140 THE CHEMISTRY OF THE SUN. [CHAP. 



say a solid and a mixture of gases giving out light, and we allow 

 the light to enter, so to speak, indiscriminately into the spec- 

 troscope, then in each part of the spectrum we shall get a 

 summation a complex record of the light of the same wave- 

 length proceeding from all. the different light- waves. But if by 

 means of a lens we form an image of the light-source, so that 

 each particular part shall be impressed in its proper place on 

 the slit-plate, then in the spectrum the different kinds of light 

 will be sorted out. 



FIG. 59. Arrangement for projecting an image of a candle flame on^the slit-plate 

 of a spectroscope. 



There is a simple experiment which shows clearly the dif- 

 ferent results obtained. If we observe the light of a candle or 

 a lamp with the spectroscope in the ordinary manner, that 

 is by placing the candle in front of the slit at some little 

 distance from it, we see a band of colour a continuous spec- 

 trum ; and in one particular part of the band we see a yellow 

 line, and occasionally in the green and in the blue parts of the 

 band other lines are observable. Now, if we throw an image of 

 the candle or lamp on to the slit the slit being horizontal and 

 the image of the candle vertical we then get three perfectly 



