[10]
![{\textstyle \displaystyle \left\{{\begin{alignedat}{2}\xi '&=\eta Z&&-\zeta Y,\\\eta '&=\zeta X&&-\xi Z,\\\zeta '&=\xi Y&&-\eta X;\end{alignedat}}\right.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/18a8832e0cd9e4c0fdc2e3f200ca81a331c84cb4)
[11]
![{\textstyle \displaystyle \left\{{\begin{alignedat}{2}\xi &=Y\zeta '&&-Z\eta ',\\\eta &=Z\xi '&&-X\zeta ',\\\zeta &=X\eta '&&-Y\xi '.\end{alignedat}}\right.}](https://wikimedia.org/api/rest_v1/media/math/render/svg/8db1fae6a34ed0017e21914e37fadc3a4138322d)
Likewise,
are proportional to
and if we set
where
denotes the radius of curvature of the line
then
By differentiating (7) with respect to
we obtain
We can easily show that
also are proportional to
In fact, [from 10] the values of these quantities are also [equal to]
therefore
and likewise the others. We set, therefore,
whence