152 ■ On the Jff'tSl'wm and Propert'tes if Light. 



inyfelf, fufpe£led the body muft be irregular m its fliape. On inverting it, however, the 

 fame thing took place ; and on turning the prifm on its axis, fo that the difFerent rays fuc« 

 ceflively fell on the fame parts, the vifible magnitude of the body varied with the rays that 

 illuminated it. This appearance is readily accounted for by the different reflcxity of the 

 rays, and follows immediately from Obfervation 2d and 3d. 



2. Sir Ifaac Newton found that the rings of colours made by thin plates, and by thick 

 plates of glafs (as he calls them), when formed of homogeneal light, varied in fize with the 

 rays that made them, being largeft in the moft flexible rays. I have had the pleafure of ob- 

 ferving feveral other forts of rings, fo extremely fimilar, and formed by flexion, that I can 

 no longer doubt of this being alfo the caufe of the phenomena obferved by Newton. I fhall 

 iirft defcribe a fpecies, to prove •' that the colours by thick and thin plates are one and the 

 fame phenomena, only differing in the thicknefs of the plates." Happening to look by 

 candle light upon a round concave plate of brafs, pretty well poliflied, fo as to reflect light 

 enough for (hewing an image of the candle, I was furprifed to fee that image furrounded 

 by feveral waves of colours, red, green, and blue, difpofed in pretty regular order. This 

 was fo uncommon in a metallic fpeculum, that I examined the thing very minutely by a 

 variety of experiments : thefe I fhall not particularly now defcribe, but give a general idea 

 of their refults. 



It muft be obferved, for the fake of clearnefs, that in the following enquiries concerning 

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

 through the centre of that ring, and terminated at both ends by the circumfere'nce ; whereas 

 the breadth means that part of the diameter intercepted between the limits of the ring, or 

 the diftance between its extreme colours, red and violet. 



In the firft place, they were formed by the fun's light in the figure of rings, furrounding 

 the centre of the fphere to which the plate was ground, at greater diftanccs increafing 

 their breadths, the colours pretty bright, though inferior in brilliancy to thofe of concave 

 j^cula. 



Secondly, the order of the colours was in all red outermoft, and violet or blue in- 

 nermoft, with a greyifh-blue fpot in the colnmon centre of the whole ; and on moving 

 the plate from the perpendicular pofition, the rings moved and broke exadlly like thofe of 

 ;^ecula. 



In the third place, homogeneal light made them of fimple colours ; they were broadeft 

 when red, narrowed when blue and violet. 



Fourthly, they decreafed in breadth from the centre ; and I found by a fimple con- 

 trivance, that they were to one another in the very fame ratio that tlie rays by fpecula 

 follow. 



In the fifth place, I compared the general appearance of the two forts by viewing them at 

 the fame time, and was ftruck with their general appearance, unlels that thefe of fpecula 

 were moft vivid and diftinft. 



Thefe things made me fufpeft that they were aftually caufed by the thin coat of gums 

 with which the furface of the plate was varniflied, called lacker. Accordingly I took it off 

 with fpirit of wine, znd found the rings difappear; on lackering it again they returned ; 

 and in like manner I caufed a well finilhed concave metal fpeculum to form the rings of 

 which we ar« fpeaking, by giving it a thin coat of lacker.; This is a clear proof that thefe 



ringa 



