  Vesto Melvin Slipher was the first astronomer to clearly define the factors
that determine the "speed" of a nebular spectrograph. This brief historical
summary recounts the way these ideas developed and how Slipher's early work on
galaxy Doppler shifts was so quickly extended in the 1930's when Milton Humason
and Edwin Hubble at Mt. Wilson Observatory began to push the velocity-distance
relationship to such a depth that no one could doubt its cosmological
significance.
