January, 1903. 



KNOWLEDGE 



its angular speed be proved actually to fluctuate syn- 

 chrouously with the eleven-year wave of solar commotion, 

 an important step will have been made towards unravelling 

 its intricacies. 



The whole subject of solar physics is, year by year, 

 taking a wider scope, and acquiring a deeper significance. 

 The complexities of the lighting and warming apparatus 

 of our system, its self-regulating power, the manifold 

 interplay of its parts, the fine poise of its equilibrium, 

 strike us with increasing wonder as we become more 

 intimately acquainted with them. More and more keenly, 

 too, we feel our ignorance as we advance in knowledge ; 

 information on many points is, indeed, still in a visibly 

 rudimentary state, but it is gradually gaining further 

 items, and while ambitious theories are constructed only 

 to collapse, something of value survives from the wreck of 

 each — a crucial circumstance jJi'eviously overlooked, an 

 original train of reasoning, a valid analogy. The gate of 

 mystery, it is true, remains closed, notwithstanding the 

 many " Open Sesames " that have been uttered in front of 

 it. Yet it hangs more loosely on its hinges than of old, 

 and science has fresh engines in reserve. 



PHOTOGRAPHS OF COMET h 1902 (PERRINE). 



By Isaac Roberts, d.sc, p.r.s. 



Six photographs of the comet have been taken at my 

 observatory with the 20-inch reflector, and five with the 

 5-inch Cooke lens simultaneously, between September 6th 

 and October 10th, 1902, and the two photograj)hs on the 

 plate annexed illustrate the appearance and show the 

 position of the comet on the days herein given. 



Fig. 1. — The comet is shown as it appeared on the tith 

 September, with an exposure of the plate during ninety 

 minutes ; the plate being held, by aid of the clock 

 mechanism, steadily on one point in the sky whilst the 

 comet moved in its course, leaving a trail on the film 

 which represents the distance it travelled in its orbit 

 during the exposure of 90 minutes. On examination of 

 the trail on the negative, by aid of a lens, it is seen 

 to be formed of a series of circular dots which closely 

 resemble the images of rather faint stars that are 

 visible on the plate around the comet ; but inasmuch as 

 the comet was travelling in its orbit at the rate of about 

 11.3 seconds of arc each minute of time during the 90 

 minutes' exposure of the plate, and only eight dots are 

 visible in the trail, it follows that a very short interval of 

 time was required to form each dot. 



How were the dots formed ? There was no movement of 

 the plate during the exposure that would account for them, 

 and the comet, we must assume, kept steadily on its course 

 along its orbit without jerking or stopping. The answer to 

 the question is, doubtless, that during the exposure of the 

 plate the transparency of the atmosphere varied frequently, 

 and the clear intervals, though of very short duration, 

 sufliced to give the highly actinic nucleus time to leave its 

 image on the film, but when less clear intervals occurred, 

 the nuclear image varied in intensity down to invisibility. 

 These facts indicate that the nucleus was of globular 

 form ; that it was highly actinic, for although siu-roimded 

 by a deuse coma of nebulous nnitter, it did not obstruct 

 the light of the nucleus so as to prevent its image being 

 formed on the film in a few minutes of time. The nuclear 

 images are round notwithstanding the rapid movement of 

 the comet in its orbit. The diameter of the nucleus 

 measured about 14 seconds of arc, and the length of the 

 trail about 103 seconds. 



Fig. 2. — The photograph was taken on the 10th 

 October, with an exposure of the platt? during 52 minutes 

 in tlii^ 20-incli reflector, ;ind it will be observed that the 



star images upon it consist of a series of zigzag trails, 

 whilst the nucleus of the comet is round. In this case the 

 nucleus was used as a fixed point for guiding purposes, 

 whilst the images of the stars are trails which represent 

 the distance in space over which the comet moved during 

 the exposure of 52 minutes. The trails were produced by 

 the frequent necessity of moving the photographic plate, 

 in order to counteract the motion of the comet, and keep 

 it, as steadily as possible, upon the crosswires in the eye- 

 piece of the guiding telescope during the whole exposure ; 

 the number of the zigzags in the trails register the 

 number of the corrections made during the exposure of 

 52 minutes. 



The comet appears, on this photograph, as if it had 

 been viewed at rest in a telescope, and the various details 

 in its structure are therefore seen with but little distortion. 



I do not consider it necessary for me to write any 

 descriptive matter here ; each reader can see for himself, 

 by examination of the photographs, the cMma, the tail 

 divided with rifts, and the separation into two tails. One 

 of them is about two degrees in length, and the other 

 about one degree. Of course some of the fainter details 

 can only be well seen on the original negatives, six of 

 which are available for examination in the event of any 

 scientific investigations requiring their use. 



THE LUNAR ECLIPSE OF 1902, OCTOBER 16." 



A series of measures was made by Prof. "W. H- 

 Pickering of the diameter of the bright spot surrounding 

 Linnc before and after the passage of the shadow of the 

 earth during the total lunar eclipse of October Iti, 1902. 

 The measures were all made in a north and south direction, 

 with the filar micrometer attached to the lo-inch 

 equatorial. The magnifying power used was 550 dia- 

 meters. The seeing was fairly good, and nearly uniform 

 throughout the observations, being about 5 on the standard 

 scale of 10 {Harmrd Annals, XXXIL, 120). After the 

 passage of the shadow, it was slightly inferior, perhaps 4. 

 Unfortunately clouds stopped observations before an 

 absolute measure of the quality of the seeing could be 

 made. Various corrections were made to the measures 

 for the thickness of the micrometer thread, the pitch of 

 the micrometer screw, and for a subjective effect described 

 in Harvard AnnaU, XXXII , page 204. During the 

 observations on October 16 the sun had been shining on 

 Linne from 8'5 to 8'7 days. 



On October 16, on the approach of the umbra, and 

 when the penumbra already lay across Linnc, the white 

 spot began to grow in size, and by the time that the umbra 

 had passed away, its diameter was very materially in- 

 creased by an amount of 2'''75. This increase was notably 

 larger than that caused by the eclipse of 1898, when Mr. 

 A. E. Douglas estimated by three different methods the 

 inci-ease to be 0"-82, 0"-73, and 0"-15 ; or than that of 

 1899, when Mr. W. H. Pickering estimated the increase 

 to be 0"'14. Mr. Pickering remarks that in the present 

 instance the great increase in size could not have been 

 due to defective seeing after totality, for the effect of bad 

 seeing on spots of this size is rather to diminish their 

 dimeusions, as shown by the measurements of artificial 

 disks, and therefore could not produce the effect observed. 

 " Moreover," he says, " this change in size was so con- 

 spicuous to tiie eye, even before I made any measures, 

 that at first I queried whether the object could really be 

 Liuue. To satisfy myself 1 re-identified it by means of 

 some small craterlets in its immediate vicinity. It is 

 certain that at no time during the observations was the 



Harvard College Observatory CirciJar, No. 67. 



