ASTEOPHYSICS IN" THE UNITED STATES BOSLER. 367 



avoid the irregular refractions produced in the lower air by the 

 heated ground, it occurred to Dr. Hale to place the coelostat upon a 

 tower some 20 meters high, sending the-rays vertically downward 

 through a lens of 30 centimeters diameter and 18 meters focal length. 

 The solar image is formed about 1.50 meters above the ground upon 

 the slit of a spectroscope (pi. 6). The light beam then descends into 

 a well 9 meters deep, traverses the dispersive system, which may be 

 rotated about a vertical axis, and the essential part of which is either 

 a Rowland 4-inch grating or a Michelsson 8-inch grating, at will, 

 and finally returns to the level of the ground to the photographic 

 plate. There is yet another spectroscope with a prism, a companion 

 to the last one, - which is of great dispersion and mounted as a spec- 

 troheliograph. This gives a monochromatic image of a portion of 

 the sun's disk, and either this or the one preceding may be employed 

 at will. 



It was with the grating spectroscope of the tower telescope that 

 Hale discovered the magnetic field in sun spots^ one of the most 

 beautiful discoveries relating to the sun in recent times. The agi- 

 tated appearance of the hydrogen flocculi about the spots suggested 

 an investigation as to whether the Zeeman effect might not be produced 

 by them. The hypothesis that the spots have electrically charged 

 matter in rotation indicated the possibility of the existence of such a 

 phenomenon. Indeed, the spectrum of the spots contains a great num- 

 ber of enlarged and reversed lines, an appearance which would be 

 produced -by a, smaller or greater degree of doubling. Therefore 

 Hale placed before the slit of his spectrograph a Fresnel rhomb and 

 a nicol serving the following purpose: The former transforms the 

 circular polarized light of the Zeeman doublets into plane polarized 

 pairs, and the second, according to its orientation, extinguishes one 

 or the other of the latter components. The experiment confirmed Dr. 

 Hale's theory. The rotation of the nicol 90° caused the disappearance 

 of the right or left line from its original position, while, the telluric 

 lines remained unchanged. Later it was found that this curious phe- 

 nomenon existed in all sun spots to a degree varying with their size, 

 and that the magnetic field seems to diminish greatly with the height 

 of the vapor above the photosphere, and later yet other peculiarities 

 were found which we can not think of describing here. A vast field 

 thus seems open for astrophysicists, and the astronomers of Mount 

 Wilson will certainly not leave it unexplored. 



In the realm of pure astronomy the Mount Wilson Observatory 

 possesses an instrument which in power is not surpassed by any 

 other in the whole world — the telescope of 1.52 meters (60 inches) 

 aperture, constructed under the direction of Ritchey, and in use 

 since December, 1908. Its mounting is interesting. The telescope 

 tube, which is of openwork construction, is carried in a forklike 



