THE WAVE THEORY OF LIGHT. 339 



with two white marks on it ten inches asunder. 

 The line on which they are marked is placed per- 

 pendicular to the line at which I shall go from 

 it. When I look at this salted spirit-lamp I see a 

 series of spectrums of yellow light. As I am 

 somewhat short-sighted I am making my eye see 

 with this eye-glass and the natural lenses of the 

 eye what a long-sighted person would make out 

 without an eye-glass. On that screen you saw a 

 succession of spedtrums. I now look direct at the 

 candle and what do I see ? I see a succession of 

 five or six brilliantly coloured spectrums on each 

 side of the candle. But when I look at the salted 

 spirit-lamp, now I see ten spectrums on one side 

 and ten on the other, each of which is a mono- 

 chromatic band of light. 



I will measure the wave length of the light thus. 

 I walk away to a considerable distance and look 

 at the spirit-lamp and marks. I see a set of spec- 

 trums. The first white line is exactly behind the 

 flame. I want the first spectrum to the right of that 

 white line to fall exactly on the other white line, 

 which is ten inches from the first. As I walk away 



Z 2 



