

THE SIZE OF ATOMS. 179 



of about the same thickness, which is transparent 

 to the electric light, as you see, and transmits 

 the beautiful green colour which you see on the 

 screen. The thickness of this gold (1*9, or nearly 

 2 x icr 5 cm.) is just half the wave-length of violet 

 light in air. This transparent gold, transmitting 

 green light to the screen as you see, at the same 

 time reflects yellow light to the ceiling. Now I 

 will show you the silver. It is thinner, being only 

 1*5 X icr~ 5 of a centimetre thick, or ths of the 

 air-wave-length of violet light. It is quite opaque 

 to the electric light so far as our eyes allow us 

 to judge, and reflects all the light up to the ceiling. 

 It is not wonderful that it should be opaque ; we 

 might wonder if it were otherwise ; but there is an 

 invisible ultra-violet light of a small range of wave- 

 lengths, including a zinc line of air-wave-length 

 3'4XIO~ 5 cms., which this silver film transmits. 

 For that particular light the silver film of 1*5 x icr 5 

 cms. thickness is transparent. The image which 

 you now see on the screen is a magic-lantern re- 

 presentation of the self-photographed spectrum of 

 light that actually came through that silver. You 



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