  We present a translation of Albert Einstein's Rio de Janeiro manuscript on
light quanta. In it, Einstein evaluates the Bohr-Kramers-Slater refutation of
light quanta, which was concurrently the subject of intense empirical scrutiny
on two continents. Written shortly before Heisenberg's discovery of quantum
mechanics, the manuscript represents Einstein's last published remark on the
constitution of light in the historical period known as the old quantum theory.
It crystallizes the fact that by 1925 the photon concept was still incomplete,
even as the corpuscular properties of light gained decisive empirical
confirmation.
