722 
Hi ley. — On the Value of D iff 'event Degrees of 
where t is the temperature in degrees Centigrade. 1 This means that the 
viscosity coeff. will be at 8o° C. a third of what it is at io° C., a result which 
is not at all in keeping with Rutgers’s observations of presentation time. 
Also Heilbronn (T2) has shown that the movable starch-grains do not fall 
quite freely, but seem to be partially controlled by the streaming proto- 
plasm. 
But quite independently of presentation time, another method of 
investigating the effect of different degrees of stimulation has been 
employed. This is the method of neutralization of the effects produced by 
the alternation of opposing stimuli. It was employed independently by 
Fitting (’05), Newcombe (’05), and Hayes (’05). 
The results which Fitting (’05) obtained with his intermittent clinostat 
are now well known. He was able, through the invention of an extension 
to Pfeffer’s clinostat, to alternate the position of plant-members between two 
directions at different angles with the vertical ; and by this means he was 
able to compare geotropic effects of various kinds in the two positions. We 
are here chiefly concerned with one aspect of the work ; indeed the funda- 
mental part on which all the other portions of this piece of research may be 
seen to be dependent. 
A plant- member, say a hypocotyl, is made to rest in the position K, 
i. e. horizontal, with the side A uppermost 
and the side B lowermost. It rests so for 
a time t. Then by a rapid half-rotation of 
the clinostat axis the hypocotyl is made to 
assume the position Z, at an angle 0 below 
the horizontal and now with the side B 
uppermost and A lowermost. Here it re- 
mains for a period /, after which the clinostat 
axis completes the revolution bringing the 
hypocotyl back to its former position. The regular alternation of 
times t and f respectively in the two positions is maintained as long as 
desired. 
When the hypocotyl lies in position K it tends to bend upwards, i. e. 
towards A ; when it has taken up the position L it still tends to bend 
upwards, but now towards B. So the tendencies in the two positions are 
opposed, and a kind of ‘ tug-of-war ’ is set up. Which tendency (i. e. to turn 
towards A or B) will overcome the other depends on the angle 6 and the 
relative lengths of the times t and / . Fitting found that the two tendencies 
would exactly neutralize each other (i.e. the hypocotyl would grow straight) 
when -/ 
t 
cos 0, 2 a result which may be expressed geometrically as follows : 
1 Thompson and Pointing : Properties of Matter. 
2 Fitting expressed his results in terms of the angle between OL and the vertical. If we call 
