202 PHILOSOPHICAL TRANSACTIONS. [ANNO 17Q7- 



had the pleasure of observing several other sorts of rings, so extremely similar, and 

 formed by flexion, that I can no longer doubt of this being also the cause of the 

 phenomena observed by Newton. I shall first describe a species, to prove " that 

 the colours by thick and thin plates are one and the same phenomenon, only differ- 

 ing in the thickness of the plates." Happening to look by candle-light on a round 

 concave plate of brass, pretty well polished, so as to reflect light enough for show- 

 ing an image of the candle, I was surprized to see that image surrounded by several 

 waves of colours, red, green, and blue, disposed in pretty regular order. This was 

 so uncommon in a metallic speculum, that I examined the thing very minutely by 

 a variety of experiments ; these I shall not particularly now describe, but give a 

 general idea of their results. 



It must be observed, for the sake of clearness, that in the following inquiries 

 concerning the formation of rings or fringes, the diameter of a ring or fringe means 

 the line passing through the centre of that ring, and terminated at both ends by 

 the circumference; whereas the breadth means that part of the diameter intercepted 

 between the limits of the ring, or the distance between its extreme colours, red and 

 violet. In the 1st place, they were formed by the sun's light in the figure of rings 

 surrounding the centre of the sphere to which the plate was ground, at greater dis- 

 tances increasing their breadths, the colours pretty bright, though inferior in bril- 

 liancy to those of concave specula. 2dly, The order of the colours was in all red 

 outermost, and violet or blue innermost, with a greyish-blue spot in the common 

 centre of the whole; and on moving the plate from the perpendicular position, the 

 rings moved and broke exactly like those of specula. In the 3d place, homogeneal 

 light made them of simple colours ; they were broadest when red, narrowest when 

 blue and violet. 4thly, They decreased in breadth from the centre ; and I found, 

 by a simple contrivance, that they were to each other in the very same ratio that 

 the rays by specula follow. In the 5 th place, I compared the general appearance 

 of the 2 sorts by viewing them at the same time, and was struck with their general 

 appearance, unless that these of specula were most vivid and distinct. 

 • These things made me suspect that they were actually caused by the thin coat of 

 gums with which the surface of the plate was varnished, called lacker. Accordingly 

 I took it off with spirit of wine, and found the rings disappear ; on lackering it 

 again they returned ; and in like manner I caused a well finished concave metal 

 speculum to form the rings here spoken of, by giving it a thin coat of lacker. This 

 is a clear proof that these rings were exactly the same with those of thick plates, to 

 use Newton's expression, for the coat of gums is, when thin, pretty transparent, 

 as may be seen by laying one on glass plates. But this coat is extremely thin, and 

 cannot exceed the 200th part of an inch ; so that the colours of thick plates are in 

 fact the very same with those of thin plates, except that the 2 kinds are made by 

 different sized plates. We cannot therefore distinguish them, any more than we 

 do the spectrum made by a prism whose angle is go from that made by one whose 

 angle is 20°. This kind of colours is not the only one I have observed of nearly the 



