ELECTRICAL UNITS OF MEASUREMENT. 109 



do now, keeping his eye at equal distances from 

 the two candles, until he sees each candle shooting 

 up out of the yellow middle of a spectrum of 

 the other candle, with no spectrum between the 

 two candles. With this condition fulfilled, he 

 measures the distance from the grating to the 

 candles. Then, by the theory of diffraction, he 

 has the proportion : as the distance from the 

 grating to the candles, is to the distance between 

 the candles, so is the distance from centre to 

 centre of the divisions on the glass, to the wave- 

 length of yellow light. This, he remembers, is 

 5*892 x icr 5 of a centimetre, and thus he finds 

 the value in centimetres of his provisional unit. 



[How easily this determination might be 

 effected, supposing the grating once made, was 

 illustrated by a rapid experiment performed in 

 the course of the lecture ; without other apparatus 

 than a little piece of glass with two hundred and 

 fifty fine parallel lines engraved on it, two 

 candles, and a measuring tape of unknown divi- 

 sions of length (used only to measure the ratio 

 between two distances). The result showed the 



