io8 POPULAR LECTURES AND ADDRESSES. 



of his thumb, and which he may take as a tem- 

 porary or provisional unit of length. He may 

 help himself to engrave the glass by means of 

 a screw cut in brass or steel, which he will easily 

 make, though he has no tools, not even flint 

 implements, to begin with. With a little time 

 and perseverance he will make the requisite 

 tools. Let him also make a temporary mea- 

 suring-rod, and mark off equal diyisions upon 

 it, which may be of any convenient length, and 

 need not have any relation to the definite pro- 

 visional unit. Let him now make two candles, 

 and light them and place them as you now see 

 those on the table, at any convenient distance 

 apart, measured on his measuring-rod. He holds 

 the piece of ruled glass in his hand, close to his 

 eye, as I hold this, and sees two rows of coloured 

 spectrums, each with one of the candles in its 

 centre. He turns the glass round till the two 

 rows of spectrums are in the same line, and 

 adjusts the parallelism of its plane, so as to make 

 the distance from spectrum to spectrum a mini- 

 mum. He moves backwards and forwards, as I 



