Dec. 13, 1883] 



NA TURE 



157 



flight. The only question was whether the velocity im- 

 pressed by the electric action could be made relatively 

 sufficiently great. This depended upon the magnitude 

 of the electric density at the surface of the electrified 

 body, and for this reason Righi used a very sharp point for 

 the discharge. Fig. i shows the arrangements for ob- 

 taining the electric shadows by Righi's process, a b is 

 an ordinary retort-stand of iron, and upon it are clamped 

 three adjustable arms of ebonite. The uppermost of 

 these carries a short metal rod, pointed below and 

 terminated above in a metal ball. The intermediate sup- 

 port carries the object, C, which is to cast the shadow. 

 The lowermost arm is fashioned as a clip in which can be 

 held a disk, d, to receive the shadows. This disk is pre- 

 ferably of ebonite backed on its under side with brass or 

 tinfoil. In certain cases a metal disk varnished on the 

 upper surface is used. Fig. 2 shows a favourite form of 

 object for casting a shadow — a floral or cruciform design 

 cut from thin metal and mounted on a stem of ebonite or 

 glass. To produce the shadow-figures a Leyden jar is 

 charged to such a potential as to be able to yield a spark 

 of I to 2 centimetres' length. The outer coating is put in 

 communication with the lower surface of the disk D, and 

 the knob of the jar communicating with its inner coating 

 is then brought into contact with the top of the pointed 

 rod. The jar discharges itself rapidly and almost noise- 

 lessly. Then there is immediately sifted over the disk, 

 from a box covered with muslin, some mixed powders of 

 minium and flowers of sulphur, in the usuil manner of 



procuring Lichtenberg's figures. .\ shadow of the inter- 

 posed object is at once revealed by the powders. If the 

 discharge has been a positive one, the shadow of the cross 

 will appear in red surrounded by a neutral region, outside 

 which there will be a region tinted yellow with sulphur. 

 The colours will be reversed with a negative discharge. 

 The shadow is depicted in Fig. 3. The size of the shadow 

 varies with the distance of the object. Righi recommends 

 that the object should be three centimetres above the 

 disk, and the point twelve centimetres, or less, above the 

 object. The lines of discharge appear to be hyperbolic 

 in form. If a disk of ebonite only be placed at d, and the 

 brass disk below it be lowered down, the effects are less 

 distinct. If a narrow strip of foil or thin brass be placed 

 below the eb )nite di=k, the shadow becomes compressed 

 laterally and shows itself only on the region over the strip, 

 and takes the form shown in Fig. 4. .Similar shadows 

 can be obtained according to Righi, on metallic disks 

 covered with non-conducting varnish, but in this case by 

 the use, not of the Leyden jar, but of an influence 

 machine. This inethod is identical with one of Holtz's 

 suggestion. Righi also finds that if the metal disk be 

 previously coated v/ith a conducting powder, such as 

 finest zinc filings, minium, or even powdered glass, a 

 shadow can be obtained. This method affords indeed 

 very sharp shadows, so that thin wires and even wire 

 gauze can be projected in shadow on the disk. Righi has 

 gone still further, and by substituting a sensitive gelatino- 

 bromide plate, has photographed the shadows produced 

 during a five or ten minutes' discharge. In this experi- 



ment two figures showed themselves : one, the genuine 

 electric shadow ; the other, the genuine photographic 

 shadow cast by the opaque object under the faint star of 

 light emanating from the electrified point above. 



If the object whose shadow is to be thus obtained is 

 itself electrified, a curious effect is observed. If it be 

 electrified with a charge of the same sign as that of the 

 point above it, the shadow swells out. If electrified with 

 a charge of opposite sign, the shadow becomes attenuated. 

 Connecting the object to earth has the same effect as in 

 the latter case. The presence of an electrified body on 

 the right or left of the region in which the discharge is 

 taking place has the effect of causing the shadow to be 

 displaced. In fact the presence of such a body alters the 

 equipotential surfaces, and therefore alters the lines of 

 electric force in the field. If the discharge takes place 

 through two points placed side by side at a short distance 

 apart over two objects respectively beneath them, the two 

 electric shadows are mutually repelled from the positions 

 wheie their geometrical shadows lie. Similar observations 

 of electrostatic influence were made two years ago by 

 Messrs. Fine and Magie of Princetown, New Jersey. 



Much as has been done of late years, especially by the 

 late Mr. Spottiswoode in conjunction with Mr. J. F. 

 Moulton, by Drs. De La Rue and Hugo M tiller, by 

 Crookes, by Goldstein, and others, to elucidate the phe- 

 nomena of electric discharges, there probably still 

 remains much to be discovered, and to be e.xplained. 

 The phenomena of electric shadows are amongst the 

 matters best worthy of study in this rapidly progressing 

 department of science. Silvanus P. Thompson 



NOTES 



We give this week a further instalment of notices of the 

 strange coloured effects recently observed in the skies, and our 

 readers in all parts of the world will render a service if they will 

 communicate any similar facts they may have observed, giving, 

 as far as possible, accurate dates. In an article in Saturday's 

 Times, Mr. Norman Lockyer shows that the body of evidence 

 already to hand connects them with the eruption of Krakatoa 

 but, to place the matter beyond doubt, further information is 

 required. The study of direction and of dates, and the facts 

 touching the variation in the phenomena from August to 

 December, all point in the same direction. 



No one will be surprised, though allmust regret, that his state 

 of health and advanced years have compelled Prof. Owen to 

 resign his appointment as Superintendent of the Natural His- 

 tory Department of the British Museum. Prof Owen's pre- 

 euinent services to science, pure and applied, are too well known 

 to require recapitulation in these columns, especially as very 

 recently we referred to them in detail in connection with his 

 portrait as one of our " Scientific Worthies." Advanced in years 

 as he is, the venerable naturalist's interest in science seems as 

 strong as ever ; to each of the last two meetings of the Royal 

 Society he contributed an important paper : we hope they will 

 be by no means the last of such contributions. 



We learn with the greate-t pleasure that Prof Sylvester has 

 been appointed to succeed the late Prof. Henry Smith in the 

 Savilian Chair of Geometry at Oxford. No more worthy suc- 

 ces or to the late Savilian Professor could have been found, and 

 it is satisfactory to know that at last the services of one of our 

 greatest living mathematicians have been permanently secured 

 for his native country. 



The ceremony of distributing the prizes to the successful students 

 of the Finsbury Technical Collegeand the South London Technical 

 Art School took place on Monday evening in the Hall of the 

 Clothvvorkers' Company, Mincing Lane. The Lord Mayor presided, 

 supported by the President of the Royal Society, the Sheriffs, Sir 



