6 INORGANIC EVOLUTION. 



as in the case of the candle, but simply by 



[CHAP. 



We see one image of the needle coloured in yellow. 



We have passed from the spectrum of polychromatic to that of 

 monochromatic light from white light to coloured light from light of 

 all wave-lengths to light of one wave-length ; from an infinite number 

 of slit images giving a continuous band of every colour, to one image 

 of the slit produced by light of one refrangibility, the colour of the 

 image depending upon the refrangibility. What we shall see in pass- 

 ing from the spectrum of the candle to that of sodium vapour in the 

 spirit lamp is shown in the accompanying woodcut. 



Candle-flame spectrum. 



Straight slit.^j g . it 



>lamp flame 

 King slit. J wif 



FIG. 3. A continuous and a discontinuous spectrum. 



That we are truly dealing with an image of the needle (or a slit) 

 can be proved by using a slit of any shape. This can be shown by 

 slightly altering our needle experiment. Take a piece of glass and a 

 piece of tin-foil 1J inches square, cut out of the centre of the tin-foil a 

 disc slightly larger than a threepenny-piece, and gum the remainder on 

 the glass. In the centre, where the disc has been cut away, gum a 

 threepenny-piece. The interval between the threepenny-piece and the 

 tin-foil constitutes a circular slit. Let it replace the needle, and ex- 

 amine the flame of the spirit lamp charged with salt through it with 

 the prism as before. 



It will readily be grasped, from what has been stated, that in the 

 case of coloured flames, the light passing through the spectroscope 

 being only red, or yellow, or green, as the case may be, will go to build 

 up an image of the slit in the appropriate part of the spectrum, and 



