PROGRESS IN ASTROPHYSICS — ABBOT. 179 



The nature of sun spots has long been a subject of investigation. 

 In the last few years comparatively satisfactory conclusions have 

 been draAvn. It appears that sun spots are cooler than the surround- 

 ing surface of the sun. This is shown in several ways. In the first 

 place, a delicate electrical thermometer, called the bolometer, in the 

 hands of Langle}'^ and subsequent investigators, has shown a de- 

 creased temperature Avhen exposed to the rays from sun spots, as 

 compared with its temperature Avhen exposed to the rays of the sur- 

 face of the sun close b3^ In the second place, the spectrum of the 

 sun spot is found to differ from the spectrum of the solar surface in 

 the immediate neighborhood in certain very characteristic ways. 

 This difference has been investigated by the Mount Wilson Solar 

 Observatory. A photographic map of the sun-spot spectrum as com- 

 pared with the spectrum of the sun's surface has been published by 

 that observatory. The accompanying illustration (pi. 2) is taken 

 from an interesting portion of such a spectrum map. 



It shows in the first place that a large number of lines are found 

 in the sun-spot spectrum which are either very indistinct, or not to 

 be seen at all in the spectrum of the sun's surface. It shows in the 

 second place that certain lines are broadened, or made double, in the 

 spectrum of the sun spot as compared with the spectrum of the sur- 

 roundings. In the third place, that some lines are weakened and 

 some strengthened in sun spots, as compared with those of the sur- 

 roundings. The cause of the numerous additional lines in the sun- 

 spot spectrum has been found to be the presence of certain compound 

 substances, such as calcium hydride, magnesium hydride, and certain 

 oxides, as, for example, that of titanium. The cause of the different 

 intensity of certain lines in the spectra of the spot and of the sur- 

 roundings is shown by Hale, Adams, and Gale to be the decreased 

 temperature of the sun spot. This conclusion they confirm, line for 

 line, by noting the behavior of the lines of the corresponding chemi- 

 cal elements when observed at different temperatures, by the aid of 

 the spectroscope, in the laboratory. 



The doubling or widening of the lines of the sun-spot spectrum was 

 found by Hale to be due to the presence in sun spots of a magnetic 

 field. This observation depends on the discovery of Zeeman that the 

 spectrum lines of the chemical elements, when produced in a strong 

 magnetic field, are often doubled or trebled or made even more com- 

 plex. The component lines, so produced, depend as regards their 

 position, number, and the polarization of their light, upon the 

 strength and direction of the magnetic field through which they are 

 observed. The relation of magnetization to the polarization of the 

 light was the feature of the matter which laid the subject of the 

 widening of lines in sun spots open to Hale's investigation. By the 

 use of proper apparatus for the polarizing and analyzing of light. 



