(22 



NA TURE 



[August 2, 1894 





1893 



Dec. 



1893 



Dec. II 



9 iS 



1894 ... Jan. 28 ... 6 



I. is near transit, following, it 

 appears slightly elongated 

 toward-; Jupiter. 



I., II., 111., each is round, IV. 

 seems a little deficient on 

 following side, as if a slight 

 phase existed, iS:c. 



I., II., III. are round, IV. is 

 slightly deficient on follow- 

 ing side, as if a slight phase 

 or a dark area existed in it. 



The time referred to above is Standard Pacific time, 

 S hours slow on Greenwich. 



The notes, however, are generally of the following type, 

 a few of which may be mentioned here. 



h. m. 

 1893 ... .Aug. 28 ... 14 30 ... All four are round. 



-Ml four are round. 

 111. is perfectly round. 

 All four are round and clearly 



defined. 

 I. and 111. perfectly round. 

 III. is beautifully round. 

 III. is perfectly round. 

 All four are round. North 

 Pole of III. is white. 



These observations show that either the satellites on 

 the whole appear generally round, or that the Lick 

 observers have been so unfortunate as to observe them 

 just at those times when the circular discs were to be 

 seen. This seems at first thought to be very improbable, for 

 the reason, as Prof. Barnard himself remarks, that the 

 Arequipa observations indicate distortions that are 

 apparently so very conspicuous. 



In the June number of Astronomy and Astro-Pliysics 

 (p. 423), Prof. Pickering gives in reply a short note to 

 the observation made by Prof. Barnard. There are here, 

 also, some measures of the position angle of the elonga- 

 tion of the first satellite as secured by himself and Mr. 

 Douglas upon si.\ different nights, a copy of which is 

 below. 



/. Satellite. 



Each measure is the mean of six readings, taken alter- 

 nately in opposite directions. These measures show 

 that the observations are fairly concordant, only that 

 there seems to be a mean personal correction of about 

 7-1. The first column gives the date of observation, the 

 second the observed position -angle of the major axis of 

 the disc, the third the average deviation of the readings 

 which combined give the individual measures, the fourth 

 the observers, the fifih the differences between these re- 

 sults, and the sixth these differences corrected by the 

 constant angle 7'l. 



In the last column, the mean 2"2 indicates the average 



NO. I 2Q2, VOt,. 50] 



difterence between the corrected measure of the two 

 observers upon any night, which shows a remarkable 

 accordance between the measures. 



There can be no doubt that there was some peculiarity 

 about this satellite that was under measurement at the 

 time, a peculiarity which, as Pi of. Pickering says, was 

 apparent when it was not in transit, but which vanished 

 at regular intervals of 6h. 32m. Why Prof. Barnard has 

 not been able to witness what seems to be a very distinct 

 phenomenon, if it be not really " personal,'' seems to raise 

 considerable surprise. .Although Prof. Pickering does 

 not deny the existence of the equatorial belts, yet he will 

 not accept it as an explanation of the changes of form 

 noticed, for he says, '" such a belt could not have pro- 

 duced the effects observed by us in Arequipa." 



A further investigation on the forms of the discs has 

 recently been made by Prof. Schaeberle {The Astro- 

 nomical Journal, No. 321, p. 70), with the intention of 

 detecting, if possible, the rapid change of phase which 

 Prof. I ickering's observations so strongly advocate. The 

 results showed, however, that, by tabulating the ratios of 

 the measured major and minor axes of the several ellip- 

 tical discs, a practically constant form for the outline was 

 always obtained. We may mention here that to Profs. 

 Campbell and Schaeberle the first satellite appears round 

 only when "it is near to or projected on the disc of 

 Jupiter, and elongated in the direction of the planet's 

 equator in all other positions." 



In judging between the weights that ought to be ap- 

 plied to observations made at .-Arequipa and Mount 

 Hamilton, a fact here is mentioned that is by no means 

 insignificant m showing the superiority, in this case at 

 any rate, of the Mount Hamilton observations over those 

 made at .Vrequipa. 



The observations in question relate to the abnormal 

 forms of the shadows of the satellites, the true forms of 

 which were observed at the Lick Observatory by Prof. 

 Schaeberle and by other observers at different places and 

 times. 



The shadows of these satellites as they pass before 

 the disc of Jupiter become at times apparently distorted 

 to observers on the earth's surface, owing to the oblique 

 illumination of the satellites in some positions of the 

 earth, and to the spherical nature of Jupiter's surface. 



These distortions reach sometimes very considerable 

 proportions, more considerable, ir. fact, than the changes 

 of shapes of the satellites, as observed by Prof. Pickering. 

 This being so, it is curious indeed that Irom .'Vrequipa we 

 have, as far as is known, no mention of such shadow 

 distortions at all, and as I'rol. Schaeberle remarks, " one 

 would naturally buppose that an observer, after having 

 discovered, as he believed, a periodic variation in the 

 form of a satellite, would seek to verify his results by 

 examinations of the satellite's shadow during its transit 

 across the disc of the planet." 



An idea of the size of the distortions alluded to may 

 be gathered from Prof. Schaeberles statements that, 

 at the time of the .Arequipa observations in January of 

 last year, the longest (longitudinal) diameter of every 

 shadow just after the ingress on the visible disc of 

 Jupiter was "more than twice the breadth of the 

 shadow, while at egress just the reverse condition of 

 things existed." 



Such, then, is the present state of affairs. Prof. 

 Pickering sees these small bodies regularly changing 

 their shapes ; Prof. Barnard sees them always perfectly 

 round ; while Profs. Schaeberle and Campbell see them 

 only lound when near to or projected on the disc, and at 

 all other times constantly elongated. 



The only conclusion that can be drawn, if one is at 

 liberty to draw any at all, is that in the estimation of the 

 shapes of such small bodies a great amount of personal 

 error is liable to creep in, and the estimation of such 

 must be left for (uture determination. 



