December 1, 1892.] 



KNOWLEDGE 



229 



the moths, have the body tenninating in a single point, 

 ■while still others, the " prominents," have the usual pair 

 of terminal claspers, but make no use of them, seeming as 

 though they were gradually losing the power of doing so. 



(To be continued.) 



PHOTOGRAPHS OF SWIFT'S COMET. 



By Prof. E. E. Barnard, of the Lick Observatory. 



THESE three photographs, selected fi-om a series 

 and enlarged 2^ times, clearly show the remark- 

 able changes the comet was subject to when near 

 perihelion. A brief description of the pictures, 

 pending a more detailed discussion, may not be 

 out of place here. 



These were made with the (> in. Willard lens, strapped 

 on to the tube of the Oi in. equatorial, which was used as 

 a following telescope. The focus of the Willard lens is 

 31 inches. As the comet was moving rapidly among the 

 stars, the clock-work, which corrects for the ordinary 

 diurnal motion, would not serve to follow the comet. The 

 nucleus was therefore brought to the intersection of cross 

 wires, a high power eye-piece being used on the 6i in., 

 and the telescope was constantly shifted by hand with the 

 slow motion rods, so that the nucleus was always kept 

 perfectly bisected, the clock simply correcting for the 

 diurnal motion. The light of the comet seems to have 

 been very strongly actinic, as will be seen fi'om the amount 

 of detail shown with comparatively short exposui-es, very 

 little of which could be seen with the eye and telescope. 

 Cloudy weather interfered greatly throughout the time of 

 the comet's greatest brightness, -\fter a long cloudy spell, 

 I had examined the comet on April 3rd, when its tail 

 was traceable for 20- with the naked eye. The 12 in. 

 achromatic showed the tail, near the head, to consist of 

 two thin streams of slightly divergent light, with scarcely 

 any nebulosity between. There was certainly no third 

 stream. 



The Photographs. 

 April 4th, l-5h. 30m. — 16h. 30m. It will be seen in this 

 picture that a third branch had .made its appearance since 

 the 3rd, in between the two previously seen. This central 

 branch is shown in the photograph to be crested with a 

 fine bright Une, more or less cun^ed and broken. For a 

 short distance from the head a second line lies close by 

 and parallel to it. To the north of the northern branch, 

 there are one or two hair-like, dark spaces, which appear 

 darker than the sky near ; they are doubtless simple rifts 

 in faint diffused nebulosity surrounding the comet. It 

 will be seen that the tails of the comet readily break up 

 into quite a number of separate branches at some distance 

 from the head. The star trails, representing the direction 

 and amoimt of motion of the comet during the exposure, 

 are pear-shaped in this picture. This is due to the fact 

 that the exposure was begun when the comet was near 

 the horizon and the light of the stars was more or less 

 absorbed by the dense atmosphere. This peculiarity 

 serves to mark the beginning of the exposure and the 

 direction of motion of the comet, the motion being in the 

 direction of the large end of the trad. 



April 6th, 1.5h. 85m. — 16h. 40m. This picture shows 

 the remarkable changes undergone by the comet in the 

 short interval of two days. There is very httle or no 

 resemblance in the comet for the two dates. It will be 

 seen that the separation in the taU makes a rather quick 

 bend near the head. There are unequal bright areas in 

 the main taU, some of which are suggestive of the remark- 

 able changes seen in the picture for AprO 7th. There is a 



little incident connected with the history of this photo- 

 graph of the 6th of April that may be of popular interest. 

 On this date it was necessary for me to go to San -Jose to 

 fulfil an engagement to lecture that night (being one of a 

 series of six lectures for the University extension). After 

 the lecture, at lOh. 30m. p.m. I hired a horse and buggy 

 and drove up the mountain, 27 miles, amving at the 

 observatory at 2.30 a.m., and between that time and dawn 

 secured this photograph. On this date, to the naked eye, 

 the tail was about 2.i^ long, the head being equal to a star 

 of the third magnitude. 



April 7th, loh. 4.5m. — 16h. 3om. This photograph was 

 made partly in moonlight and partly in dawn. I need 

 scarcely call attention to the unique appearance of the tail 

 on this occasion. There is still less resemblance to the 

 former appearance of the comet in the short interval of 

 twenty-four hours. A number of narrow branches are now 

 shown in the tail near the head, the middle or main one of 

 which has a curve in it some distance from the head. The 

 most remarkable and unique phenomenon, however, is the 

 apparent development of a secondary comet 2° behind the 

 head in the main branch. There is at this point a great 

 enlargement or swelling, which is gradualh" brighter in 

 the middle, and from which a new system of tails seems to 

 branch out. A photograph made the next morning iu full 

 moonlight showed only a portion of the tail close to the 

 head, the sky being too bright for the photography of faint 

 objects. The scale of these enlargements is 1 inch = 

 1-2=. 



It seems to me that these photographs are a revelation 

 to us. We are familiar with the rapid changes that 

 comets sometimes undergo, but if these three pictures, so 

 close together in point of time, had been drawn by the 

 most competent observer, most astronomers would pro- 

 bably have attributed their remarkable differences to the 

 unskilful hand of the artist, for there is absolutely no 

 resemblance among them. The accuracy of the photo- 

 graphic plate, however, is unquestionable, and these 

 pictures therefore give us an insight into the rapidity and 

 vastness of cometary changes little dreamed of before. 



In examining this series of photographs the idea has 

 been very forcibly impressed upon me that there was 

 possibly, in the case of this comet, a rotation of the taU 

 upon an axis through the nucleus, in a comparatively 

 short period. It is to be regretted that cloudy weather 

 broke the series to such an extent that it is not possible to 

 settle this question ; still it is a point that must be closely 

 looked after in our next large comet. 



Mount Hamilton, August 31st, 1892. 



ON THE FORMS OF COMETS' TAILS. 



By A. C. Eanyard. 



THE beautifol photographs, for which we have to 

 thank Prof. Barnard, show some remarkable 

 irregularities in the streams of matter driven away 

 from the nucleus and from the sim. There is not 

 merely a variation in the brightness or density of 

 different parts of the tail, such as might be caused by the 

 matter being driven away from the nucleus in varying 

 quantities at successive instants, but the edges of some of 

 the streamers appear to be distmctly curved or bent Ln an 

 irregular manner. This is particularly noticeable in the 

 southern edge of the great streamer, photographed on the 

 6th of .\pril. 



We do not know how far the photographic record 

 obtained may be affected by the motion of the matter of 

 the tail during the 65 minutes for which the photographic 



