134 COLOURS OF THIN PLATES. 



and in water, it will be found that the corresponding thick- 

 nesses are as four to three, or in the inverse ratio of the refrac- 

 tive indices. 



(147) We have hitherto spoken only, of the reflected rings. 

 There is another system of rings formed by transmission, but 

 much fainter than the former. The transmitted rings are 

 found to observe the same laws as the reflected rings, with 

 this remarkable exception, that the colour transmitted at any 

 particular thickness of the plate is always complementary to 

 that reflected at the same thickness ; so that, in homogeneous 

 light, the bright transmitted ring is always at the same dis- 

 tance from the centre as the corresponding dark one of the 

 reflected system. 



(148) The phenomena of thin plates are exhibited, under 

 a modified form, in the following experiment : 



A little fine soap is spread upon a plate of black glass, 

 and is distributed uniformly by rubbing the surface lightly 

 with a piece of soft leather. If then we blow on the surface, 

 thus prepared, through a short tube, taking care to direct 

 the tube always to the same part of the plate, the vapour 

 of the breath will be deposited in a thin film, whose thick- 

 ness diminishes regularly from the point to which the tube 

 is directed. This film will accordingly display a series of 

 coloured rings analogous to those formed by the plate of 

 air between two object-glasses, with this difference, how- 

 ever, that the order of the rings is reversed, the outermost 

 ring corresponding to the centre of Newton's scale. This 

 little apparatus, contrived by Mr. Head, is denominated by 

 him an iroscope. 



(149) For the physical explanation of these phenomena. 



