produced ly two Plates of Glass of equal Thickness, 355 



Owing to the rapid diminution of the fringes, by increasing 

 the angle formed bv the plates, I could not with any degree of 

 accuracy determine their breadth at greater angles of incHna- 

 tion ; and therefore it still remains to be ascertained whether it 

 varies with the sine, tangent, or secajit of the angles. 



If the light of the circular object, instead of falling perpendi- 

 ^cularlv upon the plates, is incident at difi'erent obliquities, so 

 that the plane of incidence is at right angles to the common 

 section of the plates, no fringes are visible across any of the 

 images. But if the plane of incidence is parallel to the common 

 section of the plates, the reflected images increase in brightness 

 Vvith the obli(iuitv of incidence, and the coloured fringes become 

 more vivid. When the angle of incidence increases from 0" to 

 90°, the images that have suffered the greatest number of re- 

 flections are crossed bv other fringes inclined to them at a small 

 angle. At an angle of about 44*^, the image formed by four re- 

 flections is covered with interfering fringes ; but it is not till the 

 angle of incidence is greater, that this irregularity is distinctly 

 seen on the image formed by two reflections. 



Hitherto I had observed no fringes upon the first or bright 

 image, which is obviously composed of light that has not suf- 

 fered reflection from the second plate of glass. By concealing, 

 however, the bright light of the first image, so as to perceive the 

 image formed by a second reflection, within the first plate, and 

 bv viewing this image through a small aperture, which I found 

 of the greatest service ih giving distinctness to all the pheno- 

 mena, I observed fringes across the first image, far surpassing 

 in precision of outline, and in richness of colouring, every ana- 

 logous phaenomenon which I had seen. When these fringes were 

 concealed, I also observed other fringes on the image immediately 

 behind them, and formed by a third reflection, from the interior 

 of the first plate. I now concealed the second image, upon which 

 the fringes were extremely bright, and very faint stripeg were 

 seen upon the one immediately behind it. 



On examining these phsenomcna a little more attentively, I 

 observed that the size of the fringes in the first image varied 

 with the distance of the eye from the plates, while those on the 

 second and fotirth image diminished with that distance. 



The magnitude of this change will be understood from the 

 following experiments: 



iSluuibnof Frin;;es across Number of Fiingcs across 

 tlie first Iiiiai^e. second Image. 



