266 MITCHELL— THE FLASH SPECTRUM. 



eclipse of 1870, and his was the first eye to perceive it. The sudden- 

 ness of the change caused him to name the bright line spectrum the 

 "flash spectrum." At the beginning of totality, the flash spectrum 

 lasts for only a few seconds while the moon is advancing in front 

 of the rather shallow layer. A second flash spectrum is seen at 

 the end of totality. 



The first attempt to photograph the flash was in 1893, but with 

 very imperfect results. Shackleton in 1896 was more successful, 

 though the first photographs with good definition were those of the 

 eclipse in India in 1898 by Evershed. 



In the year 1900, the American astronomers had an opportunity 

 in their own country at the eclipse of May 28. For the first time 

 in eclipse work, Rowland gratings were used. Gratings are ruled 

 on both plane and concave surfaces, and it is possible to use them 

 either with or without a slit. The great advantages of gratings 

 over prisms are the increased dispersion, but particularly their 

 normal spectrum. When used in the ordinary Rowland mounting, 

 the concave grating shows marked astigmatism. Although work on 

 stars had shown that the astigmatism of the concave grating when 

 used directly without slit was exceedingly small, most observers in 

 1900 were apparently afraid to use a concave grating without a slit. 

 Those who did use slits found their photographs entirely without 

 lines, the light that went through the slit being entirely too feeble to 

 leave any impression on the photographic plate. Gratings used 

 without slits gave more satisfactory results. 



Since 1900, the eclipses most easily observed were the Sumatran 

 eclipse of 1901, the Spanish eclipse of 1905, the Flint Island eclipse 

 of 1908, the Russian eclipse of 1914, and the American eclipse of 

 1918. 



When great dispersion is desired, the concave grating without 

 slit is perhaps the most desirable form of spectrograph. Its mount- 

 ing is very simple. Light from the coelostat mirror falls direct on 

 the concave grating where it is diffracted and brought to a focus on 

 the photographic plate. It might not be out of place here to call 

 attention to the great importance, and also the great difificulty, of 

 obtaining sharp focus with the slitless instrument. The best results 



