no POPULAR LECTURES AND ADDRESSES. 



distance from centre to centre of consecutive 

 bars of the grating, to be thirty-two times the 

 wave-length of yellow light. The breadth of the 

 span on which the two hundred and fifty lines 

 of the grating were ruled was thus measured as 

 (250 X 32 x 5*892 x io~ 5 = ) 0-47136 centimetre. 

 According to the instrument-maker this space 

 was said to be O'5 of a centimetre.] 



Thus you see, by this hurried experiment with 

 this rough-and-ready apparatus, we have been able 

 to measure a length to within a small percentage 

 of accuracy. A few minutes longer spent upon 

 the experiment, and using sodium flames behind 

 fine slits instead of open candles blowing about 

 in the air, with more careful measurement of the 

 ratio of the distances, might easily have given a 

 result within one-half per cent, of accuracy. 

 Thus the cosmic traveller can easily recover his 

 centimetre and his metre measure. 



But how is our scientific traveller to recover his 

 mean solar second, supposing he has lost his 

 tuning-fork ? He may think of the velocity of 

 light, and go through Foucault's experiment. 



