240 



SCIENCE. 



[Vol. II., Xo. 29. 



In 1S76 M. Andre, the astronomer in cliarge of the 

 French expedition to Noumea, in 1874, announced 

 that "the bridge, blaciv ligament, or blaclc drop, as 

 it is variously called, is a necessary phenomenon under 

 certain circumstances, and not merely accidental." 

 He noticed, liowever, that "it is always possible to 

 get rid of the ligament, and reduce the phenomenon 

 to geometrical constants, either (a) by reducing suf- 

 ficiently the intensity of the source of light, or aug- 

 menting the absorbing power of the dark glass 

 employed; or (6) by covering the object-glass with a 

 dark diaphragm composed of rings alternately full 

 and empty, all very thin, and bearing a certain pro- 

 portion to the focal length of the lens." 



These results and opinions of M. Andr^ were not 

 generally known at the time of the transit of Mercury 

 in 1878; although his theories were confirmed by his 

 obsers'ations at Utah at that date, the results being 

 published by him in 1881. The black drop was seen 

 and recognized in 1878 by many observers of Mer- 

 cury ; some evidently regarding their success in find- 

 ing it as a proof of accuracy of observation, others 

 apologizing for failing to perceive the phenomenon. 



The author of this paper regards it as noteworthy, 

 that every observer, so far as ascertained, who got, 

 by means of shade-glasses, the best definition of the 

 sun's limbs, with an illumination less than the eye 

 could easily bear, did not see any trace of the black 

 drop. Before seeing any account of M. Andrei's ex- 

 periments, and having given little attention to his 

 deductions announced by Father Perry, the author 

 became independently convinced, after observation 

 of the transit of Mercury in 1878, that the theory of 

 a necessary black drop was fallacious. 



While, in 1874, many American observers perceived 

 the black drop, none appear to have seen it, among 

 the eight American parties organized by the transit- 

 of-Venu3 commission of 1882. 



The paper winds up with an account of the ob- 

 servations of contact at the transit-of- Venus station 

 at Cedar Keys, Fla., last December. The observa- 

 tion of first contact was prevented by a cloud covering 

 a part of the sun's disk. On the disappearance of 

 the cloud, the illumination was reduced by a sliding 

 shade-glass, till easily endured by the eye. The defi- 

 nition of the sun's limb was perfect. When haze 

 or cirri interfered, a less density of shade-glass was 

 permitted; the steadiness and definition of the limb 

 remaining, and tbat of Venus being 'all that could 

 be desired,' with no modification, at the edge of the 

 disk, of lis dense black color. 



Before the second contact, the entire disk of Venus 

 ■was visible for several minutes. The portion beyond 

 the sun's disk was bordered by a narrow line of light 

 much less bright than the limb of the sun, and of a 

 lighter tint. About one minute before contact, the 

 apparent motion of the cusps of the sun, as they closed 

 around the planet, noticeably increased, although the 

 movement was perfectly steady. The cusps swept 

 around the planet in a line of sunlight of the same 

 tint as adjacent parts of the sun. This line was as 

 narrow as could be seen with the power used, ^216 

 diameters, — and was free from tremors or pulsations. 



There was no agitation in the limb of either body 

 near the point of contact, no trace of black drop, 

 ligament, or band, no change of tint or color on the 

 limb of Venus, and no indication of any clinging of 

 the limbs. The contact was as easily, and perhaps 

 as accurately, observed as the transit of a star within 

 8° of the pole, under the best conditions. The un- 

 certainty of noting the time of the visible contact 

 could not have been greater than three-tenths of a 

 second. The phenomena at the third contact were 

 similar to those at the second, but, of course, in a 

 reversed order. 



In conclusion, the author urges his belief, founded 

 upon his own experience as well as on study of thi", 

 work of other observers, that, with a properly arranged 

 telescope and shade-glass, no observer need have 

 trouble from any phase of the 'black drop.' To 

 attain this end satisfactorily, the observer of contacts 

 must have no other purpose in view than such ob- 

 servation. The study of any branch of solar physics, 

 or searching for some new thing, may, and probably 

 will, detract from the accuracy of his work, which 

 should be confined to obtaining the re.cord of a good 

 definition of the sun's limb, as a reference-point in 

 the passage of the limb of the planet. 



An improved method of producing a dark-field 

 illumination of lines ruled upon glass. 



BY PBOF. W. A. KOGERS OF CAMBRIDGE, MASS. 



By repeated and careful tests the author found that 

 by letting the light, which is held at an angle of 45°, 

 into the telescope, and then splitting the rays by 

 means of two opposite mirrors, throwing them on the 

 horizontal line, an almost perfect light is secured. 

 Thereby It becomes practicable to see with distinct- 

 ness stars of the smaller magnitudes upon a dark field. 



Other astronomers present expressed a preference 

 for the use of red light. Professor Rogers claimed 

 that his method was better for minute observation. 



Physical phenomena on the planet Jupiter. 



BY G. W. HOUGH OF CHICAGO, ILL. 



The rapid motion of revolntlan of the planet, by 

 changing the positions of the markings on the surface 

 to our line of sight, makes great apparent differences 

 in their shapes and sizes. This has perhaps been the 

 occasion of reports of sudden and great changes upon 

 the surface. The changes are not sudden, but are 

 gradual; and many of the features are permanent. 

 Minor changes are constantly in progress in the 

 equatorial belts. The author recently observed the 

 belt drifting down toward the red spot; but although 

 it partly surrounded it, they did not coalesce, and the 

 spot forced a scallop into the belt, — a very curious 

 phenomenon. The author saw a satellite pass over 

 this red spot, though the satellites are not visible 

 when on the white part of the disk. He had also had 

 a chance to eomijare shadows of satellites on the disk 

 and on the spot, and both are dark. The red spot 

 has seemingly retrogaded during the past four years; 

 that is to say, the rotation of Jupiter has seemingly 



