63 
the slope of the curve becomes, ie. the greater « becomes. 
We call the energy on the anterior and posterior sides respectively 
va and x,, the retardations of growth y, and Yq. 
growth retardation 
Fig. 1. energy 
m—1 
M.C.S. the difference 
In a straight with «.—a, constant = b 
m 
in growth retardation y.—y, is always = q (fig. 1). 
3 : m—1 
In a curve with decreasing slope, with «,—«#, constant — 6—— 
m 
M.C.S. the difference in growth retardation ya—yp is smaller, 
according as xq is larger (fig. 2). 
Now aq will become greater by omnilateral fore-illumination, for 
—l 
eo ze + 6 M.C.S. The absolute energy difference 6 MES. oF 
n m 
rather the energy of 6 M.C.S. applied unilaterally, will therefore give 
a smaller difference of growth retardation i.e. a smaller curvature, 
according as the energy of fore-illamination a is greater. In order 
nevertheless to obtain a constant difference of growth retardation, 
m—l 
giving a just visible curvature, 6 and therefore also 6 will have 
to be greater. 
If the energy of fore-illumination becomes 
greater, the threshold value will rise, as soon 
as the curve of growth retardation is a curve 
with decreasing slope. Here we need not yet 
assert anything about the further relation be- 
tween the magnitude of the slopeand «. 
