Skptkmber 1, 1900.] 



KNOWLEDGE. 



207 



PRODUCTION OF COLOUR EFFECTS. 



A VERY singuhu' cxperimeut illustrating the pro- 

 duction of colour effects by means of alternate, after- 

 images of black and white, has been devised by Mr. 

 C. E. Benham, of Colchester. It is an extremely simple 

 application of the principle of his already well-known 

 <u-tificial spectrum top, in combination with a wheel of 

 life arrangement. To see the effect a circular piece 

 of card has three slits cut iu it. These slits radiate 

 from the centre, and for the best effect should be within 

 a quarter segment of the circle, and the width of the 

 slits should not exceed the width of the pupil of the eye. 

 Opposite the central slit a black line, dot)<, crosses, or in 

 fact any other design in black, is traced. The whole 

 of the back of the cai'd is also blacked. The most 

 perfect dead bhu'k for this purpose is made by mixing 

 equal parts of methylated spirit and French polish with 



some vegetable black. The disc is suj)ported on a pin 

 passed through the centre and revolved iu a strung liijlit 

 in front of a min-or, the blackened side being towards 

 the eye. On looking through the slits at the image in 

 the mirror, the line, dots, or other design traced on the 

 face will be seen threefold, and the three images will be 

 seen to be coloured. The colours differ with different 

 people, and with different rates of rotation, but in 

 general the effect is red (the lowest), green, and dark 

 blue, when the disc revolves from left to right ; and 

 dai-k blue (the lowest), gi-een, and red, when the 

 disc revolves from right to left. Colours are also seen 

 with other dispositions of the slits, and also when there 

 are only two, or when there are more than three, though 

 it will ba found that the arrangement described gives 

 the most brilliant effects. The more slits, the slower 

 must be the rate of revolution, and vice versa. With only 

 one slit there is of course only a single image, apparently 

 stationary, and uncoloured. Two slits give two images, 

 generalh' red and dark blue, and changing places as to 

 their colour when the direction of rotation is reversed. 



EROS AND THE ASTROGRAPHIC CONFERENCE. 



Foe the fourth time the International Permanent Com- 

 mittee met at Paris on July 19, to discuss the future 

 proceedings of the Astrographic Catalogue and Chart. 

 Of the eighteen observatories which gave promise of 

 support at the inception of the Astrographic Chart, three 

 have not been able to fulfil their promises. These 

 observatories, of the mutable South American Republics 

 of Rio Janeiro, La Plata and Santiago (Chile), it is pro- 



posed to replace in the scheme by those of Perth (West 

 Australia), Cordoba, and Monte Video. The moot 

 questions of the publication of rectilinear co-ordinates 

 with plate constants, or R.A. and declination of stars; 

 of the measuring the diameters of stars by xmie or by 

 measuring screw ; of giving a single exposure of 40 

 minutes or three exposures of 30 minutes each to tlu' 

 second series of chart plates; were still left to the 

 several Directors of the eighteen observers to decide as 

 it pleased them individually. Neither was it decided 

 whether the chart plates should be reproduced by helio- 

 gravure, as is being done by the Paris Observ.itory, or 

 by some less expensive method. For the moment it 

 was resolved that copies should be made, hut tiu' method 

 of so doing was not decided upon. 



But a special committee was also ajipointed to take 

 charge of the planet Eros during its approaching 

 opposition. Tiiis committee consisted of the President, 

 M. Lawy, and of IMM. Andre (Lyons), Bakhuyzen (Ley- 

 den), Christie (Greenwich), Elkiu (Ncwliaven, U.S.A.), 

 Gill (Ca])e of Good Hope), Ilartwig (Bamberg), Prosper 

 Henry (Paris), Trepied (Algiers), and Weiss (Vienna), 

 and they submitted the following resolutions to the 

 general Conference: — 



(I.) It is desirable that the determination of the 

 pai-allax of the planet Eros should be maxle by micro- 

 metric, by heliometric, and by photographic measures. 

 («) By means of observations of the planet made 

 when in the east and in the west of '.he same 

 observatory ; 

 {h) By the co-operation of the observatories of 

 Europe and North America; 



(c) By the co-operation of the observatories in the 

 northern and southern hemispheres. 



(II.) During the periods of observations of parallax, 

 the daily motion of Eros should be determined as exactly 

 as jjossible by niicrometric, heliometric, and photo- 

 graphic measui-es. 



(III.) The Committee recommend to observers of 

 {a) and {b) that they should make measures on every 

 morning and evening available, and to profit by the 

 favourable atmospheric conditions to operate through 

 as large hour angles as possible — this will diminish the 

 error from the uncertainty of the planet's motion, and 

 {r) that the mean times of observation should not gi'eatly 

 differ from the time of meridian passage of the planet 

 at the southern station. 



(IV.) It is necessary to take a special series of photo- 

 graphic plates of the regions surrounding Eros so as to 

 determine the places of the comparison steirs. 



M. Hartwig will draw up a programme for heliometer 

 obsei-vations of the planet; MM. Andre and Prosper 

 Henry undertake to make researches on the atmospheric 

 dispersion ; and MM. Lanvy, Brown (Washington), and 

 Bakhuyzen will undertake the task of executing reso- 

 lutions concerning niicrometric and photographic 

 resolutions. 



The observatories taking part are Algiers, Athens, 

 Bamberg, Bordeaux, Cambridge (England), Cambridge 

 (U.S.A.), Cape of Good Hope, Catania, Cordoba, Chicago 

 (Yerkes), Edinburgh, Greenwich, Heidelberg, Leyden, 

 Leipzig, Lyons, Marseilles, Minneapolis (U.S.A.), Mount 

 Hamilton, Nice, Paris, Potsdam, Rome (Roman College), 

 San Fei-nando, Strasbourg, Tacubaya, Toulouse, Upsala, 

 Vienna (Ottakrino), Vienne (Wahring), and Washington. 



