24 



THE CHEMISTRY OF THE SUN. 



[CHAP. 



angles. When the line-slit, say twenty inches off, is observed 

 with the refracting edge of the prism parallel to its length, a 

 very brilliant spectrum of the candle is obtained, even though it 

 be wanting in definition. This latter can be improved if a 

 narrower slit is employed ; for in spectra almost all impurity 

 comes from the overlapping of images. The operations of 

 nature are so delicate, that it seems as if a pure colour, such as 

 a pure blue or a pure red, will for ever remain an abstraction ; 

 for, however great the dispersion, the adjacent rays will remain 

 commingled, arid commingled rays define a compound colour. 



FIG. 8. Showing arrangement of slit and prism (c slit). 



Instead of reducing the width of the slit, if it be not 

 connected with the prism by means of a tube as it may 

 conveniently be, the slit can be removed further from the prism. 

 In this way we get apparently a narrower slit without any 

 reduction in the quantity of light which passes through it to the 

 eye. A gas-flame or a candle placed in front of this slit is all 

 that is necessary to produce that great purification of the 

 spectrum first effected by Wollaston. 



We may now go a step further. A spectacle lens may be 

 inserted in the beam as it leaves the prism. Here we are 



