July 21, 1923] 



NATURE 



115 



the same as that which was apphed to the action 

 of pumps, with the difference presumably due to the 

 density of the liquid. 



The problem was not the fact but its significance. 

 The Aristotelians held that it had been definitely 

 established that the atmosphere had no weight, and 

 what is quite certain is that no means of discovering 

 its weight, if it had any, then existed. It is therefore 

 strikingly analogous to the case of the hypothetical 

 ether when physicists were engaged in devising means 

 of revealing its presence. Descartes, though entirely 

 opposed to the Aristotelians, yet held on a priori 

 grounds that the universe was a plenum. He needed 

 the concept in order to explain the vortex motion 

 which in his view accounted for the variety of material 

 forms. The apparent vacuum in the Torricelli tube 

 he supposed to be due not to an absence but to the 

 presence, or rather to the entrance under the conditions 

 of the experiment, of a very subtle matter. Pascal, 

 on the other hand, to quote M. Painleve, " s'inspirant 

 de Galilee et Torricelli, entre la science d'Archimede 

 et la science moderne, jette un pont par-dessus vingt 

 siecles. La presse hydraulique, le barometre observe 

 a diverses altitudes, ce sont les illustrations d'une 

 statique nouvelle qui embrasse a la fois, dans les 

 memes principes, I'equilibre des liquides et celui des 

 gaz." Let us imagine, he went on to say, that 

 through some disaster everything which we now know 

 about Pascal had been lost to us save only his scientific 

 writings. In what light would he appear to us ? 

 We should be right to point him out as the first of 

 the positivists methodically disengaging facts from 

 the confusion of words, discrediting purely verbal 

 definitions, " cet air subtil qui aurait des inclinations," 

 " cette lumiere qui est un mouvement luminaire de 

 corpuscules lumineux." 



Other memorable discourses followed, in particular 

 one by M. Picard, before we sat down to the sumptuous 

 banquet which the Municipality offered its guests. 

 When this was concluded the President of the 

 Republic rose and to the enthusiastic applause of the 

 company, though, it must be admitted, to the general 

 surprise of those who were thinking of Pascal, 

 delivered an impassioned and truly eloquent speech on 

 the politics of the hour, which was immediately 

 transmitted round the world. The rain ceased, and 

 we made the return journey to Clermont favoured by 

 a clear sky and splendid view over the grand Auvergne 

 country. The city was gaily decorated for another 

 celebration in the square over which the statue of 

 the seated Pascal presides. H. Wildon Carr. 



An Advance in Photometry. 



TJ ERR E. STEINKE, in an investigation into the 

 ^ *■ accuracy of the Wien-Planck law in the ultra- 

 violet region of the spectrum, has recently made use 

 of Elster and Geitel's potassium photo-electric cell, 

 as neither the thermopile nor the bolometer was 

 sufficiently sensitive to measure the minute quantities 

 of radiant energy involved {Zeits. f. Physik., ii. 

 4 and 5, pp. 215-238, 1922). 



Herr Steinke has found it possible to increase the 

 sensitiveness of the cell enormously, by increasing 

 the voltage between the potassium anode and the 

 platinum cathode ; and has carefully investigated 

 the behaviour of the cell under these conditions. For 

 red light, X = 63omm, an increase in voltage from 20 

 to 210 multiplies the sensitiveness by 1045, and for 

 violet light, X^462yu/i, by 1595. This is due to the 

 increased ionisation from collisions Ijetween the greatly 

 accelerated electrons and the argon contained in the 

 cell. 



NO. 2803, VOL. 112] 



It was not possible to apply such a high voltage to 

 the cell suddenly without a luminous discharge ; it 

 was necessary to raise it gradually for hours or even 

 days at a time, and it was then possible to reach the 

 210 volts already mentioned. At 212 volts luminous 

 discharge took place, after which the cell discharged 

 at 162 volts, recovering if left to itself for some days, 

 so that the potential could again be raised to 200 

 volts. In forming the cell, as above, for high voltages 

 it was found that each time the voltage was increased 

 there was a strong " darkness " current at first ; 

 which diminished in the course of time to zero for 

 lower voltages, and at 210 volts to a moderate value 

 after several days. 



When the cell is prepared in this way, and is 

 illuminated, a fatigue effect is observed, the time 

 required for the thread of the Lutz-Edelmann electro- 

 meter to move from a certain division on the scale 

 to another (85 to 65) gradually increasing to a limit- 

 ing value, which in one experiment was reached in 

 about fifteen minutes. This limiting value was deter- 

 mined in all the experiments. These were always made 

 in the order of increasing illumination, as it was found 

 that a high illumination produced a kind of after 

 action, and a small illumination measured shortly 

 afterwards showed a higher value than the real one. 

 For the high voltages employed the limiting value of 

 the photo-electric current was not proportional to the 

 illumination, as it was found to be by Elster and 

 Geitel for moderate voltages ; but the following 

 relation was proved to exist, I^ =ML ; where I is 

 the photo-electric current, L the flow of radiant 

 energy, and z and M are constants. The cell con- 

 stant z varies with the wave-length, and with the 

 voltage applied to the cell ; for small voltages it 

 scarcely differs from unity, and the law becomes 

 identical with that of Elster and Geitel ; for \ = 3i6,ti/a 

 z was observed to be 1-3495, with 208 volts on the 

 cell terminals, and intermediate values were found for 

 other wave-lengths and voltages. 



Using the method described, Herr Steinke has 

 measured the exponent c^ in Planck's formula 



C I 

 EX = j^ CjAT _ ^^^ ^ number of different wave-lengths, 



including 340,^/^. and 3i6/;t/i in the ultra violet. The 

 mean value is 14,385, the largest variation from this 

 being 0-63 per cent., and the mean probable error 

 o-i6 per cent. The actual probable error is rather 

 greater than this, owing to the difficulty of deter- 

 mining the wave-length of the rays employed, which 

 were separated from the light of a carbon in- 

 candescent lamp by filters. The value agrees satis- 

 factorily with that determined by previous observers 

 for visible radiations. The observations also show 

 that carbon radiates, throughout the range of wave- 

 lengths investigated, as a grey body. Herr Steinke 

 proposes to continue the investigation, and to measure 

 the constant C in the above formula. 



It would appear that the improved method of 

 using the photo-electric cell should prove of the highest 

 value in work on the spectrum, particularly in the 

 ultra violet. 



International Conference on Nature 

 Reserves. 



T^HREE Associations in France entitled, respect- 

 ^ ively, the Societe Nationale d'Acclimatation de 

 France, the Ligue Fran^ais pour la Protection des 

 Animaux, and the Societe pour la Protection des Pay- 

 sages de France, recently invited the Royal Society for 

 the Promotion of Nature Reserves to send delegates 

 to attend a conference " Pour la Protection de la 



