the received Theory of Newton's Rings. 4>53 



has been deduced from the theory of undulation by Young, 

 Fresnelj Poisson, Herschel, Airy, and others. My grounds 

 of dissent are as follow : — 



1. It is theoretically impossible that a reversion of the 

 aether- vibrations, reflected at the lower limit of the one glass 

 as compared with those reflected at the upper limit of the 

 other, can take place when there is no layer of air between 

 the glasses. 



2. If, in order to preserve the received theory, a layer of 

 air so thin that its depth is very small even in comparison 

 with the length of an undulation, be assumed to exist at the 

 point where the dark spot commences to show itself, this 

 could not account for the continuance of the spot after the 

 glasses have been pressed so strongly together that no air can 

 possibly exist between them. This assumption therefore does 

 not help us ; the central spot cannot be referred to the inter- 

 ference of the rays reflected at the respective limits of the 

 glasses, though this is the explanation which has heretofore 

 been accepted. 



3. In reflected daylight the central spot appears a deep 

 black. Its origin, therefore, even should we grant the exist- 

 ence of an intermediate air-layer, can by no means be referred 

 to the principle of interference; for it is well known, that, 

 owing to the difference in the lengths of the respective undu- 

 lations, the interference of rays compounded of all colours 

 must result in a tinting more or less vivid, Since therefore 

 the central spot cannot be referred to the existence of a thin 

 layer of air, nor for the reason first assigned be referred to 

 the principle of interference, we are driven to seek a different 

 explanation of the phenomenon from that heretofore given. 



4. Because, with homogeneous light, if the distance of the 

 glasses from each other be equal to nought, and a reversion 

 of the vibrations consequently impossible, the phases of the 

 rays reflected from the two glasses at the point of contact 

 must correspond, and a dark spot cannot be the consequence. 



5- Because, in the case under consideration, the phseno- 

 mena of colour which are exhibited in all analogous cases, 

 among which may be reckoned the figures due to diffraction, 

 are entirely absent. 



From all these grounds it follows, that the middle of the ring- 

 system, when the distance between the glasses is nought, cannot 

 appear dark in reflected light ; but that, in direct opposition to 

 all assertions heretofore made, it ought to appear bright. 



The shortest proof for the so-called law of Young, I find in 

 FresnePs expression for the velocity of oscillation of a reflected 

 ray polarized in the plane of incidence, and of a reflected ray 



