( 1031 ) 
eliminated the degree of curvature at the moment when straightening 
out begins, can perhaps serve as measure. 
If one wishes to investigate the influence of some external condition 
on the sensitiveness, one can determine what quantity of energy 
gives a definite degree of curvature each time this condition is varied; 
in which case it is very convenient that the moment at which 
such a curvature becomes visible, is constant. 
What was formerly understood by reaction-time but what has 
now been found to be almost exclusively curvature-time, is constant 
for a definite quantity of energy. This curvature-time greatly 
increases according as the energy of stimulation is smaller, a fact 
clearly shown by the following curve. 
In conclusion I wish heartily to thank Dr. Brauw and in parti- 
cular Professor Weryt for their kind interest and advice. 
Utrecht. Bot. Laboratory. 
Crystallography. — “On the orientation of crystalsections.” By 
J. Scumutzer. (Communicated by Prof. C. E. A. Wicamann). 
(Communicated in the meeting of February 25, 1911). 
When determining the orientation of a secant-plane from the 
angles that the traces of three unparallel planes not lying in one 
zone include together, one generally obtains a biquadratic equation 
in cos 2e, furnishing as maximum 4 compatible roots. As now angle 
20 can be supposed at the same time in two quadrants, it follows 
that one finds 8 values for eo. With these values correspond 8 values 
of 5. If however three erystal-planes and a definite erystal-section 
are given, the secant-plane is entirely determined; which value of 
o and of © comes in consideration here, can be decided with certainty, 
if one takes into account the circumstance that a erystal-plane is at 
the same time the boundary-plane of the mineral substance. 
Being admitted a plane (Ak/) (ef. fig. 1) the pole of which lies in 
p, and forming with the plane C’ (4 c-axis) a secant-line AZ, then 
7 T « > e s 
the angle DBE —=a< = a8 filled with mineral-substanee, the obtuse 
angle HA/ on the contrary is not. Now one can suppose the projection- 
globe divided into 8 oetants of which 4 are lying above and 4 beneath 
the projection-plane C, and of which the first two (BOD ==, 
DOA = 1) contain the acute plane-angle DBM. If one fastens sto 
the coordinates « = ~ BM, 6 = ~ Ms, then s lies in the 15* globe- 
octant, calculated from AB; for a plane 7” (hkl) s lies in octant IIL, 
