GROOVED SURFACES OF METALLIC AND TRANSPARENT BODIES. 315 
I feel great difficulty in assigning a satisfactory cause for their production. 
That they are not owing to the diffraction and interference of the rays reflected 
from two or more of the surfaces n, considered as narrow slits or apertures, is 
obvious ; for in that case they would be affected by the distance of the luminous 
object and the distance of the eye, and the colours would form bands parallel 
to the direction of the grooves. 
In my experiments on the production of the complementary colours by the 
metallic reflexion of polarized light, I have shown that one reflexion from a 
plate of silver, &c. is equivalent in its action to a given thickness of a crystal- 
lized film, and that the tints descend in the scale by increasing the angle of 
incidence as if the equivalent film had diminished in thickness. That these 
colours are produced by the interference of two pencils, one of which suffers 
reflexion later than the other, cannot be doubted ; but whether these two por- 
tions are reflected within the sphere of reflecting activity, at such distances 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 polished 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 inferred 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 diminution of the spaces n 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 fluids of 
much higher refractive power, colours would be produced when the spaces n 
were much larger, and when the fluid approached in refractive density to that 
2 s 2 
