624 Professor George E. Hah [May 14, 



cases where a dark central line is accompanied by wings, others in 

 which lines are thinned or completely obliterated, etc. 



I have already referred to the importance of applying in astrono- 

 mical research the methods of the physicist. During the last quarter 

 of a century the study of spectroscopic phenomena in the laboratory 

 has been completely transformed. It may well be said that this 

 transformation, which has involved such discoveries as spectral series, 

 the effect of pressure on wave-length, and the Zeeman effect, has 

 been directly due to tlie use of Rowland's concave gratings, of great 

 focal length, arranged for photography. In astronomical spectroscopy 

 great advances have also been made, but the spectroscope has 

 continued to occupy the place it formerly held as an attachment of 

 the telescope. Although Rowland used a long-focus concave grating 

 for his classic study of the solar spectrum, the heliostat and lens 

 employed with this instrument gave so small a solar image on the 

 slit that the investigation of sun-spots and other details was impos- 

 sible. We thus see that while in the observatory the spectroscope 

 continued to be used as an accessory of the telescope, in the labora- 

 tory the parts were exchanged, and the telescope was employed 

 simply as an accessory of the spectroscope. It seemed obvious that 

 a great opportunity for advance lay open to the investigator who 

 would comlDine a long focus spectroscope with a long focus telescope. 

 As it would be difficult or impossible to use for photography a suffi- 

 ciently long spectroscope attached to the tube of an equatorially 

 mounted telescope, some form of fixed telescope was plainly essential. 



The tower telescope on Mount Wilson (Fig. 5) is designed to 

 accomplish this purpose. It consists essentially of a 12-inch refract- 

 ing telescope, of 60 feet focal length, mounted in a fixed position, 

 pointed directly at the zenith . The ordinary telescope tube is replaced 

 in this case by a light steel tower, firmly held in position by steel 

 guy ropes. The 12 -inch objective lies horizontally at the summit of 

 the tower, and sunlight is reflected into it from the second of two 

 adjustable plane mirrors. The first of these mirrors is mounted as a 

 coelostat, and is rotated by an accurate driving-clock about a polar 

 axis at such a rate as to counteract the apparent motion of the sun. 

 Thus a beam of sunlight is reflected from the coelostat mirror to the 

 second mirror, which sends it vertically downward through the ob- 

 jective. In the focal plane, 60 feet below the objective, an image of 

 the sun, about 6 ' 6 iuches in diameter, is formed on the slit of a 

 spectrograph, at a height of about three feet above the surface of the 

 ground. After passing through the sHt, the light of any desired 

 portion of the solar image (a sun-spot, for example) descends 

 vertically into a well about 30 feet deep, excavated in the earth 

 beneath the tower. Thirty feet from the slit the diverging rays 

 encounter a 6-inch objective, through which they pass. After 

 being rendered parallel by the objective, the rays fall upon a Rowland 

 plane grating, ruled with 14,438 lines to the inch. The grating 



