the Cause ^ toloured concentric Rings, 199 



much fight that I could nxrt perceive them. The co- 

 lour of the glasses, in this way, coming directly to the eye, 

 gave a strong tinge to the centres of the rings, so that in- 

 sreadof a pure white I had a blueish white, a greenish white, 

 and so of the rest ; but the form of the rings was no less 

 perfect on that account. 



XXVIII. Of the Action of the fourth Surface. 



We have already seen that a set of rings may be com- 

 pletely formed by reflection from a third surface, without 

 the introduction of a fourth : this, at all events, must provt 

 that such a surface is not essential to the formation of rings ; 

 but as not only in direct transmission, but also When two 

 sets of rings are to be seen, one of which may be formed by 

 transmission, this fourth surface must be introduced, I have 

 ascertained by the following experiments how far the same 

 has any share in the formation of rings. jotll 



In direct transmission, where the light comes from l)e- 

 low, the fourth surface will take the part which is acted by 

 the first, when rings are seen reflected from a metalline 

 mirror. Its office therefore will be merely to afford an en- 

 trance to the rays of light into the substance of the subjii^ 

 cent glass ; but when that light is admitted through the 

 iirst, second, and third surfaces, the fourth takes the office 

 of a reflector, and sends it back towards the poirit of con- 

 tact. It will not be required to examine this reflection, 

 since the light thus turned back again is, with respect to 

 the point of contact, in the same situation in which it was 

 after its entrance through the first surface when it proceeded 

 to the same point ; but when two sets of rings are to be 

 formed by rays, either coming through this point dirfcCtly 

 towards the fourth surface, or by reflection from the same 

 point towards the place where the secondary rings are to be 

 seen, it will then be necessary to examine whether this sur-^ 

 face has any share in their formation, or whether these rings, 

 being already completely formed, are only reflected by it to 

 the eye. With a view to this, I selected a certain polished 

 defect in the surface of a piece of coach-glass, and when 

 :MAA. N4 a 26-inch 



