Januarv 1, 1895.] 



KNOWLEDGE 



13 



The entire spectrograph, the two slits of which are 

 firmly connected together, moves before the motionless 

 image of the sun. It is driven by a clepsydra, and 

 transmits a proportional movement to the sensitive plate 

 by a single system of levers. 



This arrangement has many advantages : 



1. The small dispersion of the spectrograph, which is 

 a necessary condition from the optical point of view, 

 permits the greatest delicacy of detail to be obtained in 

 the image, together with the exact intensity of the chromo- 

 spheric flames, and also allows the flames of the centre, 

 and the flames of the disc near the limb, which may be 

 lost with a greater dispersion, to be equally well represented. 

 2. With the two slits fixed together, it is easy to obtain 

 circular and not elliptical images, and there is no need to 

 give the second slit a greater breadth than that of the line 

 which it is desired to isolate. 'A. The great focal length 

 of the objective of the spectrograph makes the aajnstment 

 of the second slit easier, and gives an enlarged image 

 without loss of light. 



These photographs do not represent the faculre, the 

 images of which are slightly diiferent, but the chromosphere 

 itself above the solar disc. 



The boundary of tlie photographs of the entire disc is 

 not the true limb of the sun, but a slightly greater circle 

 which includes the lowest and most intense parts of the 

 chromosphere. When the exposure is sufficiently long, as 

 in the present case, the prominences of strong and medium 

 intensity also appear on the plate. They are better seen 

 on the original negatives. 



The two photographs showing the limb were taken with 

 a longer exposure, and show all the prominences, and all 

 parts of the chromosphere above the middle layer. The 

 disc of the sun has been covered by a diaphragm, slightly 

 smaller than it in diameter, to avoid diffusion of light. 



PERIODICAL COMETS DUE IN 1895. 



By W. T. Lynn, P..A., F.R.A.S. 



TWO comets of short period are due to return to peri- 

 helion in the course of the present year ; but whereas 

 one of these, which is in view whilst we write, has 

 been seen at no fewer than twenty-six returns and 

 consecutively since that of lSlS-19, when it acquired 

 its name from that of the illustrious astronomer who 

 investigated its motions and calculated its orbit, the 

 other has hitherto been seen at only one appearance. 



The first comet is, of course, our old friend Encke, 

 •which was first discovered by Mechain at Paris on 

 the i7th of -January, 1786, and first seen at the present 

 appearance on the 31st of October, IkOI, at Nice, being then 

 in the constellation Pegasus, very near the place predicted 

 for it in the ephemeris of Dr. Backlund {Astrunomischr 

 Naclirichtin, No. 8203), who, we regret to notice, states that 

 this, is the last time that he will be able to undertake its 

 calculation. The comet, he finds, will pass its perihelion 

 on the 4th of February : the last time it was in that 

 position was on the 18th of October, 1891, on v.'hich 

 occasion it made one of its very near approaches to the 

 planet Mercury. 



The other comet due in IS'.io was discovered at its 

 first appearance on the 16th of July, 1881, by Prof. 

 Barnard, now of the great Lick Observatory in California, 

 but who was then at Nashville, Tennessee. Although 

 thus discovered in the northern hemisphere, the comet 

 remained throughout that appearance in the southern. 

 Dr. Gill and his assistants afterwards obtained a number of 

 observations of it at the Cape of Good Hope, and it was 



also observed at Melbourne and other places, but was 

 always very faint and diificult of observation. Its orbit was 

 determined by Herr Berberich to be one of short period, 

 amounting to only about five and a half years ; but its 

 position at the return expected in the winter of 1880 was 

 exceedingly unfavourable, and it was not seen. Another 

 appearance will be due in the summer of the present year. 

 Herr Berberich calculates (Astrdnoinisilw XacliricJUen, No. 

 3260) that it will then be somewhat brighter than at the 

 return when it was discovered in 1884, and that its 

 perihelion passage will probably take place on the 3rd of 

 .June. Its distance from the sun, when least, is nearly 

 equal to that of Mars. 



Urttrrs. 



[The Editor does not hold himself responsible for the opinions or 

 statements of cori'espondents.] 



• 



61 CVGXI. 



To the Editor of Kxo^'ledge. 



Sir, — There are some points connected with these stars 

 which I think astronomers have not sutHciently borne in 

 mind. They must be physically L-onnected. In one sense, 

 of course, all stars are physically connected ; but in this 

 case it seems certain that the two stars are at no great 

 distance from each other, and that no other bright star is 

 situate within a considerable distance of either. (There 

 may be dark stars.) This is proved by their parallaxes. 

 These have been so frequently determined, with but small 

 difl'erence in the results, that we can say with tolerable 

 certainty that it lies between 0-4" and 0-5". And it appears 

 to be almost the same for both stars. Consequently, one 

 of them does not lie at any great distance beyond the 

 other ; indeed, it is doubtful which is the more distant. 

 But two stars with nearly equal parallaxes of (say) 0'4", 

 and an angular distance of (sa\) 20", cannot be very 

 remote — hardly as much so, I think, as some of our known 

 periodical comets when at their greatest distances from 

 the sun. And as these comets accompany the sun in its 

 flight through space, so the pair 61 Cygni are evidently 

 accompanying each other in a very rapid flight in which 

 no neighbouring star partakes. Like the comets, they 

 may move in very elongated ellipses or, possibly, even in 

 hyperbolic orbits. But the motion can hardly be parabolic 

 unless the combined mass of the stars is very small, for 

 their relative motion is not considerable. For the same 

 reason, a very elongated ellipse is unlikely. The apparent 

 elongation is probably due, to a considerable extent, to 

 projection. 



Should Dr. Wilsing's results prove correct, we shall 

 have one explanation of why a satisfactory orbit has 

 not hitherto been computed. The distances vary periodi- 

 cally from some cause independent of the orbital motion 

 which we are trying to compute, and the law of description 

 of equal areas in equal times is probably not fulfilled. If 

 one of the pair has a dark satellite, it is the centre of 

 gravity of the star and satellite which describes equal 

 areas in eijual times round the other star, and otherwise 

 fulfils the condition of the orbit. But, apparently, Wilsing 

 found nothing but this oscillation in the distance. The 

 mean distance seemed to have remained constant during 

 his observations. If so, the orbital motion of the pair 

 may be regarded as almost certain. Their distance has 

 undoubtedly been increasing, and if the motion were recti- 

 linear it would continue to increase. If the increase has 

 been arrested the motion is orbital, and the distance is 

 about to commence diminishing. A few years will tell. 



