on a new Method of viewing the Chromosphere. 223 



The image of the sun is brought to focus on a diaphragm having a 

 circular disk of brass (in the centre) of the same size as the sun's 

 image, so that the sun's light is obstructed and the chromospheric 

 light is allowed to pass. The chromosphere is afterwards brought 

 to a focus again at the position usually occupied by the slit of the 

 spectroscope; and in the eyepiece is seen the chromosphere in 

 circles corresponding to the "C"or other lines. The lens D is 

 used to reduce the size of the sun's image, and keep it of the same 

 size as the diaphragm at different times of the year; and the 

 lenses F are used in order to reduce the size of the annulus of 

 light to about -| inch, so that the pencils of light from either side 

 of the annulus may not be too divergent to pass through the prisms 

 at the same time, and that the image of the whole annulus may be 

 seen at once. There are mechanical difficulties in producing a 

 perfect annulus of the required size, so one -J- inch in diameter 

 is used, and can be reduced virtually to any size at pleasure. 



The proposed photographic arrangements are as follows : — 



A large Steinheil spectroscope is used, its usual slit being replaced 

 by the ring one. 



A solar beam is thrown along the axis of the collimator by a he- 

 liostat, and the sun's image is brought to focus on the ring-slit 

 by a 3J-inch object-glass, the solar image being made to fit the slit 

 by a suitable lens. 



By this method the image of the chromosphere received on the 

 photographic plate can be obtained of a convenient size, as a tele- 

 scope of any dimensions may be used for focusing the parallel 

 beam which passes through the prisms on to the plate. 



The size of the image of the chromosphere obtained by the 

 method adopted will be seen from the accompanying photograph, 

 taken when the ring-slit was illuminated with the vapours of copper 

 and cadmium. 



December 6, 1872. at 11.30. 



December 7, 1872, at 11.30. 



f\ 



/ \ ] 



M 



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Outer circle 100" from inner one. Chromosphere at normal height, except 

 where prominences marked. 



