72 THE CHEMISTRY OF THE SUN. [CHAP. 



When delicate and detailed observations have to be made, the 

 amount of dispersion afforded by one prism is insufficient, and 

 has to be increased by the addition of more prisms. The way 

 in which this is effected is shown in Fig. 29, which represents 

 the instrument with which Kirchhoff made his elaborate maps. 



By placing a mirror, or better still a heliostat, in front of this 

 spectroscope, light from the sun, or a cloud illuminated by the 

 sun in case the quantity of light which enters the instrument 

 when turned directly towards the sun is too great to allow of 

 easy observation, may be reflected into the instrument. 



Using his measuring apparatus, Kirchhoff was enabled to 

 construct his map showing the positions of the lines observed 

 in the solar spectrum by moving his observing telescope along 



FIG. 30. Steinlieil's slit, showing reflecting prism (a, b). 



the spectrum, as it were, the telescope being furnished with 

 a delicate cross wire, or some properly-contrived means for 

 defining the exact position of each line. Indeed he prepared 

 this map with the object of providing himself with a scale of 

 extreme value for the future work which he then laid out for 

 himself. 



The future work was this : He wished to determine the 

 positions of the bright lines given by the different chemical 

 elements; having got this information, he wished to put the 

 same question to the solar spectrum with regard to each of those 

 elements as already had been done in the case of sodium. How 

 then did he propose to do this ? He made an addition to the 

 slit of the spectroscope, such as was then employed. He put a 



