How did Bertrand Russell first learn about Gottlob Frege’s work?

Bertrand Russell first learned of Gottlob Frege’s work near the end of his own research for his book The Principles of Mathematics (1903).1

Upon discovering that Frege had already anticipated his own logicist philosophy of mathematics in the earlier work The Foundations of Arithmetic (1884), Russell acknowledged Frege's prior discoveries by adding an appendix to his own book. In this appendix, Russell discussed Frege's contributions and noted that the German mathematician had written "many things which I believed I had invented".1