( 128 ) 



CHAPTEE VIII. 



COLOURS OF THIN PLATES. 



(140) THE colours of thin plates were first noticed by 

 Boyle and Hooke. They are displayed in every instance in 

 which transparent bodies are reduced to films of great tenuity. 

 Boyle succeeded in blowing glass so thin as to exhibit the 

 phenomena : they are more readily developed in mica, and 

 some other transparent minerals which possess a lamellar 

 structure ; but the most familiar instance of their exhibition 

 is in the froth of liquids, the fluid envelopes of the bubbles 

 which compose it being in general of extreme thinness. 



These colours vary with the thickness of the film, and dis- 

 appear altogether when it passes certain limits. "When the 

 film exceeds a certain thickness, all the colours are equally 

 reflected, and the reflected light is therefore white. On the 

 other hand, when the thickness falls below a certain limit, no 

 light whatever reaches the eye, and the surface of the film 

 appears absolutely black. 



(141) The foregoing facts may be observed in the common 

 soap-bubble, when properly defended from the disturbing in- 

 fluence of currents of air. If the mouth of a wine-glass be 

 dipped in water, which has been rendered somewhat viscid 

 by the mixture of soap, the aqueous film which remains in 

 contact with it after emersion will display the whole suc- 

 cession of these phenomena. "When held in a vertical plane, 

 it will at first appear uniformly white over its entire surface ; 

 but, as it grows thinner by the descent of the fluid particles, 



