30 



Vc 



opened into the joint tube 0, where the wire was in 

 contact with mercury, which in its turn was in contact 

 with the wire K, which is continually 

 immersed in the mercury. 



In the thick-walled capillary B, ope- 

 ning at the top in a glass funnel E, 

 the platinum-wire F, passed through 

 the cork L, can be pulled up and down 

 to adjust the regulator at a definite 

 température. The wire F is connected 

 with the other pôle of the accumulators,* 

 with which the resistance-box and the 

 interruptor, for breaking the main cur- 

 rent are also connected. 



The regulator as a whole is covered 

 by a mantle G with legs H and I in 

 order to support the glass-frame. At the 

 height of the mercury-reservoir fourrows 

 of openings in the copper jacket are made. 

 The daek room. 



Ail experiments took place in the dark 

 room of the Botanical Laboratory at 

 Utrecht; this dark room has been used 

 exclusively for experiments on geotro- 

 pism and phototropism, during the last 

 few years. The area of this dark room 

 is 3X3 mètres, its height is 4i mètres, 

 it is wholly painted dead-black inside. 

 The gasmain has been out of use for 

 several years, but the room has one 

 permanent electric light and there are three switches 

 available. A double door with half a meter space between 

 the two doors makes it possible to enter or to leave the 

 room without any light entering. 



Fig. 4. Scale 1 : 2,6. 

 Total height 50 cm. 



