The advance of photography : its history and modern applications (1911)

300 THE ADVANCE OF PHOTOGRAPHY spot is allowed to fall on the slit on either side. By this means the spot spectrum is obtained between two strips of the ordinary solar spectrum, and a comparison of the lines obtained can then be made. The exposure required for sunspots may last from a few minutes to over one hour, and is from 5 to 20 times as long as that required for the solar spectrum. The spectrum of a sunspot was first examined by Lockyer as far back as 1866, and he found that many of the lines of the solar spectrum were widened where they crossed the sunspots. Since that time the observations of these widened lines have been carried on in a systematic manner. Among the chief workers in this field have been Young and Mitchell, who worked with the 23-inch Princeton refractor, and a powerful grating spectroscope. They found that many of the lines in the sunspot spectrum really became doublets. Mitchell afterwards made a special study of these doublets. With the Tower Telescope and spectroscope just described these widened and doubled lines have been photographed, and Professor Hale and his staff have investigated the question as to the cause of this, by comparison with results obtained under known conditions in the laboratory at Pasadena, which is attached to their observatory. By this means they have arrived at the conclusion that the doubling of the lines is due to the presence of a magnetic field in the sunspot regions, in fact, that in this they are in such cases dealing with what is known as the Zeeman effect. Not only have they done this, but by comparison with the known magnetic conditions in the laboratory experiments, they have been able to measure the strength of the magnetic field which causes the doubling of the lines. The strongest fields which they have yet measured by means of these photographs of sunspots is about 4500 gauss. They also find that the