1§4? DR - young's account of some cases 



lion at its entrance, and that much of the light palling through 



the air will be fcattered by refraction at the fecond furface. 



For thefe reafons, the fringes are feen when the plates are not 



directly interpofed between the eye and the luminous object ; 



and, on account of the abfence of foreign light, even more 



diftinctly than when they are in the fame right line with that 



object. And, if we remove the plates to a confiderable diftance 



out of this line, the rings are (till vifible, and become larger 



than before ; for here the actual route of the light palling 



through the air, is longer than that of the light palling more 



obliquely through the water, and the difference in the times 



of paflage is leflened. It is however impoffible to be quite 



confident with refpect to the caufes of thefe minute varia? 



tions, without fome means of afcertaining accurately the 



forms of the diffipating furfaces. 



The arguments In applying the general law of interference to thefe colours, 



from the general as we j[ as to thofe of thin plates already known, I muft confefs 



velocity of un- that it is impoffible to avoid another fuppofition, which is a 



dulation is p ar t of the undulatory theory, that is, that the velocity of light 



mediums? "^ * s tne g rea ter, tn€ rarer the medium ; and that there is alfo a 



condition annexed to the explanation of the colours of thin 



The central plates, which involves another part of the fame theory, that is, 



foap bubble°is that where one of the portions of light has been reflected at the 



produced by un-furface of a rarer medium, it muft be fuppofed to be retarded 



€dfromtbe fl cra" one half of the appropriate interval, for inftance, in the cen- 



fines of a denfe tral black fpot of a foap-bubble, where the actual lengths of 



and a rare me- ^ pat h s verv near ly coincide, but the effect is the fame as if 

 dium in circum- r * • J • ' « « , , 



Aances todeftroy one or the portions had been io retarded as to deltroy the other. 



each other : From confidering the nature of thiscircumftance, I ventured to 



predict, that if the two reflections were of the fame kind, made 



at the furfaces of a thin plate, of a denfity intermediate between 



the denfities of the mediums containing it, the effect would be 



reverfed, and the central fpot, inftead of black, would become 



white ; and I have now the pleafure of ftating, that I have fully 



a contrary effeft verified this prediction, by interpofing a drop of oil offaflafrafs 



is produced when between a prifm of flint-glafs and a lens of crown glafs : the 



ttf^f^r central f P ot feen bv refle ^ ed Ji g ht wa s white, and furrounded 

 suedium. by a dark ring. It was however neceflary to ufe fome force, in 



order to produce a contact fufficiently intimate ; and the white 

 fpot differed, even at laft, in the fame degree from perfect 

 vyhitenefs, as the black fpot ufually does from perfect black- 

 nefs. 



The 



