340 POPULAR LECTURES AND ADDRESSES. 



from it I see it is now very near it ; it is now on it. 

 Now the distance from my eye is to be measured, 

 and the problem is again to reduce feet to inches. 

 The distance from the spectrum of the flame to 

 my eye is thirty-four feet nine inches. Mr. 

 President, how many inches is that? 417 inches, 

 in round numbers 420 inches. Then we have the 

 proportion, as 420 is to 10 so is the length from 

 bar to bar of the grating to the wave-length 

 of sodium light. That is to say as forty-two is 

 to one. The distance from bar to bar is the four 

 hundredth of a centimetre : therefore the 42nd 

 part of the four hundredth of a centimetre is the 

 wave-length according to our simple, and easy 

 and hasty experiment. The true wave-length 

 of sodium light, according to the most accurate 

 measurement, is about a I7,oooth of a centimetre, 

 which differs by scarcely more than one per cent 

 from our result ! 



The only apparatus you see is this little grating 

 a piece of glass having a space four-tenths of 

 an inch wide ruled with 400 fine lines. Any of 

 you who will take the trouble to buy one may 





