  A history is given of the discovery between 1914 and 1935 of stars of
intermediate luminosity between giants and dwarfs with spectral types between
G0 to K3. The Mt Wilson spectroscopists identified about 90 such stars in their
1935 summary paper of spectroscopic absolute magnitudes for 4179 stars. Called
"subgiants" by Str\"omberg, these 90 stars defined the group at the time. The
position of the Mt Wilson subgiants in the HR diagram caused difficulties in
comparisons of high weight trigonometric parallaxes being measured and with
Russell's prevailing evolution proposal, and critics questioned the reality of
the Mt Wilson subgiants. We compare, star-by-star, the Mt Wilson spectroscopic
absolute magnitudes of the 90 stars defining their sample against those
absolute magnitudes derived from Hipparcos (HIP) trigonometric parallaxes. We
address concerns over biases in the Mt Wilson calibration sample and biases
created by the adopted methodology for calibration. Historically, these
concerns were sufficient to discredit the discovery of subgiants in the Mt
Wilson sample. However, as shown here, the majority of the Mount Wilson stars
identified as subgiants that also have reliable HIP trigonometric parallaxes do
lie among the subgiant sequence in the HIP HR diagram. Moreover, no significant
offset is seen between the M(V) brightnesses derived from the Mt Wilson
spectroscopic parallaxes and the M(V) values derived from Hipparcos
trigonometric parallaxes with a fractional error of 10%, which confirms in an
impressive manner the efficacy of the original Mt Wilson assessments. The
existence of subgiants proved that Russell's contraction proposal for stellar
evolution from giants to the main sequence was incorrect. Instead, Gamow's 1944
unpublished conjecture that subgiants are post main-sequence stars just having
left the main sequence was very nearly correct but was a decade before its
time.
