March 31, 18S1] 



NA rURE 



519 



M. Grandidier gains the Saviguy prize fur researches on the , 

 fauna of Zanzibar and Madagascar ; while the Thorel prize is 

 awarded both to M. Vays-ieres and M. July, for observations 

 proving a small aniaial found in streams to be the larva of an 

 insect of the family of Ephemeraiis. 



The Montyon prize for statistics gies to Dr. Ricoux for his 

 " Figured Demography of Algeria." 



We further note that M. Birckel receive; 1500 francs on the 

 Montyon foundation, for an improvement in the Davy lamp, 

 and that M. Dupuis receives the Delaland-Guerineau prize for 

 his expl>rations in Tonkin. 



The published list of subjects for prize competition in 1S81, 

 1SS2, 1 88 J, and 1885 comprises the following subjects (briefly 

 stated) aaiong others: — Motor for tramways; physiology of 

 champignons; influence of environment on plant-organs; struc- 

 ture and development of cork ; internal organisation of European 

 edriophthalmale crustaceans ; cure of Asiatic cholera ; genito- 

 urinary organs ; revision of the theory of Jupiter's satellites ; 

 elasticity of crystalline IjDdies ; origin of atmospheric electricity, 

 and ciuses of electric phenomena in thunderstorms ; inoculation 

 as a prophylactic for domestic animals ; coloured parts of the 

 tegumentary system of animals, and fecundating matter of 

 animated beings ; marine, lacustrine, and terrestrial deposits on 

 the French coast since the Roman epoch ; botany of the North 

 of France ; diagnostic signs of death and prevention of premature 

 burial. 



MEASURING THE INDEX OF REFRACTION 



OF EBONITE ' 

 ■pROF. BELL found that when an intermittent beam of light 

 ■*■ fell on a sensitive selenium cell the sound pri'duced in a 

 telephone (which with a battery was attached to the selenium) 

 was not»entireIy de-troyed by interpo;in'J a thin sheet of ebjnite 

 in the path of the intermittent rays of light, or, in other words, 

 that ebonite was slightly transparent for invisible rays that 

 affected selenium. It occurred to us some months ago that if 

 such invisible rays were at all df the nature of light, they probably 

 suffered retardation in passing through the eb >nite, or that 

 refraction would take place if the sheet of elwnite vvere replaced 

 by an ebonite prism or lens, a result we have been able experi 

 mentally to confirm, and at the same time to measure the index 

 of refraction. 



A H is a glass lens concentrating a parallel beam coming from 

 a lime-light on to one hole H in a rapidly revolving brass disk 

 cr>. This disk we have constructed manv times as thick as the 



one employed by Prof. Bell, and have thus succeeded in eliminat- 

 ing all the sound produced by the syren action of the disk, so 

 disturbing in delicate experiments. E F is a stationary zinc 

 screen with a hole in it smaller than the holes in the rotating 

 disk. 



1. We first tried to focus these intermittent rays on a selenium 

 cell by means of an ebonite lens, and so determine the focal 

 length of the lens ; but as our lens was then not mourned on an 

 optical bench, so as to be moved parallel to itself, or rotated 

 through known angles, and as the rays were invisible, so that our 

 eyes could not of course guide us as to the proper position in 

 which to put the lens, we failed to succeed in this very delicate 

 experiment, which however our subsequent experiments, now to 

 be described, show must ultimately succeed with the lens 

 properly mounted. 



2. A small portion of the intermittent light which passed 

 through the hole H in the rotatory disk was allowed to fall on 

 an ebonite prism K L, by passing through a slit in a zinc screen 



' Note communicated to the Royal Society by Profesiors Ayrton and 

 Perry. 



G J, the slit being arranged parallel to the edge of the ebonite 

 prism. The prism employed had an angle of 27°'5. M N is 

 another zinc screen with a slit in it also parallel to the edge of 

 the prism, and pl.aced in front of a sensitive selenium cell s (the cell 

 described by us in the accinnt of our experiments on " Seeing by 

 Electricity"). This screen M N was moved parallel to itself, while 

 an experimenter listened with a telephone to each ear, and who 

 was placed in another room, so as not to be influenced by seeing 

 what clvmLjes were being made in the position of the screen or 

 in the position of the ebonite prism. The telephones had each 

 a resistance of 74 ohms, and the battery nn electromotive force 

 of about 40 volts. No direct light falling on the selenium, the 

 listener at the telephones heard nothing for the majority of posi- 

 tions of the screen M N, but in one position representcil in the 

 figure a faint distant sound was distinctly heard, which was 

 entirely cut off by interposing the hand in front of the selenium, 

 or by moving away the prism. 



The invisible rays that affect selenium after passing through 

 ebonite are consequently refracted, and some preliminary experi- 

 ments, when the ebonite prism was arranged for minimum devia 

 tion, gave I'7as a first rough approximation for the index of 

 refraction of these rays by ebonite. It is interesting to notice 

 that the square, 2'89, of this index of refraction is between the 

 highest and lowest limits obtained by different experimenfers for 

 the specific inductive capacity of ebonite, so far agreeing with 

 Maxwell's electromagnetic theory of light. 



We are now having prisms of ebonite and of other opaque 

 substances of different angles mounted on a goni imeter stand, 

 to enable us to measure the indices of refraction accurately. 



MOLECULAR ELECTROMAGNETIC 

 INDUCTION^ 

 'THE induction-currents balance which I had the honour of 

 ■^ bringing before the notice of the Royal Society (Proc. 

 Roy. Soc. vol. xxix. p. 56) showed how extremely sensitive it 

 was to the slightest molecular change in the compo>ition of any 

 metal or alloy, and it gave strong evidence of a peculiarity in 

 iron and steel which its magnetic properties alone failed to 

 account for. We could with all non-magnetic metals easily 

 obtain a perfect balance of force by an equivalent piece of the 

 same metal, but in the case of iron, steel, and nickel it was 

 with extreme difficulty that I could obtain a near approach to 

 a perfect zero. Two pieces of iron cut off the same bar or wire, 

 possessing the same magnetic moment, never gave identical 

 results; the difficulty consis'ed, that notwithstanding each bar or 

 wire could be easily made to produce the same inductive reaction, 

 the time during which this reaction took place varied in each 

 bar ; and although I could easily change its balancing p.iwer as 

 regards inductive f irce by a change in the ma^s of ihe m^-tal, by 

 heat or magnetism, the zero obtained was never equal to that 

 obtained from copper or silver. 



This led me to suppose the existence of a peculiarity in mag- 

 netic metals which could not be accounted for except u.jon the 

 hypothesis that there was a cause, then unknown, to produce 

 the invariable effect. 



Finding that it would be impossible to arrive at the true 

 cause without some new method of investigation, which should 

 allow me to observe the effects from an electrical circuit, whose 

 active portion should be the iron wire itself, I constructed an appa- 

 ratus or electro-magnetic inducti m balance, consisting of a single 

 coil reacting upon an iron wire in its axis, and perpendicular to 

 the coil itself ; by this means the iron or other wire itself became 

 a primary or secondary, according as the current passed through 

 the coil or wire. Now with this apparatus we could induce 

 secondary currents upon the wire or coil, if the coil was at any 

 angle, except when the wire was absolutely perpendicular ; in 

 this state we could only obtain a current from some dis'urbing 

 cause, and the current so obtained was no longer secondary but 

 tertiary. 



The whole apparatus however is more complicated than the 

 general idea given above, as it was requisite not only to produce 

 effects, but to be able to appreciate the direction of the electrical 

 current obtained, and have comparative measure^ of their value. 

 In order to fully understand the moie of experiments, as well 

 as the results obtained, I will first describe the apparatus 

 employed. 



The electro-magnetic induction balance consists — (i) of an 



' Paper read at the Royal Society, March 17, by Prof. D. E. Hughes, 

 F.R.S. 



