22 



SCIENCE-GOSSIP. 



shadows over the face of the planet. The satellites, 

 when they first enter upon the disc, usually appear 

 as tiny, bright round spots ; as they get farther on 

 the disc, very frequently I. and II. seem to disap- 

 pear altogether, although at times they may be 

 seen bright across the disc, and always appear 

 as such as they near the western limb. III. and IV. 

 usually very soon disappear as bright spots, and 

 then quickly reappear as dark spots, and so cross 

 the planet until they nearly reach the western 

 limb, when they first disappear and then become 

 bright. The shadows, before opposition, cross the 

 planet on the western side of the satellites throw- 

 ing them, but after opposition they may be seen on 

 the other side. The shadow very often seems 

 larger in diameter than the satellite throwing it, 

 doubtless an effect due to the penumbra or partial 

 shadow surrounding the shadow itself. An 

 ill-defined edge has been observed to these 

 shadows which confirms this explanation. A 

 phenomenon not so easy of explanation has some- 

 times been observed. A shadow has been seen to 

 be grey instead of black. Likewise when the 

 planet is some three months past opposition the 

 shadows have sometimes been noticed to be some- 

 what elongated east and west on first entering upon 

 the disc, due to the positions of the Earth and Sun 

 with respect to the planet. Satellites III. and IV. 

 have been seen when in transit to be almost, if not 

 quite, as dark as the shadows they threw, and very 

 occasionally IV. has been seen to enter upon the 

 disc as an almost black spot. 



The illustration showing satellite III. and its 

 shadow in transit on August 31st, 1879, is from a 

 drawing by W. Harding, of Bournemouth, as seen 

 with a 3-inch Wray achromatic, and the shadow of 

 II. is also shown just entering on the disc in one of 

 Eev. T. E. E. Phillips' drawings on p. 281, vol. vii. 

 If a transit of one of the satellites occurs when, at 

 the time of opposition, the Earth happens to be 

 nearly in a straight line between the Sun and Jupiter, 

 the Moon may be seen to occult its own shadow. 

 Eecords of such phenomena occur under the dates 

 of January 14th, 1872, at midnight, when Mr. 

 F. M. Newton saw the shadow of I. reduced to a 

 black crescent; on May 13th, 1876, Mr. G. D. 

 Hirst, of Sydney, Australia, observed the shadow 

 of this same satellite reduced to the form of a 

 crescent. In 1893 Professor Barnard made a very 

 similar observation. 



If an observer could be on Jupiter on June 30th, 

 1901, our Earth would actually appear in transit 

 across the Sun's disc. On that date satellite I., as 

 seen from the Earth, transits over the disc, and will 

 cover the central portion of its own shadow. 



Against the date August 30th, 1877, at 8 p.m., 1 

 have the note that the shadow of III. was much 

 the larger and blacker, and that of I. seemed to pale 

 and fade away as it drew near the limb. Several 

 observers have fancied the shadow of II. to be 

 often very ill-defined. 



The reason of the variations in brightness in the 

 satellites is undoubtedly caused by the presence 

 of spots upon their surfaces, the spots having 

 frequently been drawn by observers using tele- 

 scopes of considerable aperture. Numbers!, and 

 IV. are sometimes found to have irregular shape. 



Very occasionally, as on November 2nd (old 

 style), 1681, and on August 21st, 1867, Jupiter was 

 seen as apparently without satellites, all of them 

 being either in transit or eclipsed. 

 (To ve eontimted.) 



CONDUCTED BY B. FOULKES-WINKS, M.R.P.S. 



EXPOSUEE TABLE FOE JUNE. 



The figures in the following table are worked out for plates of 

 about 100 Hurter & Driffield. For plates of lower speed number 

 give more exposure in proportion. Thus plates of 50 H. & D. 

 would require just double the exposure. In the same way, 

 plates of a higher speed number will require proportionately 

 less exposure. 



Time, 8 A.M. to i p.m. 



Between 7 and 8 A.M. and 4 and 5 p.m. double 



the required exposure. Between 6 and 7 A.M. and 



5 and 6 P.M. multiply by 4. 



Subject 



F. 5-6 



F.8 



F.ll 



F. 16 



F.22 



F.32 



F.45 



F.64 



Sea and Sky . . 



si- 



T50 



T2o 



T?o 



5S 



i 



1 



8 



l 



4 



Open Landscape 

 and Shipping 



}^ 5 



oV 



h 



TV 



J 



* 



I 



1 



Landseape,with 

 dark fore- 

 ground, Street 

 Scenes, and 

 Groups 



I* 



A 



5 



1 

 i 



I 



1 



2 



4 



Portraits in 

 Booms 



1* 



4 



8 



16 



32 



- 



- 



- 



Light Interiors 



4 



8 



10 



32 



1 



2 



4 



8 



Dark Interiors 



16 



32 



1 



2 



4 



8 



16 



30 | 



The small figures represent seconds, large figures minutes. 

 The exposures are calculated for sunshine. If the weather is 

 cloudy, increase the exposure by half as much again ; if gloomy, 

 double the exposure. 



Plate Tester. — We have received a descrip- 

 tive pamphlet of the new " Chapman-Jones Plate 

 Tester" made by Messrs. Sanger, Shepherd & 

 Co., of 5, 6, and 7 Gray's Inn Passage, Eed Lion 

 Street, Holborn, W. The fact of this instrument 

 emanating from such a well-known house is in 

 itself sufficient guarantee of exactness. The object 

 of this apparatus is to provide means of ascertain- 

 ing, within a sufficient degree of accuracy to be of 

 practical value, the relative working characteristics 

 of photographic plates and films. It consists 

 essentially of a screen plate 4J- inches by 3| inches, 

 containing: — A series of 25 tints • of graduated 

 densities; a series of coloured squares and a strip 

 of neutral grey, all five being of approximately 

 equal luminosity ; a series of four squares of special 

 colours, each colour passing light from a definite 

 portion of the spectrum ; and a square of a line 

 design over which is superposed a half-tone 

 negative. In order to use the instrument, a 

 quarter-plate of the brand to be tested is simply 

 exposed behind the screen for a few seconds, 

 developed, fixed and washed. An examination of 

 (his plate will show sensitiveness, or speed, range 

 of gradation, possible range of exposure, sensitive- 

 ness to colour, comparative size of grain of plate. 

 amount of halation, the most suitable light for 

 development. By further exposures, the instrn- 



