February 6, 191 3] 



NATURE 



6-' I 



experiences differ as to the visibility of objects seen 

 through a dewed plate. There are two reasons for 

 this. One is that the test objects were quite different. 

 His was a gas flame, while mine was a landscape, 

 an object much more easily rendered invisible than 

 a flame. The second reason for our difference is that 

 he dewed his plate by breathing on it, and I can 

 quite imagine it would be difficult to get sufficient 

 obscurity by that process. I, on the other hand, 

 dewed mine by cooling the back of the plate with a 

 piece of ice, by means of which any amount of deposit 

 may be obtained. I have lately tried a test object 

 similar to Lord Rayleigh's, using an incandescent gas 

 light. Bv continuing the cooling and condensation 

 long enough, it was found possible gradually to 

 diminish the visibility of the mantle until it all 

 vanished and only an irregular bright, undefined 

 centre of light remained. John Aitkex. 



Ardenlea, Falkirk, January. 



An Electrical Phenomenon. 



The form of high-resistance Bell telephone receiver 

 which of recent years has been evolved for use in 

 some systems of wireless telegraphy, gives us an 

 extraordinarily sensitive means of detecting variable 

 electric currents of a very minute description. 



Using one of these instruments, made by Mr. 

 H. W. Sullivan, which is wound with so many turns 

 of very fine wire that it has a resistance of 5000 ohms, 

 I have found at my residence, 40 Chester Square, a 

 somewhat surprising state of things. 



Holding one terminal of the telephone in one's hand 

 so as to connect the electrostatic capacity of one's 

 body, and applying the other terminal, which one 

 must be careful not to touch, to _any metallic object of 

 considerable size in the house, one immediately hears 

 in the telephone a singing noise. The larger the 

 metallic object and the higher up it is in the house, the 

 louder the sound. The sound is also made louder if 

 one's own electrostatic capacity is increased bv contact 

 with another person or with a metallic sheeting. 

 The metal of all the fireplaces gives the sound most 

 distinctly, as does also an iron bedstead on the third 

 floor, which stands on glass castors free from contact 

 with the wall. The sound can be obtained even from 

 the brass stair-carpet rods only about 3 ft. long, par- 

 ticularly towards the top of the house, and it jan 

 also be got from the gilt of picture frames. All the 

 curtain-poles give it loudly. 



The position of the object from which the sound 

 \i obtained does not appear to matter, excepting that, 

 as mentioned, the higher up in the house the louder 

 is the sound. 



There seems to be no question that the cause is 

 some form of electrical induction, or wave, from the 

 electricity supply, which is direct current, from the 

 Westminster Company. That this is so is evidenced 

 by the fact that an exactly similar note can be 

 obtained from the supply mains. The sound appears 

 to be due to a ripple superimposed on the continuous 

 current by the commutators of the dynamos, and at 

 times the beat of the fast-running engines, such as 

 do, in fact, supply the current at certain hours, can 

 distinctly be heard. Furthermore, the phenomenon 

 described above ceases the moment the supply is cut 

 off at the house by opening the double-pole main 

 switch. 



It should be mentioned that on the top of the house 

 there is an aerial, consisting of a piece of wire-netting 

 20 ft. by 4 ft. in area, supported on four insulators 

 about 10 ft. above the roof, with a wire coming down 

 from this outside the house and entering a room on 

 the ground floor, the whole being destined for use as 



NO. 2258, VOL. 90] 



a wireless receiving station. It is possible that this 

 aerial may have some influence, but the fact that 

 putting it to earth or leaving it insulated does not 

 seem to have the slightest effect upon 'the results 

 obtained seems rather to point to an opposite con- 

 clusion. 



It should, however, be mentioner* that with the 

 telephone receiver connected between the wire leading 

 to the aerial and the water-pipe, the singing noise is 

 still very distinctly audible, even after the main 

 double-pole switch is opened, which points to there 

 being a field of force operating on the aerial either 

 from the street mains or from the electric wiring in 

 the neighbouring houses. 



Similar experiments at this office, which is also 

 supnlied by the Westminster Company, give a very 

 high-pitched but rather faint whistling sound when 

 the telephone is applied either to the water- or the gas- 

 pipe, while the metal of the fireplaces also gives a 

 similar sound, but so very faintly that it is barelv 

 discernible. This sound, too, is no doubt due to 

 the high-speed commutators of the turbo-generators 

 which supply this portion of the Westminster system. 

 A. A. Campbell Swinton. 



66 Victoria Street, Westminster, S.W., 

 February 4. 



Luminous Halos surrounding Shadows of Heads. 



I REMEMBER when I was a boy, more than eighty 

 years ago, that I used to notice this luminous halo 

 surrounding the shadow of my head on the water 

 when I was fishing from a bridge in the meadows 

 below Salisbury. I think it was in some way con- 

 nected with the ripple on the water, which was so 

 clear that I could see the fish. I mention this because 

 similar conditions could be easily met with. 



O. Fisher. 



Graveley, Huntingdon, January -;o. 



This phenomenon may sometimes be seen in this 

 country when one's shadow falls on grass. It is not 

 necessary that the grass should be wet, if the leaves 

 have a shining cuticle; but the general direction of 

 the blades (which grow usually more or less parallel 

 to one another) in relation to the position of the sun 

 at the time must be such that its rays strike their 

 surface at an angle approaching a right angle. Under 

 these conditions the blades of grass from which most 

 light reaches the observer's eye are those upon which 

 the sun's rays fall, and are reflected to him, most 

 nearly perpendicularly, and the rays which do so are 

 those which pass closest to his head without being 

 intercepted by it. Hence there appears to him a 

 ring of brighter illumination immediatelv surrounding 

 the shadow of his head, the effect being heightened 

 by contrast. Farther from the shadow, as the angle 

 of incidence becomes more oblique, the luminous ring 

 becomes gradually merged into the general illumina- 

 tion. The reason why the bright ring is not seen 

 round the lower parts of the body or around the 

 heads of other persons is that these are not so nearly 

 in the direct line of incidence. 



The phenomenon "A Shadow and Halo" is de- 

 scribed in N.WURE in 1888 by several correspondents 

 (vol. xxxviii., pp. 540, 589, 619), and its production 

 by reflection from dewy grass is explained on the 

 lines I have mentioned. 



An analogous phenomenon is the striped appear- 

 ance of a lawn or grass field which has been rolled 

 by a roller passing alternately in contrary directions. 

 Where the roller has travelled in a direction from the 

 position of the observer the blades of grass are bent 



