200 Astronomical and Nautical Collections. 



is reflected within the denser medium or without, the velo- 

 city of the particles which constitutes, the undulation is posi- 

 tive or negative respectively ; so that all the corresponding 

 motions must have contrary signs in the two cases. 



We may now return to the phenomenon of the coloured 

 rings, and we ma}'' suppose, to simplify our reasoning, that 

 the reflected light, which is observed, is perpendicular to the 

 surface, or very nearly so ; and that one of the systems of 

 undulations is thrown by the illuminating object on the first 

 surface of a plate of air, which is also the second surface of 

 the upper glass: and what will be said of this system is 

 applicable to every other. At the moment when it arrives at 

 the surface of separation of the glass and the air, it suffers a 

 partial reflection, which diminishes, in some measure, the 

 intensity of the light transmitted to the air, and excites 

 within the first glass another system of undulations, of which 

 the intensity is greatly inferior to that of the transmitted 

 light ; so that this light, being very little weakened by this 

 first reflection, produces, when it arrives at the second sur- 

 face of the plate of air, a second system of reflected waves of 

 an intensity nearly equal to that of the waves which are de- 

 rived fi-wn the first reflection : and hence their interference 

 produces colours so bright in white light, and dark and light 

 rings so distinct in hom.ogeneous light. The two surfaces of 

 the plate of air being nearly parallel, in the neighbourhood 

 of the point of contact, where the coloured rings are formed, 

 the two systems of waves follow the same path : but that 

 which has been reflected at the second surface, will be found 

 retarded, in comparison with the other, by an interval equal 

 to twice the thickness of the plate of air which it was twice 

 crossed. We must, besides, remark that there is another 

 difference between them; the first having been reflected 

 within the glass, which is the denser medium, while the 

 second has been reflected without the second glass ; whence 

 arises, according to the principle already established, an 

 opposition in the direction of the elementary oscillations. 

 Thus when, from the difference of the paths described, the 

 two systems of undulations ought to agree with each other, 

 and so perform their motions in the same direction, we are 



