220 



KNOWLEDGE & SCIENTIFIC NEWS. 



[Sept., 1904. 



much need for the yellow screen at sunset as at midday. 

 The changes in light obviously affect all coloured objects. 

 If a photograph in natural colours is made so success- 

 fully that it is an exact reproduction of the original as 

 seen by full daylight, it may be different from the 

 original when they are compared by evening light, be- 

 cause the colours of the photograph are only imitations 

 of those of the object, and they may, and probably will, 

 be differently affected by the change in the character of 

 the light. If anyone desires a good illustration of the 

 effect on colours of daylight of different kinds, he has 

 only to get one of the separating black papers from a 

 " premo-film-pack " as supplied by the Kodak Company, 

 and see how the red printing on it appears by ordmary 

 daylight and again by twilight. He may find in the 

 latter case that the inscription has apparently vanished, 

 or, if he can see traces of it, he will probably be unable to 

 decipher it. If the red constituent of the light has gone, 

 a pure red will appear black and be indistinguishable 

 from it. 



These changes in the light that reaches us from the 

 sun are generally ascribed to the terrestrial atmosphere, 

 particularly the aqueous vapour in it, and the fact that 

 the light from the sun has to pass through more and 

 more of the atmosphere as it sinks lower and lower. 

 But the sun also has an atmosphere, and it is possible 

 that variations in this may contribute to the changes that 

 we observe. Professor Langley, who has worked at this 

 subject for about thirty years, especially by means of his 

 bolometer, has recently stated that there is " an increasing 

 probability that the solar radiation itself varies in a 

 degree appreciable to our present means of daily observa- 

 tion, and a strengthening of the belief that it probably 

 varied through much greater ranges in the past, and may 

 do so again in the future." 



Thi Keeping of Sensitive Plates. — The time that sensitive 

 material can be relied upon to maintain its good qualities 

 is of great practical interest. Plates in England, if stored 

 so that they shall be reasonably free from foul air, will 

 last a long time in good condition if the emulsion is not 

 very rapid. I recently had occasion to use some " spec- 

 trum " plates that are six years old, and found that they 

 had a full red and green sensitiveness, that they worked 

 clean, and, generally, were in good condition. They are 

 rather slow, for the most rapid spectrum plates are six or 

 eight times as fast. Slow plates of all kinds, if well 

 made, will keep in good condition for an astonishing 

 length of time. Ordinary fast isochromatic plates I have 

 found when a year or so old to require about double the 

 exposure they did when new, but otherwise satisfactory. 

 The ultra rapid plates, whether colour sensitised or not, 

 should be used as soon as possible after purchase. I 

 have found such plates when a few months old to be only 

 half as fast as at first, and to show considerable fog. It 

 is obvious that a higher degree of sensitiveness must 

 mean a want of stability, for sensitiveness and stability 

 are directly opposed to each other. While, therefore, it 

 is the makers' aim to provide plates that will keep well 

 under all ordinary conditions, the user of them should 

 bear in mind that high sensitiveness in plates means that 

 they are affected by very feeble forces, and as it is im- 

 possible to keep them isolated from adverse influences 

 whatever care is taken in their preservation, the more 

 sensitive a plate, other things being equal, the shorter its 

 life. 



The Thornton I'ickard Co. has sent us a prospectus of their Annual 

 Competition, open to users of their apparatus. The prizes this 

 year consist of twenty equal amounts of £i in cash. The Com- 

 petition closes on October i, End full particulars and entry forms 

 may be had free on application to them at Altrincham. 



ASTRONOMICAL. 



The Ninth Satellite of Saturn. 



It will be remembered that five years ago Prof. W. H. Picker- 

 ing announced the discovery of a new and faint satellite of 

 Saturn with a period of about a year and a half. The satellite, 

 to which he gave the name of Phcebe, was discovered upon 

 photographs taken with the 24-inch Bruce telescope. Eleven 

 photographs, taUen by Mr. Frost at the Arequipa Observa- 

 torv, under the direction of Prof. Bailey, have enabled Prof. 

 Pickering to follow the satellite from April 16 to June 9 of the 

 present year, and to correct its epbemeris ; and a full discus- 

 sion of its orbit will appear in a few weeks, in a forthcoming 

 volume of the Annals of the Harvard College Observatory. 



Comet 1903 (Borrellyi and Light = Pressure- 

 in a paperin the " .Astrophysical Journal " forJuly.Mr. S. A. 

 Mitchell deals with the question of the formation of cometary 

 tails by the influence of light-pressure. The researches of 

 Bredichin had shown comets' tails to be of three different 

 tvpes according to the intensity of the repulsive forces which 

 Bredichin explained as electrical in nature. This Lebedew 

 showed not to have a sound physical basis, but .-^rrhenius has 

 recently substituted the pressure of light. For a little cube of 

 water with an edge of one micron, the pressure of the sun's 

 light on it, at the sun's surface, is exactly equal to its weight ; 

 for a smaller cube the pressure would he greater than the 

 weight, and hence the particle would be repelled. Measures 

 of the angles between the tails of Comet Borrelly and its 

 radius vector, made by Mr. Sebastian .Albrecht on thirty-two 

 photographs taken between June 22 and .\ugust 18, J903, gave 

 somewhat discordant results for the principal tail, but the 

 mean of the best values gives the repulsive force as i8'47 

 times gravity. The values for the secondary tail agreed much 

 better, and their mean was i'S24; the last four values gave a 

 mean of 1-460, seeming to show the existence of a third tail, 

 and this appeared to be corroborated from the photographs of 

 August 12 and 15. The size of the particles forming the tails 

 would be respectively o-i, i, and 1-33 microns. Mr. Mitchell 

 concludes that there seemed to be a lagging even behind the 

 direction given by the repulsive force : in other words, that 

 the value of the repulsive force may increase as the comet 

 approaches the sun. This increase, he considers, is in part at 

 least real, and due to the more violent action of the gases 

 liberated as the comet approaches the sun. 



The Position of the Galactic Plane. 



A most important and lucid paper by Professor Simon New- 

 comb has been published on the position of the galactic and 

 other principal planes toward which the stars tend to crowd. 

 He states the problem thus : •' It is well known that the sky 

 appears to us poorest in stars in the regions around the poles 

 of the galaxy, and that it continually grows richer at a rate 

 which is slow at first but more rapid afterwards, from the 

 poles to-vard the galactic circle." Within the galactic girdle, 

 the thickness of the stars in space is approximately constant, 

 but in the Milky Way itself it is obvious that it consists of 

 agglomerations of stars which have often fairly well defined 

 boundaries ; the stars here are much thicker than outside the 

 girdle. The chief object of this paper is to determine this 

 principal galactic plane, and abo to determine whether the 

 non-galactic stars condense towards this same plane or towards 



