462 ASTROPHYSICS 



doubts even as to ultimate success were quite generally felt and freely 

 expressed. The beginnings of success were made by Huggins * and 

 Pickering, 2 in showing that photography reveals, with great clearness, 

 the delicate spectral lines which the eye in purely visual observations 

 is unable to see at all. In 1888, Vogel 8 applied this knowledge in the 

 first photographic attempt to measure radial velocities, and his work 

 inaugurated a new era. His observations, obtained with a small tele- 

 scope and imperfect spectrograph, were not sufficiently accurate to 

 meet the needs of the principal sidereal problems, but they led to 

 several brilliant discoveries at Potsdam, and were invaluable in mark- 

 ing out the path of progress. It was not until 1896 that the use of 

 a powerful telescope, equipped with an efficient spectrograph, gave 

 results accurate enough to satisfy present requirements. 4 In fact, the 

 accuracy obtained exceeded our most hopeful expectations. 



It is not surprising that thirty years were required to develop 

 successful methods. The work is so delicate that, unless suitable pre- 

 cautions are taken at every point in the process, the errors introduced 

 may readily be larger than the quantities sought for. With the Mills 

 spectrograph, for example, a speed of nine kilometers per second dis- 

 places the lines only 0.01 mm. The probable error of a velocity deter- 

 mination for the best stars, such as Polaris, 5 is but one fourth of a 

 kilometer per second, corresponding to a linear displacement of 0.0003 

 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 improve- 

 ments 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. 6 



(3) The use of a longer collimator, permitting a wider slit, and 

 requiring larger prisms, with greater resolving power. 7 



(4) The use of simple prisms, of better glass, with better optical 

 surfaces. 8 



(5) Care in collimating, to insure that the star light and comparison 

 light traverse identically the same part of the collimator lens. 9 



Atlas of Representative Spectra, plate n. 



Annals, Harvard College Observatory, various volumes. 



Publications, Potsdam Astrophysical Observatory, vol. vn. 



Astrophysical Journal, vm, 123. 



Ibid, xiv, 60. 



Ibid, vm, 146. 



7 Ibid, vin, 125 



8 Astronomy and Astro-Physics, xn, 45. 



* Publications, Lick Observatory, m, 177. 



