Page:Popular Science Monthly Volume 66.djvu/316

312 mm., or 0.00001 inch. In view of the newness of the subject, the richness of the field, and the fact that the more active great telescopes are now nearly all applied to this work, I append a list of the improvements which have contributed most powerfully to recent progress:

1. A realization of the fact that a spectrograph is an instrument complete in itself. The telescope to which it is attached serves only to collect the light and to deliver it properly upon the slit.

2. The development of a method of reduction which permits the use of all good stellar lines, irrespective of whether they correspond to, or lie between, the comparison lines.

3. The use of a longer collimator, permitting a wider slit, and requiring larger prisms, with greater resolving power.

4. The use of simple prisms, of better glass, with better optical surfaces.

5. Care in collimating, to insure that the star light and comparison light traverse identically the same part of the collimator lens.

6. The adoption of a compact and rigid form of spectrograph mounting, designed in accordance with good engineering practise.

7. The elimination of flexure effects by supporting the spectrograph, in connection with the telescope, in accordance with engineering principles. The conventional spectrograph had been supported entirely at its extreme upper end; the instrument projected out into space, unsupported, boldly inviting flexure under the varying component of gravity.

8. The use of a constant temperature case around the instrument.

9. Precautions taken to eliminate many sources of error from the measures of the spectrograms.

Up to December, 1900—the last month of the departing century—the speeds of 325 stars had been determined with the Mills Spectrograph in the northern two thirds of the sky. Omitting several stars whose lines could not be measured accurately, and some thirty spectrograph and visual binaries for whose centers of mass the velocities were still unknown, 280 stars remained available for deducing the relative motion of our solar system. The observational data were distributed symmetrically in right ascension, and the result for this coordinate of the apex agreed with Newcomb's proper-motion result within a small fraction of a minute of arc. The data were extremely unsymmetrical in declination, as there were few observations between −15° and −30° declination, and none whatever south of −30°. The solution placed the apex 15° south of Newcomb's position. The deduced speed, 20 km. per second, is no doubt close to its true value.

There is a question whether the direction of the solar motion can be determined more accurately from proper motions or from radial velocities, an equal number of stars being available in the two cases; but as to the speed, no doubt of the very marked superiority of the spectrographic method can exist. This, however, is but incidental,