398 



NATURE 



[August 2 2, 1889 



at the meeting of the Association next year at Limoges. 

 That they did not forget past events was proved by the 

 name of the restaurant in which they met, but he firmly 

 beheved that " Le temps prime la force," and he drank 

 " to the peace of the whole world." 



MM. van Beneden, Dekterev, Istrati, Hartog, Ramon 

 de Luna, Llaurado, Stephanos, Timiriazeff, and Watson, 

 spoke in the name of their respective countries, and drank 

 to the cause of peace and to the prosperity of France. 



On Friday, August 16, a general excursion took place 

 to the paper works at Essonne, and to the works of M. 

 Decauville (the constructors of the narrow-gauge railway 

 in the Exhibition), who offered a lunch to their guests. 



Thus concluded this brilliant and successful session of 

 the French Association. 



All foreigners must have carried away with them a 

 pleasant memory of their welcome. Special thanks are 

 due to Prof Gariel, the Secretary of the Association, 

 and to the Vice-Secretary, Dr. Cartaz, for the remark- 

 able kindness and courtesy they showed to the foreign 

 guests. 



EXPERIMENTS ON ELECTRO-MAGNETIC 

 RADIATION, INCLUDING SOME ON THE 



PHASE OF SECONDARY M^AVES. 

 T N continuation of some experiments which were de- 

 ■*■ scribed in Nature, vol. xxxix. p. 391 (" Repetition of 

 Hertz's Experiments and Determination of the Direction 

 of the Vibration of Light ") attempts were made to obtain 

 periodic reflection of electric radiation from plates of 

 different thicknesses, analogous to Newton's rings, with 

 the view of further identifying these radiations with 

 " light." 



It was there described how a sheet of window-glass 

 refused to reflect the Hertzian waves, but how a masonry 

 wall reflected them readily. The non-reflection from the 

 thin sheet is due to the interference of the reflected waves 

 from each side which takes place owing to a ehange of 

 phase of half a period on reflection at the second surface, 

 as in t.ie black spot of Newton's rings. 



By making the reflecting plate such a thickness that 

 the reflection from the ba.k has to travel half a wave- 

 length further than that from the front, the two reflections 

 ought to be in accordance, for they differ by a whole 

 period, half arising from difference in path, and half from 

 change of phase on reflection ; but if the chff"erence in 

 paths were made a whole wave-length by doubling the 

 thickness of the plate, there ought again to be inter- 

 ference, and so on. 



The first plan tried with this end in view, was to fill a 

 large wooden tank to different depths with water or other 

 liquids. On gradually filling the tank reflection should 

 be obtained, and at a certain depth equal to 1/4 (\ sec r)//Li, 

 reach a maximum ; further addition of the liquid then 

 should diminish the reflection, and at double the above 

 depth the reflection should reach a minimum, the two 

 waves interfering. 



The mirrors for concentrating the radiation had for 

 this purpose to be suspended over the tank as shown in 

 the figure. The tank was first tried empty, but unfortu- 

 nately the wooden bottom was found to reflect, thus it 

 was useless for the purpose intended. I then tried what 

 ought to have been tried before constructing the tank — 

 namely, whether ordinary boards, such as flooring, 

 reflected. The floor was found to reflect readily. This 

 was attributed to moisture in the wood causing it to con- 

 duct, speciafly as wood was found not to polarize by 

 reflection. Experiments were then undertaken to deter- 

 mine if water reflected, even though in thin sheets. A 

 large glass window was placed beneath the mirrors and 

 flooded with water ; this was found to reflect well, both 

 when the mirrors were in the position shown and when 

 rotated to the position "at right angles." Thus water 



also acts like a metal, reflecting the radiation however 

 polarized. The glass had to be hardly more than damp 

 in order to get some reflection. 



The wooden tank being unsuitable, a glass tank was 

 thought of, but was given up for solid paraffin, which, 

 being in slabs, could be easily built up into a vertical 

 wall of any desired thickness. Through the' kindness of 

 Mr. Rathborne a large quantity of this was lent for the 

 purpose. 



A thin sheet of paraffin about 2 centimetres thick was 

 found not to reflect, as was expected. Next a wall 1 3 centi- 

 metres thick (180 centimetres long, 120 centimetres 

 high) was tried, and found to reflect, this being the 

 thickneis required in order to add another half period to 

 the retardation of the wave reflected from the back at an 

 incident angle of 55', the wave-length being taken as 

 66 centimetres, and the index of refraction being taken as 

 I '5 1, the square root of 2'29, the value taken as the 

 specific inductive capacity of paraffin. 



Then a wall twice the thickness was tried, but it also 

 reflected, contrary to expectation. While in doubt as to 

 the cause of this, it was decided to make a determination 

 by direct experiment of the index of refraction of paraffin 

 for these waves, by a method suggested in Nature (vol. 

 xxxix. p. 393), which consists in interposing a sheet or 

 wall of paraffin between the resonator and the metallic 

 reflection in the Hertzian experiment of loops and nodes 

 which are formed by the interference of the reflected wave 

 with the direct radiation ; the ratio of the velocity in the 

 wall to that in the air being easily found from the observed 

 shifting of the loops and nodes towards the screen. 



Fig, 



In this way the index of refraction for the radiation of 

 the period employed was found to be about 18, so that 

 the paraffin walls which had been used were too thick ; 

 The proper thickness being about 10 and 20 centimetres 

 — exactly so for an incident angle of 51°. On making 

 this alteration I fancied I could detect a slight difference 

 between the reflections from the thick and thinner walls ; 

 still the difference was not sufficient to be at all satisfactory. 

 The nature of the observing apparatus makes it almost 

 impossible to say if the reflection on one occasion is more 

 intense or less so than on another so long as sparks can 

 be obtained. This is due to the sparking-point in the 

 receiving apparatus continually requiring readjustment 

 when working with small sparks, as the distance between 

 them changes either from shaking or from the points 

 getting burnt up. Dust and moisture from the observer's 

 breath are also troublesome.^ Thus it might be quite 

 possible that the points had always to be much closer 

 with the 20 centimetre wall than with the 10 centimetre 

 wall in order to get sparks, and yet the difference escape 

 detection ; the thing observed being whether sparks can 

 be obtained or not, the eye being incapable of comparing 

 with any degree of accuracy the intensity of hght on one 

 occasion with that on another. 



However, if it had been possible to suddenly change 

 the wall, while viewing the sparking, from being 10 to 

 20 centimetres, it would have been easy to detect any 



I With 7iery ^;«a// sparks the thermal expansion must be counteracted by 

 unscrewing. 



