r)?c . 1904.1 



KNOWLEDGE iS: SCIENTIFIC NEWS. 



2<S7 



I'jK i ..ii,.ii, lui ^Iculnlcss this possibility has received 

 attention. 



Liimierc's Siar:li McthoJ of Tlircc-Coloiir Fliolograpliy. 

 — This process, which was described about six months 

 ago, contrasts \cry emphatically with Konij^'s method 

 in the simplicity of the necessary manipulation. NO 

 colour screens or filters are needed, there arc no films 

 to stain, no colours to produce of the correct intensity 

 to match one another, no separate nesjatives \\ ith sub- 

 sequent printinsjs, but merely one exposure, ordinary 

 development, and then, instead of fixint;, the silver 

 image is dissolved out and the remaining siKer salt 

 reduced to the metallic state. Rut if the work of the 

 photographer himself is simple, it is because of the 

 complex character of the prepared plate, and pre- 

 sumably it is the difficulties of manufacture that have 

 led to the delay in putting the prepared plates on the 

 market. The plates are made by selecting starch 

 granules of from 15 to 20 thousandths of a millimetre 

 in diameter, staining quantities of them red, green, and 

 violet respectivelv, dr\ing them, mixing them so that 

 neither colour predominates but that the whole pre- 

 sents a neutral gray tint, and spreading the mixture on 

 glass one layer thick. The interstices are filled in with 

 a fine black powder, and the layer is fixed and pro- 

 tected by a coat of varnish. On this is put a film of 

 suitably colour-sensitized emulsion. Exposure is gi\ en 

 through the glass, and the subsequent treatment of 

 the plate is as described above. The dyed starch 

 granules form an irregularly grained three-colour 

 screen, which ser\es the double purpose of taking antl 

 viewing. 



It is easy to describe such a process, Jjut besides the 

 obvious mechanical difficulty of prepiiring the plates, 

 there must be many compromises made before the re- 

 sult can be passablv satisfactory. The best three 

 colours for the exposure are not the best three for 

 viewing the picture, but in this case they have to be 

 the same. If the stained starch granules are mixed 

 to the most neutral tint possible, it appears that a 

 perfectly orthochromatised sensiti\e film would be 

 necessary. The imperfections of the film in this 

 matter must be neutralised as far as possible. Indeed, 

 the difficulties of which the photographer is relieved 

 have to be overcome by the manufacturer, and in this 

 particular case they are so many and complex that if 

 it had not been stated that results have been obtained 

 in the manner described, we might \erv well doubt tin- 

 possibility of it. 



Lantern Demonstrations. — Optical lanterns are so often 

 unskilfully used, even on occasions when the best 

 methods of demonstration might well be expected, and 

 .sometimes when they are handsomely paid for, that I 

 wish to take the present opportunity of calling atten- 

 tion to one matter now, and shall refer to other matters 

 at a subsequent dale. .\t scientific lectures it is often 

 necessary to introduce a small piece of apparatus on to 

 the stage of the lantern, such as an electroscope or 

 thermometer, for example, in order that its changes 

 during an experiment may be clearly seen by the audi- 

 ence. The lecturer must have ready access to the 

 lantern to superintend or conduct the experiment. 

 When there is no special provision for such demonstra- 

 tions, the usual way is to fix up a sheet at the back of 

 the platform, and to have the lantern at one side near 

 the front of it. The lantern has to be tipped up to get 

 the image above the level of the lecture table, its stage 

 therefore, is sloping, and apparatus put on it is very 

 likely to shift its position, if not to fall over; the 

 lecturer when at the lantern is sure to be between the 



sheet and some ol tin- autlience, how L'ver he may contort 

 his body to get it out of the wa\; and as the lantern 

 is tipped up, and generally to one side of the sheet, the 

 disc of light is far from circular, antl it is impossible 

 to focus more than a small p.iit of the nbjccl, even 

 when it is fiat, at any oni' lime. All llusi' and other 

 .innovances m;iy be oxercome by the use of a small 

 translucent screen on the lecture-table with the l;nilern 

 centrallv placed behind it so as to gi\e a two or three 

 foot disc, taking care that the lantern is [)ro|)erly pro- 

 tected bv screens to avoid the possibility of .anv light 

 that inav leak out from it glaring into the eves of the 

 audience. The lectmcr would find this arrangcnu-nt 

 much more conxcnient, .and the audience wotdd sc;' the 

 projected ima^ge much more clearK-, the l.ict that It 

 would be smaller than olhciwisc usual liciiig an un- 

 qualified advantage. 



Phoebe, Satvxrrv's Ninth 

 Satellite. 



Hv A. C. I). ('K().\nn I i\. 



Tiil-KK is no question that the disi-overy of I'hoebe 

 reflects the greatest credit on Prof. W. 11. Pickering. 

 It was no mere accident, but the result of a (U-liberate 

 sL-arch for additional satellites which he has been 

 carrying on for manv years, h'ven after the existence 

 of the satellite is known it is :i tedious matter to 

 identify it on a photograph, but to lia\e disc-overed it 

 in this way — one little grev dot among myriads of 

 others — is, indeed, astonishing. I'rof. Pickering ex- 

 plains the long delay in the contirniation of the origin.il 

 announcement by the fact that his attention was called 

 oft by his photographic work on the moon. In .iddi- 

 tion to this the unexpectedly large eccentricity of tlu- 

 orbit {q.12, foiu' times that of our moon) made it iiuiih 

 more difficult to detect Phoebe on the [jiates taken at 

 .\requipa in 1900. 



.At length the idea occurred to him to extend the 

 search to a greater distance from .Satiun than he had 

 hitherto thought sufTicient, and thert% in fact, I'hoebe 

 was found, near elong.ation, some ^^t' from its 

 primary, indicatini; a dislance vA nine millions of miles. 



It v\as not till a few months ago that the most 

 surprising leature of all — the retro<;rade motion round 

 .Saturn — was (lisco\ ered. 



It is well known that in doubl • star orbits it is fre- 

 quently impossible to say whether the upper or the 

 lower half of the orbit is nearer to us ; in the case of 

 some bright stars, like .Sirius or a Centauri, the 

 spectroscope has settled the matter, but in other cases 

 it remains insoluble. In the same way we could not 

 tell from a single year's observations of Phoebe whether 

 its motion was direct or retrograde, and it was e\en 

 found possible to construct ;i direct orbit which would 

 represent the observations of 1898 .and igoo without 

 any large errors. Hut in 1904 the position of Saturn 

 has ch.anged so much that the direct and retrograde 

 orbits are at once distinguishable, just as the question 

 v\ould be settled in the case of a double star il ue 

 could transport ourselves to ;i \ery distant standpoint, 

 where we might view the system in another direction; 

 and it is to be noted that the retrograde motion is 

 indicated in two entirely different ways : (i), by com- 

 paring the position of the Perisaturnium, f)r nearest 



