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LXVI. Olservalions and Experiments on the new Species of 

 coloured Fringes discovered by Dr. Brkavstek, in the Light 

 reflected between two Plates of parallel Glass of eqnnlThick- 

 ness. By M. BioTj Member of the National Inslilule, &c, 

 &c. &c* 



X HE discovery of a new species of coloured fringes by M. Brewster 

 being extremely reinarkable, I was anxious to verify it by ex- 

 periment. For this pur]jose, I first nuide use of two plates of 

 glass cut from the sides of a plane mirror with parallel surfaces 

 \vliich had been ground and polished by M. Cauchoix. Each 

 of these })lates ha<l a thickness of about three millimetres (a lit- 

 tle more than 1-Sth of an inch). I then placed them one upon 

 another, keeping them sojvarate, at their extremities, by small 

 pieces of card of the same thickness, which I could nuiltiplv at 

 pleasure when I wished to increase the distance or the inclina- 

 tion of the plates. In this way I obtained the coloured fringes 

 which M. Brewster has announced. I obtained them even when 

 the distance of tlie plates was at least two millimetres (nearly 

 l-13th of an English inch): but it ought to be remarked, that 

 the experiment is very delicate when the thickness of the glasses 

 is great, and their distance considerable; for it is then very easy 

 to go beyond the limits of inclination within which the phaeno- 

 menon is produced. The fringes were more easily obtained with 

 thin plates of glass about half a millimetre in thickness (l-50th 

 of an inch). It then appeared to me, that at a given distance 

 the phenomenon began to be produced by these plates when the 

 incident ray formed a much more considerable angle with their 

 surface. It appeared also, that in proportion to the thinness 

 of the plates the parallelism of their surfaces was no longer a 

 condition rigorouslv necessary; for those which I used liad not 

 been wrought for this purpose, and were a little prismatic. It 

 is, besides, easy to convince oneself that tiie light which produces 

 the fringes has been reilected several times from one plate to 

 the otlier ; which explains why the fringes cease to be produced 

 when one of the exterior surfaces is wetted, as M. Brewster has 

 remarked. 



These fine experiments liave obviously a great connexion 

 with those which Newton has described at the end of his Optics, 

 relatively to the coloured rings formed with thick plates of glass 

 whose two surfaces were spherical and nearly concentric. In 

 the experiments of Newton the incident ray falls at first perpen- 

 dicularly uj)on the plate, and traverses it once. A great part 



* From the Bulletin dcs Sciences pur lu Sociclc Philumatique de Paris, 

 Wars 1815, p. 44. 



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