A REMARKABLE SCIENTIFIC APPARATUS, WITH WHICPI TllE VISITOR CAN SEE THE 

 BENDING OE A H:EAVY STEEL EAR BENEATH THE PRESSURE OF ONE FINGER 



This bar is supported at each end and a small mirror is fixed at the center. Above it 

 is' a frame bearing another partially silvered mirror, both of which reflect the light of a 

 sodiurn burner, the lower mirror showing a series of purple and yellow concentric rings. 

 The slightest pressure on the bar — even the weight of a visiting card or a pin — causes these 

 circles to expand outward, forming, as it were, a series of ripples like those made when a 

 stone is dropped into the center of a still pond. The pressure of one finger on the bar 

 causes the formation of five or six new circles, showing that the bar has been bent about 

 one twenty-thousandth of an inch, as each new circle means a movement of one hundred- 

 thousandth of an inch (see illustrations, page i6i). 



B C Photos from Bureau of Standards 



THE LIGHT WAVES ARE USED AS UNITS OF MEASURE 



If all the standards of length in the entire world were by some accident destroyed, the 

 meter could be exactly reproduced from the red line in the spectrum of cadmium, as it is 

 mvanable (see page 165). This illustration is a photograph of the spectrum of cadmium 

 vapor, showing the three lines (marked A, B, and C) used to determine the international 

 meter, the world's standard of length. These lines never vary and are exactly 64,384,696, 

 50,858,219, and 47,999,087 millimicrons long respectively. A millimicron is a thousandth part 

 of a micron; a micron is a millionth part of a meter. 



160 



