208 • PHILOSOPHICAL TRANSACTIONS. [ANNO 1797. 



and then undergoing a second, or, in other instances, without this second re- 

 flexion; and that the other fringes are produced exactly as described above, from 

 the necessary consequences of the theory. I shall only add, that nothing could 

 have been more pleasing to me than the success of this experiment; not only be- 

 cause in itself it was really beautiful from its variety, but also because it was the 

 most peremptory confirmation of what followed from the theory a priori, and in 

 that point where the singularity of its consequences most inclined me to doubt its 

 truth. 



Let us now attend to several conclusions to which the foregoing observations 

 lead, independently of the propositions, viz. the first 5, which they were made to 

 examine. 



1. We must be immediately struck with the extreme resemblance between the 

 rings surrounding the black spots on the image made by an ill polished speculum, 

 and those produced by thin plates observed by Newton ; but perhaps the resem- 

 blance is still more conspicuous in the colours surrounding the image made by any 

 speculum whatever, and fully described in observ. 10 and 11. The only difference 

 in the circumstances is now to be reconciled. The rings surrounding the black spot 

 on the top of a bubble of water, and those also surrounding the spot between 2 

 object glasses *, have dark intervals, exactly like those rings I have just now described, 

 and the fringes surrounding the shadows of bodies; but these intervals transmit 

 other fringes of the same nature, though with colours in the reverse order; from 

 which Sir Isaac Newton justly inferred, that at one thickness of a plate the rays 

 were transmitted in rings, and at another reflected in like rings. Now it is evident, 

 that neither reflexibility nor refrangibility will account for either sort of rings, 

 because the plate is far too thin for separating the rays by the latter, and because 

 the colours are in the wrong order for the former; and also because the whole ap- 

 pearance is totally unlike any that refrangibility and reflexibility ever produce. To 

 say that they are formed by the thickness of the plates, is not explaining the thing 

 at all. It is demanded in what way ? and indeed we see the like dark intervals and 

 the same fringes formed at a distance from bodies by flexion, where there is no 

 plate through which the rays pass. The state of the case then seems to be this: 

 " when a phenomenon is produced in a particular combination of circumstances, 

 and the same phenomenon is also produced in another combination, where some of 

 the circumstances, before present, are wanting; we are intitled to conclude that 

 the latter is the most general case, and must try to resolve the other into it." In 

 the first place, the order of the colours in the Newtonian rings is just such as 

 flexion would produce; that is, those which are transmitted have the red inner- 

 most, those which are reflected have the red outermost ; the former are the colours 

 arranged as they would be by inflexion, the latter as they would be by deflexion ; 

 and here by outermost and innermost must be understood relative position only, or 

 position with respect to the thickness of the plate, not of the central spot. 2dly, 



* Optics, Book 11. P. l.—Orig. 



