OF THE PRODUCTION OF COLOURS. 183 



a more minute examination, I found that the magnitude of 

 . the portions of air and water was by no means uniform, and Thefe were not 

 that the explanation was therefore inadmiffible. It was, J^g*^ an J Te" 

 however, eafy to find two portions of light fufficient for the flection but by 

 production of thefe fringes ; for, the light tranfmitted through ^J^"/ the 

 the water, moving in it with a velocity different from that of light through 

 the light paffing through the interfaces filled only with air, the watcr and ai *» 

 two portions would interfere with each other, and produce 

 effects of colour according to the general law. The ratio of 

 the velocities in water and in air, is that of 3 to 4 ; the fringes 

 ought therefore to appear where the thicknefs is 6 times as 

 great as that which correfponds to the fame colour in the 

 . common cafe of thin plates ; and, upon making the experi- 

 ment with a plane glafs and a lens (lightly convex, I found Proofs of this 

 the fixth dark circle actually of the fame diameter as the firft theory, 

 in the new fringes. The colours are alfo very eafily pro- 

 duced, when butter or tallow is fubftituted for water; and 

 the rings then become fmaller, on account of the greater re- 

 fractive denfity of the oils : but, when water is added, fo as 

 to fill up the interfaces of the oil, the rings are very much 

 enlarged ; for here the difference only of the velocities in 

 water and in oil is to be considered, and this is much fmaller 

 than the difference between air and water. All thefe circum- 

 flances are fufficient to fatisfy us with refpect to the truth of 

 the explanation ; and it is fall more confirmed by the effect 

 of inclining the plates to the direction of the light ; for then, 

 inftead of dilating, like the colours of thin plates, thefe rings 

 .contract : and this is the obvious confequence of an increafe 

 of the length of the paths of the light, which now traverfes 

 both mediums obliquely ; and the effect is every where the 

 fame as that of a thicker plate. 



It muft however be obferved, that the colours are not The curved 

 produced in the whole light that is tranfmitted through the fi S u f e of the 

 mediums : a fmall portion only of each pencil, paffing through m UC h of the 

 the water contiguous to the edges of the particle, is fufficiently l| ght to deviate 

 coincident with the light tranfmitted by the neighbouring por- eg- e ^ s# l y 

 tions of air, to produce the neceffiary interference ; and it is 

 eafy to fhow that, on account of the natural concavity of the 

 furface of each portion of the fluid adhering to the two pieces 

 of glafs, a confiderable portion of the light which is beginning 

 to pafs through the water will be diffipated laterally by reflec- 

 tion 





