous zenith-telescope observations 

 of pairs of stars. The details of the observations are in the 

 press, and will shortly appear; meanwhile we extract fr.mi 

 No. 59 of the Notices of the Hydrographic Office the positions 

 of the more important points — 



Z«/'i?«— Centre of dome of the Royal Observatory. 



Lat. 38° 42' 3i"-3 N. ... Long. oh. 36m. 44-683. W. 

 Func/ml (Madeira)— Flagstaff of Fort St. Jago. 



Lat. 32° 38' 4"-i N. ... Long. ih. 7m. 35-565. W. 

 Pcrnambuco — Lighthouse near Fort Ricao. 



Lat. S° 3' 22"-2 S. ... Long. 2h. igm. 27-775. W. 

 Bahia — San Antonio Lighthouse. 



Lat. 13° o' 37"-4 S. ... Long. 2h. 34m. 8-375. W. 

 Rio de Janeiro — Centre of dome of Imperial Oljservatory. 

 Lat. 22° 54' 23"-8 S. ... Long. 2h. 5203. 41 -41s. W. 

 Montevideo— CecAx^ of south-east tov.-er of Cathedral. 



Lat. 34° 54' 33" -o S. ... Long. 3h. 44-01. 49-023. W. 

 Buenos Ayres—Ccwixft of Cupola of Custom-house. 

 Lat. 34° 36' 29"-8 S. ... Long. 3h. 53m. 28-955. W. 



Four-Figure Logarithms and Anti-Logarithms.— 

 Messrs. Layton, Fleet Street, have lately published tables of 

 logarithms of numbers to four places from 1000 to 9999, and 

 anti-logarithms -0000 to -9999, arranged by General Hannyng- 

 ton, similarly to the modern six and seven-figure tables. All the 

 figures are printed, and the value sought is consequently found 

 by mere inspection. The logarithms of numbers o to 999'would 

 have requu-ed two more pages only, and probably would have 

 been more used than any of the other pages. A complete manual 

 of four-figure logarithms of numbers and trigonometrical functions 

 IS much to be desired, and would suffice for eclipses, occultations, 

 star-corrections, and many subordinate astronomical calculations ; 

 Jt is to be regretted that the logarithms of trigonometrical 

 functions are not given in the present publication. 



ACTION OF PHOSPHORESCENT LIGHT ON 



SELENIUM 1 

 P^ FEW weeks ago, when listening to Mr. Heaton's lecture on 

 Balmain's luminous p.iint at the Society of Arts, it occurred 

 to mc to try whether the faint light of phosphorescence would 

 exercise any sensible effect upon the electric conductivity of 

 selenium. I lately made some experiments in this direction, f jr 

 which I adopted the following arrangement :— One of Dr. 

 Werner Siemens' selenium [^reparations, of the kind described 

 by rne in vol. vi. of the Society of Telegraph Engineers' 

 Journal, was placed in the circuit of two Daniell cells, together 

 %vith a delicate Thomson's reflecting galvanometer. The sele- 

 ' This article was sent to us by Dr. Obach in April last.— Ed. 



nmm was put into one end of a blackened brass tube, which 

 was placed inside] a dark box provided with a sliding door in 

 front of the open end of the tube. The source of phosphores- 

 cence was a sheet of glass 20 X 30 centimetres, painted at the 

 back with some phosphorescent material. 1 This luminous sheet 

 was placed before the opening of the box, usually at a distance 

 of about 60 centimetres from the selenium plate. 



In the first series of experiments the phosphorescence of the 

 sheet was excited by exposing it to light from different sources. 

 The results were as follows : — 



Description of light used for exciting the 

 phosphorescence of the sheet. 



The sheet was e-xposed to : — 

 Light reflected from ceiling of semi-dark 



room for several minutes. 

 Light reflected from ceiling of moderately 



light room for fifteen minutes. 

 Light of the sky at 5 p.m. for several 



minutes. 



Light from a few inches of bnrning 



magnesium ribbon. 



Sunlight for two minntesexactly. 



Sunlight for five minutes exactly. 



Increase of con- 

 ductivity of the 

 selenium by the 

 action of phos- 

 phorescent light. 



Per cent. 

 0-7 



2-4 



4-6 

 5'i 



7-S 

 6-3 



During these experiments the phosphorescent light acted on 

 the selenium immediately after the exposure of the sheet to light. 

 After a lapse of some minutes the effect was found considerably 

 lessened ; for instance, the third experiment gave only about Jth 

 of the original value five minutes after the exposure. The sheet, 

 exposed to strong light two hours' previously, showed no per- 

 ceptible action on the selenium. The curious fact that the effect 

 is less when the sheet is exposed to sunlight for five minutes 

 than when it is exposed for only two minutes, is probably due to 

 the circumstance that the .f/; ft-/ becomes sensibly warm during the 

 longer period of exposure. 



In the second series of experiments different lengths of 

 magnesium ribbon were burnt in front of the sheet and at a 

 distance of about 15 centimetres from it. The intensity of the 

 l^liosphoresccnce increased with the time of exposure to light, as 

 the following little t.able shows : — 



In the third series of experiments a length of 20 centimetres of 

 magnesium ribbon was burnt in front of the sheet at 20 centi- 

 metres distance from it. The sheet was then, immediately after 

 its exposure to light, placed at distances of 200, 150, 100, and 

 50 centimetres respectively from the selenium. It was found 

 that the effect upon the selenium varied approximately as the 

 inverse distance of the sheet from the selenium plate, or in other 

 words, as the square root of the light intensity. The same rela- 

 tion lias been found by Dr. Werner .Siemens and others for 

 considerably stronger light intensities. 



In the fourth series of experiments the phosphorescent light 

 was made to pass through differently-coloured sheets of glass 

 before acting on the selenium. It v\as found that colourless 

 glass transmitted all the active rays. Blue glass transmitted iths 

 of the total amount ; green glass transmitted f th ; red (almost 

 monochromatic) ; and yellow glass transmitted no perceptible 

 action. 



In the fifth series of experiments the action of the shed upon 

 the selenium was compared with that of a spermaceti candle, 

 the phosphorescence of the sheet being excited by diffused day- 

 light. Two separate sets of measurements with the standard 



'A so-called Alnddiu's lamfi from Messrs. Ihlee and Home, London.] 



