444 ON A NEW SPECIES OF COLOURED FRINGES, &C. 



In all these cases, the fringes are obviously produced by a 

 refraction and a reflexion in each of the two plates, and the in- 

 terfering fringes are produced by the secondary reflexions 

 witliin the glass plates. 



The fringes, however, which appear upon the first or bright 

 images, are produced in a different manner from those formed 

 by the light that has been reflected from the plate CD ; for the 

 light of which they are composed has suffered two or more re- 

 flexions within the plate AB, as shewn in fig. 8. and two re- 

 fractions by the plate CD. These refractions are absolutely ne- 

 cessary to the production of Lhe fringes ; for they disappear 

 when the light reaches the eye, without passing through the 

 second plate. Any variation in the distance of the plates, 

 when their inclination and thickness remain the same, ought 

 obviously to produce no change in the appearance of the frin- 

 ges, as the fits will return in the same manner as before. 



In order to compai'e the preceding phenomena with the 

 Newtonian Theory of Fits, I propose to resume the investiga- 

 tion with plates of parallel glass, that differ very considerably 

 in thickness, and that have their surfaces ground as flat, and 

 polished as highly as possible ; and I have no doubt but that 

 all the results may be calculated by means of that beautiful 

 theory. 



The fundamental experiment by which I ascertained the 

 production of coloured fringes by two plates of glass of equal 

 thickness, has been repeated and verified by my friend M. Biot 

 of the Institute of France, and was exhibited at a public 

 meeting of that distinguished body. 



xvi. 



