the concave grating, and used it for his epoch- 

 making photographic studies of laboratory spectra 

 and the spectrum of sunlight. But his solar image 

 was small and unsteady and there remained a most 

 promising opportunity to apply a powerful photo- 

 graphic spectroscope to the investigation of sun- 

 spots, the chromosphere, and other details of a 

 large solar image. Solar spectroscopy was far 

 behind laboratory spectroscopy and new types of 



FIG. 7. The Snow Telescope. 



instruments were clearly demanded. It was evi- 

 dent that for efficient use in photography the 

 spectroscope of the day must be greatly lengthened, 

 thus making it too long to serve as an attachment 

 to a moving telescope. Accordingly it became 

 necessary to develop a suitable form of fixed tele- 

 scope, capable of forming a large and sharply 

 defined image of the sun on the slit of a long fixed 

 spectrograph. 



