I30 



NA TURE 



[^une 9, 1 88 1 



vidual readings, of which only the mean is useful. I do 

 not believe that, since the year 1835 at least, any person 

 in the world except ourselves has actually taken a mean. 

 As each reading contains 3 or 4 figures, there are printed 

 in each year something like 150,000 useless figures. Re- 

 liance must be placed somewhere on the skill and fidelity 

 of the observer, and (considering the severity with which 

 every figure of transit-wire and of circle microscope, and 

 of their means, is exammed here) this reliance may be 

 placed at least as well on the means as on the originals. 

 I have reason to think that the bulky volume of nearly 

 900 pages, might be reduced to about three-fifths of its 

 present size by omitting those originals. 



" I would submit for the consideration of the Board 

 whether it might not be advantageous that they should 

 hold a special meeting to consider the subjects which I 

 have indicated. The length of time at an ordinary visita- 

 tion, and the circumstances under which the Board meets, 

 are not sufficiently favourable for the discussion of broad 

 questions of Observatory policy." 



HOLTZ'S ELECTRICAL SHADOWS 



IN an extremely elegant series of researches Prof. 

 W. Holtz of Berlin has lately brought to light the 

 existence of a new class of electrical phenomena, to which 

 their distinguished discoverer has assigned the name of 

 Electric Shadow -figures. Though nearly six months have 

 elapsed since they were described in the Proceedings of 

 the Gottingen Gesellsclinft der Wissenschaften, no detailed 

 account of them has appeared in any English journal. 

 Yet the shadow-figures are remarkably easy to produce, 

 and the whole research is of extreme simplicity, as very 

 little apparatus is required beyond the simplest odds and 

 ends to be found in every physical laboratory, the only 

 large instrument necessary being one of Holtz's electrical 

 machines. 



The fundamental arrangement is that shown in Fig. i. 

 From the discharging-rods of a Holtz machine the brass 

 balls are removed. To the left rod there is attached in 

 place of the ball a circular disk of some 10 to 20 centi- 

 metres diameter, having its front face either flat or slightly 

 concave. To the right rod a point is fixed, and it is 

 drawn back till from 6 to 15 centimetres distant from the 

 disk. A piece of silk or satin of the same size as the 

 front surface of the disk is laid upon it while the machine 

 is in action, it adheres of itself to the surface, and the 

 preparation is now complete. Before the silk is placed 

 over the disk a small "brush" discharge of blueish light 

 is all that can be distinguished at the point of the right- 

 hand discharging-rod : but this now changes to a very 

 faintly glowing star. At the same moment the central 

 region of the silk-covered disk exhibits a peculiar glim- 

 mering light over a well-defined circle. The utmost care 

 is needed to shut out all extraneous light from the room, 

 otherwise the delicate appearances which follow cannot 

 be seen. It is upon this circular patch of feeble light that 

 the shadow-figures are thrown. Its pale gleam becomes 

 more vivid when the machine is more energetically 

 worked : it enlarges in area but diminishes in brightness 

 as the point is drawn back from it, and contracts with an 

 accompanying increase in brightness as the point is 

 brought nearer. It is possible to obtain a similar glim- 

 mering surface also upon a large metal ball covered with 

 silk and attached to the rod in place of the concave di^k, 

 or instead a screen made of two or three folds of silk 

 stretched over an ebonite ring may be placed between the 

 two discharging-rods, the ends of both being furnished 

 with points. In each case it is important that the silk be 

 without crease or wrinkle, otherwise an evenly illuminated 

 disk of light will not be obtained. 



If now a body of definite outline of form be interposed 

 between the point and the disk, an electrical shadow of 

 it will be cast upon the luminous circle. These shadows 



are truly electrical, not optical, for all bodies do not cast 

 them, and, more curious still, different bodies though of 

 the same shape may cast differently shaped shadows. 

 Conductors of electricity cast well-defined shadows, and 

 so do semi-conductors, such as wood and cardboard. 

 True insulators of small dimensions cast no shadows. 

 The insulation or non-insulation of the conducting bodies 

 makes no difference ia their shadow-giving power. A 

 crosscut out of cardboard casts (as in Fig. i) a well-defined 

 shadow at the centre of the field, but the exterior portions 

 are somewhat hazy. An ebonite cross casts no shadow. 

 A cross made up of two strips, one of cardboard, the 



other of ebonite, fastened with shellac, casts only a single 

 bar of shadow. Rings of tinfoil, cardboard, or wire also 

 cast shadows. Such small objects are conveniently held 

 by attaching them lo the end of glass rods. The size of 

 the shadows increases if the objects are displaced from 

 their central position to right or left. A strip of card or 

 thin metal casts the same shadow whether it be held 

 broadside or edgeways in the field. A wire grating having 

 5 millimetres width between the bars obscures the field 

 like an opaque body. Breathing on a strip of ebonite or 

 glass renders its surface a feeble conductor, and it casts a 

 transient shadow. A glass rod heated at one point casts 

 a shadow at the heated point, the shadow dying out as the 



Fig. 2. — uminous figure projected through apenure in caidboard screen. 



rod cools. No shadow is cast by a conductor whose surface 

 is completely covered by insulating material, sudh as a 

 shellac-covered wire or a glass tube containing water, but 

 dry externally. The smoke ascending from a cigar casts 

 moving shadows upoii the silken screen. If a small ball 

 be fi.xed upon the left discharging-rod in place of the point 

 the shadow on the silk is poor, but a second shadow is 

 observed upon the surface of the ball, and this is exces- 

 sively small, reminding one of the diminished erect virtual 

 optical image in a small polished ball. This one e.Nperi- 

 ment succeeds best if the ball be made the positive con- 



