7-14 PHILOSOPHICAL TRANSACTIONS. [aNNO IJQQ. 



surface, they are inflected and condensed by the second. I am not, I own, 

 quite satisfied with this account of the matter: that they are produced by in- 

 flection, the Duke's exjjeriments put beyond doubt; hut that they should be 

 formed in passing through the first surface, and reflected by the 2d, is quite in- 

 consistent with the ratio observed by their breadth, this being greater in the 

 thinnest glass, and also with the order of the colours. Besides, all the coloured 

 images which fall on the backside of the mirror, will be, by what we before 

 found when speaking of flexibility, reflected into a white focus. So that, on 

 the whole, there ajjpears every reason to believe that the rings are formed by the 

 first surface, out of the light which, after reflection from the 2d surface, is 

 scattered, and passes on to the chart. It will follow, 1, that a plane mirror makes 

 them not: for the regularly reflected light, not being thrown to a focus, mixes 

 wilh the decompounded scattered light, and dilutes it. 2. That the nearer to 

 the perpendicular the rays are incident, the more light will be reflected to the 

 focus, and consequently the less will dilute and weaken the rings. 3. That the 

 thinner the mirror is, or the nearer the 2 surfaces are, the broader will the rings 

 be. 4. That the rings ftrther from the focus will be broader. And lastly, that 

 when homogeneous light is reflected, the fringes or images will be larger, and 

 farther from one another, in red than in any other primary colour. All which 

 is perfectly consistent with the experiments of Newton and Chaulnes. There is 

 only one difficulty that may be started to this explanation: how happens it that 

 the colours, made by the mirror, are always circular? We answer, it is owing 

 to the manner of polishing the concave mirror, which is laid between a convex 

 and concave plate, and then turned round, with putty or melted pitch, in the 

 very direction in which the rings are. If it should be asked, why does the thick- 

 ness of the mirror influence the breadth ol the rings exactly in the inverse sub- 

 duplicate ratio? We answer, that to a certain distance from the point of inci- 

 dence (and the rays are never scattered far from it) this is demonstrable to hold 

 as a property of mathematical lines in general. 



Having found that the fringes by flexion are images of the luminous body, I 

 thought that, from' this consideration, a method of determining the different 

 degrees of flexibility of the different rays might be deduced, similar to that which 

 I had formerly used for determining their degrees of reflexibility. I tiierefore 

 made the following experiment. 



Obs. 12. Having let into my darkened chamber a strong beam of the sun's 

 light, through a hole ^\ of an inch in diameter, I held a hair at 4 feet fiom tlie 

 hole, and receiving the shadow at 2 feet from the hair. I drew a line across the 

 middle of the coloured images, and pointed off" in each the divisions of the 

 colours, as nearly as I could observe; and repeating the observation several 

 times and at different distances, I found, by the same way I had formerly done 

 in my experiniLiit on reflexibility, that the axis, or line, drawn through the 

 middle of each, was divitlcd inversely, according to the intervals of the chords 



