66 Sidereal Explorations
more than three magnitudes beyond the previous limit of
completeness corresponds to an extension of the survey to
four times the distance.
To accompany the chart of progress, Table II has been
compiled. It shows that nearly one hundred thousand stars
have been classified by Miss Cannon during the past eight
years. The determination of the photographic magnitudes
lags far behind, however, so that only half of the new
spectra are published. The present state of photographic
photometry is such that the determination of the magni-
tudes of these faint Milky Way stars is a more serious prob-
lem than the classification of their spectra ; but we hope to
have remedied within a few years these photometric defi-
ciencies.
Items 4 and 5 in Table II indicate zones, extending
through all right ascensions, which were especially studied
at the request of Professor Schlesinger of the Yale Observa-
tory in order to assist in his work on stellar motions. The
magnitude limit for these zones is much higher than for
other regions of the Henry Draper Extension, since the
object in view was merely to get spectra of stars of accur-
ately measured positions.
Until large representative areas in different parts of the
Milky Way have been completed, it would be premature to
analyze in detail the accumulating data on the spectra and
magnitudes of faint Milky Way stars. As an illustration,
however, of the manner in which the Henry Draper Exten-
sion contributes to our knowledge of the structure of the
local system, I give herewith a series of diagrams illustrat-
ing the distribution of the stars of different spectral classes
on the surface of the sky throughout the “Taurus” region,
and also for different parts of this region curves illustrating