CHAPTER III. 



THE FIRST GLIMPSES OF " BRIGHT LINE SPECTRA." 



THE phenomena first observed by Wollaston and Fraunho'er 

 must now be referred to in more detail ; it will be seen in the 

 sequel that they lie at the root of all the subsequent work. In 

 the first chapter I described an experiment by which it is 

 possible to convince ourselves, with the aid of no other apparatus 

 than a glass lustre-drop and a candle, that white light is a 

 composite thing. It will now be well to see whether we can 

 familiarise ourselves with the developments of the subject we 

 owe to the two men I have named, by means of very simple 

 experiments. 



Now we first learnt from Wollaston that we must use a slit, 

 or crevice as he called it. Let us then cut a fine slit, say 

 one-twentieth of an inch broad and half an inch long, care- 

 fully out of a piece of tin-foil, and gum the tin-foil with 

 this "crevice" in it on to a plate of glass. Then we can 

 make also in the same manner a narrow circular slit about 

 half an inch in diameter and vary the previous experi- 

 ment in this way. Let us place the straight slit where the 

 candle was in the first experiment, and then place the candle 

 close behind it, so that, so to speak, only a slice of the candle 

 light will pass through the slit to the eye placed as before. It is 

 immaterial on which side we place the base of the prism pro- 

 vided the light enters and leaves the refracting surfaces at equal 



