(713) 
4, Threshold value and refractory period are complex quantities 
which originate in the imperfect isolation of the reflex-are from the 
surrounding medium and in the passive resistances of the chemical system. 
5: Augmentation and summation of the effect of stimulation are the 
consequence of not compensated changes (in the sense of CLAUSIUS). 
6. The form which expresses the law of WEBER-FECHNER is a 
formula of interpolation deduced from the principle of entropy. 
Dynamics. — H. A. Lorentz. “Some considerations on the princi- 
ples of dynamics, in connexion with Hertz’s “Prinzipien 
der Mechanik”. 
In his last work Hertz has founded the whole science of dy- 
namies on a single fundamental principle, which by the simplicity 
of its form recalls Newton's first law of motion, being expressed 
in the words that a material system moves with constant velocity 
in a path of least curvature (“geradeste Bahn”). By means of the 
hypothesis that in many cases the bodies whose motion is studied 
are connected to an invisible material system, moving with them, 
and by the aid of a terminology akin to that of more-dimensional 
geometry, HERTZ was able to show that all natural motions that 
may be described by the rules of dynamics in their usual form, 
may be made to fall under his law. 
From a physical point of view it is of the utmost interest to 
examine in how far the hypothesis of a hidden system, connected 
with the visible and tangible bodies, leads to a clear and satisfactory 
view of natural phenomena, a question which demands scrupulous 
examination and on which physicists may in many cases disagree. 
On the contrary, it seems hardly possible to doubt the great advan- 
tage in conciseness and clearness of expression that is gained by 
the mathematical form Hertz has chosen for his statements. I 
have therefore thought it advisable to consider in how far these 
advantages still exist, if, leaving aside the hypothesis of hidden 
motions, and without departing from the general use in dynamical 
investigations, one considers the motion of a system as governed by 
“forces” in the usual sense of the word. 
In what follows there is much that may also be found in the 
book of Hertz. This seemed necessary in order to present the 
subject in a connected form. 
As to the authors who have, before Hertz, published similar 
investigations, I need only mention BELTRAMI, LipscuitTz and DARBOUX. 
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