786 



become very much the same and the curvature — and that is the 

 definitive phototropic curvatui-e of Phycomyces — only appears after 

 the maximum of the acceleration of growth, because the growth- 

 reaction of the less illuminated anterior wall diminishes somewhat 

 more rapidly than that of the posterior wall. 



To be on the safe side I will not extend the scope of these facts 

 further than to the positive phototropism of Phycomyces, but of this 

 phenomenon I think, that proof has now been furnished, that the 

 appearance of these curvatures is the result of an asymmetrical modl- 

 ücation of the growth of different sides of the cell, caused by the 

 asymmetrical illumination of these sides; that for this case therefore 

 UE Candoi.le'6' simple and ancient theory — more particularly in 

 contradiction to the later conception of Sachs — is reestablished and 

 with this a theory of a perception of the light-directio7i itself is 

 superfluous. 



In the meantime I do not wish to generalize, but only to await 

 the results of further investigations, which are being continued in 

 various directions, on Phycomyces and on the root and stem of higher 

 plants. Only, in my opinion, for no single one of these cases can 

 any decisive proof in the literature be found against de Candolle's 

 theory. 



Finally I desire to express my great indebtedness for the facilities 

 afforded me in the Laboratory of the Teyler Foundation, for carrying 

 out these experiments — and it is no mere formality, that I tender 

 my thanks especially to the conservator, Jhr. Dr. G. Elias, for his 

 assistance and interest in the arrangements for this investigation. 



Haarlem, Dec. 1013. 



Physics. — ''Magnetic researches. X. Apparatus for the general 

 cryomagnetic investigation of substances of small susceptibility." 

 By H. Kamerlingh Onnes and Albert Perkier. Communication 

 ■NM39rt from the Physical Laboratory at Leiden. (Continued). 

 (Communicated by Prof H. Kamerlingh Onnes.) 



§ 7. Sources of error. Sensibility. Accuracy. Disturbing magnetic 

 influences. The action of the magnet upon the carrier without the 

 experimental tube appeared to be negligible, even when the lower 

 end of the carrier was reduced to the temperature of liquid hydrogen. 

 The action upon the coil of the carrier was also imperceptible even 

 when a much stronger current /,„ was passed through this coil than 



