Type O--bright bands on a faint continuous background, with five subdivisions, Oa, Ob, Oc, Od, Oe, according to the varying width and intensity of the bands. Type B--the Orion type, or helium type, with additional lines of origin unknown as yet, but without any of the bright bands of type O. Type A--the Sirian type, the regular Balmer series of hydrogen lines being very intense, with a few other lines not conspicuously marked. Type F--the calcium type, hydrogen lines less strongly marked, but with the narrow calcium lines H and K very intense. Type G--the solar type, with multitudes of metallic lines. Type K--in some respects similar to G, but with the hydrogen lines fading out, and the metallic lines relatively more prominent. Type M--spectrum with peculiar flutings due to titanium oxide, with subdivisions Ma and Mb, and the variable stars of long period, with a few bright hydrogen lines additional, in a separate class Md. Type N--similar to M, in that both are pronouncedly reddish, but with characteristic flutings probably indicating carbon compounds. The Draper classification being based on photographic spectra, and the original Secchi classification being visual, the relation of the two systems is approximately as follows: Secchi Type I includes Draper B & A II includes Draper F, G & K III includes Draper M IV includes Draper N Pickering's marked success in organization and execution of this great programme was due to his adoption of the "slitless spectroscope," which made it possible to photograph stellar spectra in vast numbers on a single plate. The first observers of stellar spectra placed the spectroscope beyond the focus of the telescope with which it was used, thereby limiting the examination to but one star at a time. In the slitless spectroscope, a large prism is mounted in front of the objective (of short focus), so that the star's rays pass through it first, and then are brought to the same focus on the photographic plate, for all the stars within the field of view, sometimes many thousand in number. This arrangement provides great advantages in the comparison and classification of stellar spectra. When spectroscopic methods were first introduced into astronomy, there was no expectation that the field of the old or so-called exact astronomy would be invaded. Physicists were sometimes jocularly greeted among astronomers as "ribbon men," and no one even dreamed that their researches were one day to advance to equal recognition with results derived from micrometer, meridian circle, and heliometer.