p. 13: If the radius of the sphere be 1, as we shall assume throughout the discussion of this general transformation, or its equation when written homogeneously, be :
$$x^{2}+y^{2}+z^{2}-t^{2}=0$$
the equations connecting x, y, z, t and X, Y, Z, T are those indicated in the following scheme
$$(6)\quad \begin{array}{c|c|c|c|c} & X+iY & X-iY & T+Z & T-Z\\ \hline x+iy & \alpha\bar{\delta} & \beta\bar{\gamma} & \alpha\bar{\gamma} & \beta\bar{\delta}\\ \hline x-iy & \gamma\bar{\beta} & \delta\bar{\alpha} & \gamma\bar{\alpha} & \delta\bar{\beta}\\ \hline t+z & \alpha\bar{\beta} & \beta\bar{\alpha} & \alpha\bar{\alpha} & \beta\bar{\beta}\\ \hline t-z & \gamma\bar{\delta} & \delta\bar{\gamma} & \gamma\bar{\gamma} & \delta\bar{\delta} \end{array}$$
p. 15: The general transformation (6) represents the totality of those projective transformations or collineations of space for which each system of generating lines of the sphere, ##x^{2}+y^{2}+z^{2}-t^{2}=0##, is transformed into itself, and among which all rotations of the sphere are obviously included as special cases. This is the geometrical meaning of the equation
$$\zeta=\frac{\alpha Z+\beta}{\gamma Z+\delta}$$
for unrestricted values of ##\alpha,\beta,\gamma,\delta##.
But the transformation admits also of a very interesting kinematical interpretation which I shall consider at length in my third lecture. With respect to it our sphere of radius 1 plays the role of the fundamental surface or "absolute " in the Cayleyan or hyperbolic non-Euclidian geometry.