Complex sentence: A circuit is a current source only to the extent that its output current is independent of its output voltage, in the circuits of Figures 1 and 2, the output voltage of importance is the potential from the collector of Q3 to ground.
Split into simpler sentences: A circuit is a current source only to the extent that its output current is independent of its output voltage. In the circuits of Figures 1 and 2, the output voltage of importance is the potential from the collector of Q3 to ground.