The Sun 



sufficiently opened, is brought to bear tangenti- 

 ally on the edge of the sun's image, an image 

 of the corresponding part of the chromosphere 

 appears in the field of the instrument. This 

 image is formed at the place where with a narrow 

 slit the red line characteristic of hydrogen would 

 have appeared. The chromosphere is invisible to 

 us, under ordinary conditions, because the blind- 

 ing light of the photosphere overwhelms its very 

 feeble lustre, with the exception of this red radia- 

 tion, but the spectroscope relegates each radiation 

 to its own proper place and enables the chromo- 

 sphere to be observed by the most suitable light. 

 The method of observation, discovered by 

 Huggins, has rendered it possible to explore 

 systematically the chromosphere, and this layer 

 of the sun is now known in detail. We know 

 that its mean depth is very limited, about eight 

 thousand kilometres, and augments the diameter 

 of the sun by only one hundred and seventy 

 fifth part. Moulded internally on the photo- 

 sphere, its external face presents forms of extra- 

 ordinary irregularity and variety. In places it 

 is as flat as the sea; in others it is covered with 

 luminous filaments, like bright hairs all pointing 



L 181 



