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On a new Species of Coloured Fringes, produced by reflexion be- 

 tween the Lenses of Achromatic or Compound Object-Glasses. 

 By DAVID BREWSTER, LL.D., F. R. S., V. P. R. S. Ed. &c. 



(Read January 2. 1832.) 



N a paper which I communicated to this Society in 1815, and 

 which was published in the Seventh volume of their Transactions, 

 I described a new species of coloured fringes, produced between 

 two plates of parallel glass. From a consideration of the theory 

 of this class of phenomena, it was obvious that analogous, though 

 much more complicated, systems of rings should be produced be- 

 tween plates with curved surfaces, but it was not till 1822 that 

 I succeeded in detecting them ; and so completely are these rings 

 concealed by the superposition of similarly situated images, that, 

 in consequence of having forgotten my method of observation, I 

 have experienced the greatest difficulty in rediscovering them. 



My earliest experiments were performed with a double achro- 

 matic object-glass, made by BERGE, having a diameter of 2 T S 

 inches, and 30 inches in focal length. The inner surfaces of the 

 crown and flint glass lenses were ground to different radii, as 

 shewn in the section of it at AB, CD, Fig. 5. Plate IX. ; and 

 the outer surface of the flint-glass lens was concave, so that there 

 was left between the lenses a meniscus of air A 2 B 3 A. 



In order to observe the system of rings as nearly as possible 

 at a perpendicular incidence, I placed the smallest flame I could 

 procure at S, about four or five inches distant from the object- 



