OPTICS. 



35 



The colours in the preceding table 

 are those seen when the light is reflected 

 and transmitted perpendicularly ; but as 

 the incident ray deviates from the per- 

 pendicular the rings increase in size, 

 the same colour requiring a greater 

 thickness to produce it. Hence, the 

 colour of any film will descend in the 

 scale to one of a lower order, when we 

 view the plate which produces it more 

 and more obliquely. When the thin 

 plate is rarer than the ambient medium, 

 it will reflect at differently oblique inci- 

 dences all sorts of colours ; whereas if it 

 is much denser, the colours are but little 

 changed by a variation of obliquity. 



Xo explanation, entirely free from 

 objections, has yet been given of the 

 colours of thin plates. Sir Isaac New- 

 ton supposed that every ray of ligtat in 



passing through any refracting surface 

 is put into a certain transient condition 

 or state, which in the progress of the 

 ray returns at equal intervals, and dis- 

 poses the ray at every return to be 

 easily transmitted through the next re- 

 fracting surface, and between the returns 

 to be easily reflected by it. By means 

 of this principle Sir Isaac has given an 

 explanation of most of the phenomena ; 

 but, as it is entirely hypothetical, and is 

 besides of a complex nature, we shall 

 content ourselves with having merely 

 announced it. 



In the undulatory theory of light the 

 colours of thin plates are supposed to 

 arise from the interference of the li^ht 

 reflected from the second surface of the 

 plate with the light reflected from the 

 first surface. This explanation, which 



