TOTAL ECLIPSES OF THE SUN. 243 



C)ne of tlie most siiccessfiilh- observed of recent eclipses was that 

 of Ma}^ 28, ] 900. The duration of totality was only two minutes, but 

 almost perfect weather prevailed everywhere. It was visited by a large 

 number of skilled observers, and an examination of the work per- 

 formed and attempted will give a good idea of what astronomers at 

 the present day hope to learn about the sun at times of total eclipse. 

 As stated above, the ordinary solar spectrum consists of a bright band 

 crossed by dark absorption lines due to a reversing layer present in the 

 chromosphere. At the eclipse of 1870, Professor C. A. Young, who was 

 watching the spectrum of the fast disappearing sun , saw, at the instant 

 when the last bit of the photosphere was covered by the moon, the 

 solar spectrum with its dark lines replaced by a spectrum composed 

 of bright lines. This phenomenon, from the suddenness of its appear- 

 ance became known as the 'Flash.' The 'flash' spectrum is one of the 

 most interesting features of a total eclipse. The depth of the flash 

 layer is very small, and the duration of its greatest intensity very 

 brief, since it is covered by the moon after two or three seconds. To 

 obtain good photogi-aphs of this phenomenon is somewhat difficult. 

 This has been accomplished, however, at the eclipses of 1896 and 1898, 

 and, especially, by several observers, at the eclipse of 1900. Several 

 kinds of spectroscopes are in use. Ordinarily an astronomical spec- 

 troscope consists of a telescope, a narrow slit, a train of prisms, and a 

 small telescope which brings the spectrum to the eye or to the photo- 

 graphic plate. When the object which is to be examined has an area 

 like the sun the use of a slit cannot be avoided. When the source of 

 light is a point, or a narrow line of light, there is no such necessity and 

 the more simple apparatus, known as the slitless spectroscope, or 

 objective prism, may be used. This consists of a prism placed over the 

 lens of the telescope and a photographic plate at the focus. Instead of 

 the prism or prisms a diffraction grating may be used. Professor 

 Pickering, the director of the Harvard Observatory, has obtained for 

 many years fine spectra of the stars by this method, which is an 

 adaptation of the original method of Fraunhofer. An apparatus of 

 this sort used in eclipse work is known as a 'prismatic camera.' It is 

 evident that this form of spectroscope could not be successfully used 

 on the uneclipsed sun, since the resulting spectrum would be simply a 

 confused mass of colored light. There must be a slit, but in the case 

 of total eclipse, nature furnishes it. As the moon at such times has an 

 apparent diameter greater than that of the sun, it is readily seen that 

 at the instant before the moon's disc completely covers the sun there 

 will remain a very narrow crescent of light. At the instant after 

 totality has begun the photosphere will be entirely covered, but for two 

 or three seconds the thin line of chromospheric light remains in view. 

 The two spectra taken at these moments, the one an instant before 



