536 



. KNO^A;■LEDGE • 



[April 21, 1883. 



SOLAR MOTION AND COMETS FROM OUTSIDE. 



AN inKri'miiiK ciiimtioii is nuKKUxloJ ''X ll>0 ttppriioch of tlm 

 runiiiiK comet, oiio tlint Mr. rpictor cnn cluriilatK to tlie 

 rcailcrii of Knowlehok, niicl tlicroliy nolvc n |)rol>Ioin tliiit jinn pro- 

 biibly iiuKRi'atiHl iltielf to olliom lioHiilpii inygolf. It is thin: — 

 8upp(«inK ••">!. tliiH coiiiot (or niiy other) is a visitor to our Bolnr 

 »)'iitein from the outer n-Kiou" of ppiire, and is now moving,' in ii 

 direction thut will bring it within, fny, 10 millions of miles of the 

 prtftnt position of the sun by the middle of June, where will it 

 bo in relation to the sun nt that date, assuminjf that the sun is 

 travelling through space with his regular attendants at the rate of 

 nearly GtlU.dUO miles jier day? One of the components of the 

 comet's orbit, the solar gravitation, will, of course, change its direc- 

 tion with thntuf the sun, but the other, the comet's original motion, 

 remains of its original component value. 



llow do astronomers deal with this (|iie8tion, and docs the courso 

 of such a comet verify the theory of solor motion in space? 



W. Mattieu Williams. 



I The astronomer can only judge of o comet's courso by observa- 

 tions made from the earth, which shares w^hatover motion the sun 

 may have in space, lie finds the comet's orbit relatively to the 

 sun' to bo such and such, and ;whether the sun bo at rest or in 

 motion, the movements of the comet with respect to the sun and 

 solar system will be the same. If, however, wo imagine an observer 

 in space knowing the exact courso a comet is pursuing, that is 

 the exact direction and rate of the comet's motion, and knowing 

 niso tlio exact position of the sun, bot not knowing anything 

 about the snn's motion in space, it is certain that the 

 calculations of such on observer as to the comet's position 

 at any given future time, would bo altogether incorrect if 

 he assumed the sun to be at rest, and the sun is really 

 moving very rapidly through space. Ho would make no cor- 

 rection for that jmrt of the comet's motion at the moment, 

 which is equal and parallel to the solar motion, and which, there- 

 fore, must be regarded as removed before the computation of the 

 comet's courfc with respect to the san can be taken into account. 

 Thus, suppose the comet moving from A to C (Fig. 1) in space, while 

 the sun is moving from S to S' in tlio same time (if both continued 

 on an unchanging coui-se), and let CB be parallel and equal to SS'. 

 Then our imagined observer would calculate for the velocity and 

 direction represented by AC, and get a very different orbit round S 

 than ho would if, knowing of the sun's motion, ho took the right 

 Telocity and direction, viz., that represented by the line AB. The 

 observer on earth cannot make this mistake, for whether SS' be 

 large or small, and whatever its direction, the terrestrial observer 

 can only recognise the velocity and direction AB, that is the 

 comet's actual velocity and direction, corrected for the sun's. 



Fig. 1. 



Fig.: 



Another difficulty which is, I think, commonly ctperienced, is 

 this. If we imijgine a comet leaving the domain of a sun moving 

 in one direction, or perhaps nt rest, and coming within the effective 

 influence of our sun moving in another dii-ection, how docs the swift 

 motion of the snn away from the position it had had at first with 

 respect to the comet, affect the comet's eventuolcf ure ? In reality, 

 this is the same difficulty as the other. It matters not uhcre the 

 comet is when it has the motion represented by AC in our figure, or 

 how that motion was originally acquired, or tchat the direction and 

 amount of that motion may be ; that motion, corrected into motion 

 AH by the application of a motion CIJ parallel and opposite to the 

 sun's, is the motion tletermining the c*>met'8 courso with respect to 

 the sun. Of course, when the comet is e(|ually, or nearly equally, 

 attracted by the sun it ia leaving, and by our own sun, the former 

 attraction must bo taken duly into account in estimating the future 



courso of the comet. But supposo onr sun, for a momoiit, the odIj 

 sun in existence, and imagine that a c<imct is placod at rest at A 

 (Fig. 2), and left free to yield to solar ottract on. liow will it 

 behave if the snn, instead of being at rest at S. is in'iving in direc- 

 tion SS', with velocity represented by the length SS'. We apply to 

 the comet thn motion AA', eqaal and parallel to SS', but in the 

 opposite direction, and so learn the nature of the comet's orbit with 

 reference to H. It will be either a parabola or a hyperbola, having 

 vertex at A ; or an ellipse, having A as an apse (either perihelion 

 or aphelion); or a circle, according to the distance AS ond the 

 velocity AA' or SS' ; the real path of the comet in space will, of 

 course, be obtained by combining the comet's motion in tlis poth 

 round S with the advancing motion of S, and will bo either a 

 skewed parabola or hyperbola, or a series of looped or wavy cnrveg. 

 These if the comet's relative path is circular, will l>e curtate 

 or prolate cycloids, according to tho sun's velocity ; or in one par- 

 ticular case will bo the common cycloid. — En."! 



OPTICAL BLINDNESS TO RED LIGHT. 



A CURIOUS effect of bright white light upon the vi ion is 

 recorded in a recent number of the Journal de I'h^fique by 

 MM. J. Mace de Lcpinay and W. Nicati. After passing some hours 

 in a snow-field brilliantly lighted up by sunshine, it was observed 

 that at least eight hours afterwards all gaslights, candles, and arti- 

 ficial lamps appeared to be strongly coloured green. In other words, 

 the rod rays of such lights were not jicrceived. Tho reason of this 

 was supposed to be the fatigue of the retina for red, which partial 

 effect lasts longer than a similar weariness of other colours. The 

 truth of this supposition may be proved in a very simple manner 

 by obtaining three coloured glasses — red, green, and blue — of such 

 relative depth of colour that they could be seen through with about 

 equal visual effect with a given power of light. An observer fur- 

 ni.slicfl with these glasses is then to place himself at a convenient 

 distance before one of the sight-testing plaoards commonly used by 

 oculists, and consisting of a white ground printed with black cha- 

 racters of various sizes. If the room is now almost darkened, the 

 blue glass will still permit the observer to distinguish the medium- 

 sized characters on tho placard, while through the red screen not 

 even the white sheet itself is perceptible. After a time, however — 

 the same degree of semi-darkness being continued — the visual 

 acuteners through the red glass is increased, so that the larger 

 characters on the placard may be discerned. The visual percep- 

 tion through tho blue glass remains as at first. It is therefore clear 

 that colour-blindness, of a temporary nature, to tho red rays, is 

 more persistent than in respect of the blue rays. Hence may be 

 assigned to physiological reasons the well-known fact that a pro- 

 longed or even tempoi'ary exposure of the eye to the electric light 

 renders it for some considerable time afterward incapable of fully 

 estimating the illuminating power of a gas-flame, which is so much 

 richer in red rays. 



RED SNOW. 



AT a recent meeting of the San Francisco Microscopical Society, 

 Dr. Harkness presented a bottle of "red snow," which he 

 gathered last June on the Wasatch Mountains. The red snow was 

 found on the north side of a spur which rose about 10,000 feet 

 above tho sea level. When fresh, the snow has the appearance of 

 being drenched with blood, as tbongh some large animal had been 

 killed. Tho *' red snow " is caused by tho presence of a one-celled 

 plant called Protococcus nii-alis, which reproduces itself by sub- 

 division ; that is, the cell divides itself into several new cells. This 

 is done with great rapidity, and a few cells lodged in the snow, 

 under favourable conditions, soon will give it the appearance called 

 "red .«now." It was remarked that the iilienomena of red snow 

 had been observed from the earliest times, as Aristotle has a passage 

 which is thought to refer to it. The subject was, however, lost sight 

 of until brought up by the investigations of Saussure, who found it 

 on the Alps in 1700. He made chemical tests which showed him 

 that the red colour was due to the presence of vegetable matter, 

 which he supposed might be the pollen of some plant. In 1S19. an 

 Arctic expedition under Captain Ross brought some six^cimens from 

 the cliffs around Baflin's Bay, and they were examined by eminent 

 botanists, some of whom mistook the nature of the pli:nt,and there 

 was long discussion as to its proper classification, some holding it 

 to be a fungus, some a lichen ; but it was finally set at rest as one 

 of the unicellular alga?. It ia of interest also that soaio of the 

 early examiners pronounced the colour due to animalcules, but this 

 was disproved. Dr. Harkness said that during his last visit to 

 England, ho saw the original bottle of specimens brought from the 

 Arctic more than sixty years before, and in which tho protococcus 

 could still bo seen with the microscope. — Scicntifc American. 



