Oct. 12, 1876] 



NA TURE 



535 



certain resistance represents the Ohm. The Ohm is a 

 small coil of German silver wire representing the resist- 

 ance overcome by a current in a certain time." What 

 kind of conception can Mr. Beechy have of current and of 

 resistance ? 



Still, as we have already said, we are much pleased with 

 Mr. Beechy's little book. The author can readily make 

 the necessary improvements in a future edition. 



LETTERS TO THE EDITOR 



\The Editor does not hold himself responsible for opinions expressea 

 by his correspondents. Neither can he undertake to return, 

 or to correspond with the writers of, rejected manuscripts. 

 No notice is taken of anonymous communications.^ 



Action of Light on Ebonite 



It is well known to electricians that the insulating power of 

 ebonite gradually diminishes in consequence of the formation of 

 a conducting layer of sulphuric acid on the surface (produced by 

 the oxidation of the sulphur used in vulcanising). It is perhaps 

 not so well known that exposure to light facilitates this change, 

 if indeed it is not an essential condition. 



In order to put this to the test, a plate of ebonite polished on 

 both sides was cut into four pieces, each about 52 mm. long, 

 22 mm. wide, and 8-5 mm. thick, exposing therefore a surface of 

 about 3,500 square millimetres (the edges were not polished), 

 and one half of each piece was varnished with an alcoholic 

 solution of shellac. Two pieces were placed in wide test tubes 

 plugged with cotton wool, and the other two were sealed her- 

 metically in similar tubes. One of the sealed tubes and one 

 plugged with cotton wool were placed in a dark drawer, and the 

 other pair exposed to light in the laboratory, and during the latter 

 part of the experiment to direct sunlight. The experiment was 

 commenced on December 26, 1874, and after some time minute 

 drops of liquid were perceived on the ebonite exposed to light 

 and air, the remaining three pieces retaining their original ap- 

 pearance. Between September i and 21 of this year the 

 sealed tube exposed to light was accidentally broken, so that for 

 a period of less than three weeks the ebonite in it was exposed 

 to both light and air. On September 21 the tubes were opened, 

 the ebonite washed with water, and the amount of acid deter- 

 mined by standard solution of caustic soda. No trace of acid 

 could be detected on either of the pieces of ebonite which had 

 been kept in the dark ; on the one which had been exposed to 

 light in the closed tube, '343 milligrammes of sulphuric acid 

 were found, and on that exposed to light and air, 2 '646 milli- 

 grammes. 



Ey a mistake it was not ascertained whether the part of the 

 ebonite which had been varnished had become acid, but during 

 the time of exposure small drops were also perceptible on this 

 portion of the surface. When the pieces were exposed to direct 

 sunlight another change became visible, the drops being replaced 

 by what appeared to be small particles of a yellowish white 

 solid. This may have been due to the heating of the black 

 material by the sun and consequent action of the strong acid on 

 the solid. 



I was led to try this experiment by noticing that an ebonite 

 plate electric machine which had been kept in a light room had 

 changed in colour except on those ponions which had been 

 protected from light by the rubbers. The exposed surface 

 acquired a brown colour and the machine acted very badly. On 

 cleaning the plate with a hot solution of caustic soda, large 

 quantities of ammonia were evolved and the brown surface be- 

 came softened, so that it could be easily scraped off. 



I had an opportunity of noticing a remarkable instance of this 

 action a short time since in the laboratory of my friend, Mr. 

 Warren De la Rue. An apparatus with an ebonite base, with 

 three adjusting screws, was standing at some distance from a 

 window. The surface of the plate was covered with a fine dew 

 of an acid liquid, except at the parts where the shadows of the 

 heads of the screws fell. The surface at these places completely 

 retained its original polish. 



The interest of this matter must be my excuse for comrauni . 

 eating the results of an incomplete experiment. 



Royal Indian Engineering College, Herbert McLeod 

 Cooper's Hill, October 2 



Visual Phenomena 



The following quotation was written, and a stereo-slide to 

 which it was appended was sketched by myself in January last, 

 and shown at the soirie of the Manchester Mechanical and 

 Scientific Society then held : — 



"In looking through comparative darkness at any bright 

 light, the writer, who is near-sighted, sees in place of such light 

 or any number of such lights, a bright disc or discs each like the 

 stereoscopic combination of the figures here shown. 



" Are such figures seen by other myopic subjects, and do they 

 consist of the middle portions of the crystalline lenses as seen 

 from within ? 



" In order to develop the figures the source of light must be 

 sufficiently distant to subtend an angle of about one-twelfth of a 

 degree ; the discs have an apparent diameter of about 1° or 

 more, being like the pupils which seem to define their outline, 

 persistently variable in size {ie., always on the move). The 

 disc-patterns are constant in markings and position, and their 

 brighter lines irradiate the darkness (of the vitreous humour) as 

 though by refraction from the (?) denser portions of the lens." 



The discs above mentioned differ a little in each eye, but the 

 groundwork in both cases is a somewhat irregularly five-armed 

 star ; each arm has a shaded axis with bright margins, and they 

 radiate from a luminous ring inclosing a darker central spot. 

 The whole figure is well illuminated, its details being defined 

 rather by variations of hght than by dark markings, and their 

 comparative brightness inter se being not unlike that shown by 

 the various parts of the lunar surface at the full. The intervals 

 or sectors of the figures are filled with a mottled pattern not easy 

 to sketch ; one space contains a figure like a Y with the stem 

 outwards ; another a V point inwards. Some dark spots, inside 

 bright ring>, as they are exposed or excluded by the margin of 

 the figure, curiously define the varying size of the pupil as one 

 approaches or recedes from the light; at about 12 yards from 

 (say) a street lamp the disc is suddenly supplanted by the true 

 form of the gas flame. 



I see these appearances with the unassisted eyes ; a concave 

 lens at once snuffs them out. About sixteen years ago I tried 

 some experiments with convex lenses, and found that on holding 

 the lens farther from the eye than its (the lens's) focal distance, 

 the star figure suddenly became a negative — its cardinal points 

 reversed, its lights shadows, and vice versd ; the arms bright, 

 with shaded borders, and the dark spots bright, with shaded 

 rings. 



On coming from darkness into a gas-lighted street, the star 

 discs appear large for about a second, then suddenly contract, 

 but retain a slight oscillation, corresponding with the slight but 

 incessant movements of the iris. Tlie conjunction of lightning 

 and street-lamps has a curious effect; fl/?^^each flash the hundred 

 or more of discs, one at each light, suddenly contract and more 

 leisurely expand, the contraction taking about one second and 

 the readjustment about four. 



In place of Mr. Mallock's Fig. 2 (p. 350), I get a sort of very 

 acute St. Andrew's cross, its arms consisting of parallel rays 

 crossed by numberless very fine striations. 



Fig. 3 I only see as a tangled confusion, owing to the hairs 

 not being so neatly arranged as in Fig. 4; yet their foreshortened 

 crookedness seems, by way of amends, to be responsible for the 

 following : — 



In looking towards, but a little below, the sun, which should 

 be at about its winter meridian altitude, the upper field of view is 

 crossed by a sort of variegated aurora of rainbow colours, which 

 have almost a polariscope brightness, and are lined and ringed, 

 as it were, upon a sort of chain pattera foundation. 



It was in November or December last that I first found that 

 the before-named star figures were not necessarily extinguished 

 by a light sufficiently strong to allow of my sketching them ; the 

 occasion being a highly successful Manchester copy of a London 

 fog. A lucid interval and a lowered gas-jet in a large room 

 accidentally gave the requisite conditions. 



If considered of sufficient intere t, I would send copies of the 

 discs which are sketched nearly two inches in diameter. The 

 disc of a gas-lamp at 100 yards distance has an apparent 

 diameter of nearly 3 feet, and a lighted up cotton-mill is all 

 light, no wall. H. B. Biden 



Sale, Manchester 



If Mr. T. W. Backhouse (Nature, vol. xiv., p. 474) is right 

 in interpreting the phenomenon of radiance described by Mr. A. 



