60 Dr Brewster an a 7iew series of 'periodical colours ^ Sec, 



shown that one reflection from a plate of silver, &c. is equi- 

 valent in its action to a given thickness of a crystallized film, 

 and that the tints descend in the scale by increasing the angle 

 of incidence as if the equivalent film had diminished in thick- 

 ness. That these colours are produced by the interference 

 of two pencils, one of which suffers reflection later than the 

 other, cannot be doubted ; but whether these two portions are 

 reflected within the sphere of reflecting activity, at such dis- 

 tances as to produce colours by their interference, or whether 

 the one is reflected in the usual manner, while the other is not 

 reflected till it has penetrated a certain thickness of the polish- 

 ed metal, it is not easy to ascertain. 



If either of these effects takes place with polarized light, an 

 analogous effect should be produced with common light, though 

 the intensity of the interfering pencils might in this case be 

 very inconsiderable. 



If we suppose that the spaces n are smaller than the distance 

 to which the reflecting force extends, the removal of the metal 

 from the adjacent grooves must diminish the reflecting force 

 of these spaces. That this is the case may, we think, be in- 

 ferred from direct experiment. At the separating surface of 

 the steel and a fluid, we observe a certain change in the action 

 of the steel surface, which can be ascribed to no other cause 

 than the diminution of the refractive and reflective power of 

 the surface. Now it is manifest from experiment that the di- 

 minution of the spaces w has exactly the same effect, the colours 

 not only being rendered brighter by each of these causes, but 

 the minima being produced at greater angles of incidence. 



Since in a system of grooves with only 312 in an inch, oil 

 of cassia developes colours which did not previously exist, it 

 is evident, that, if we had fluidsof much higher refractive power, 

 colours would be produced when the spaces n were much lar- 

 ger, and when the fluid approached in refractive density to 

 that of the metal, we should witness the periodical colours 

 without any grooves at all on the reflecting surface ; so that 

 the phenomena would then become identical with those which 

 are developed at the separating surface of transparent bodies. 



We can scarcely, therefore, avoid the conclusion, that the 

 removal of the substance from the grooves, whether they are 



