Footnote 23: _Vide_ note 15. Footnote 24: _Vide_ note 16. Footnote 25: _Vide_ note 17. Footnote 26: _Vide_ note 19. Footnote 27: _Vide_ note 18. Footnote 28: Vide note 40. APPENDIX Mechanics and the Relativity-Postulate. It would be very unsatisfactory, if the new way of looking at the time-concept, which permits a Lorentz transformation, were to be confined to a single part of Physics. Now many authors say that classical mechanics stand in opposition to the relativity postulate, which is taken to be the basis of the new Electro-dynamics. In order to decide this let us fix our attention upon a special Lorentz transformation represented by (10), (11), (12), with a vector _v_ in any direction and of any magnitude _q_ < 1 but different from zero. For a moment we shall not suppose any special relation to hold between the unit of length and the unit of time, so that instead of _t_, _t′_, _q_, we shall write _ct_, _ct′_, and _q_/_c_, where _c_ represents a certain positive constant, and _q_ is < _c_. The above mentioned equations are transformed into _r′__{_ṽ_} = _r__{_ṽ_}, _r′__{_v_} = _c_(_r__{_v_} - _qt_)/√(_c²_ - _q²_), _t′_ = (_qr__{_v_} + _c²__t_)/_c_√(_c²_ - _q²_) They denote, as we remember, that _r_ is the space-vector (_x_, _y_, _z_), _r′_ is the space-vector (_x′_ _y′_ _z′_) If in these equations, keeping _v_ constant we approach the limit _c_ = ∞, then we obtain from these _r′__{_ṽ_} = _r__{_ṽ_}, _r′__{_v_} = _r__{_v_} - _qt_, _t′_ = _t_. The new equations would now denote the transformation of a spatial co-ordinate system (_x_, _y_, _z_) to another spatial co-ordinate system (_x′_ _y′_ _z′_) with parallel axes, the null point of the second system moving with constant velocity in a straight line, while the time parameter remains unchanged. We can, therefore, say that classical mechanics postulates a covariance of Physical laws for the group of homogeneous linear transformations of the expression -_x²_ - _y²_ - _z²_ + _c²_ (1) when _c_ = ∞. Now it is rather confusing to find that in one branch of Physics, we shall find a covariance of the laws for the transformation of expression (1) with a finite value of _c_, in another part for _c_ = ∞. It is evident that according to Newtonian Mechanics, this covariance holds for _c_ = ∞ and not for _c_ = velocity of light. May we not then regard those traditional covariances for _c_ = ∞ only as an approximation consistent with experience, the actual covariance of natural laws holding for a certain finite value of _c_. I may here point out that by if instead of the Newtonian Relativity-Postulate with _c_ = ∞, we assume a relativity-postulate with a finite _c_, then the axiomatic construction of Mechanics appears to gain considerably in perfection.