194 Dr BREWSTEE on a new Species of Coloured Fringes 



are more easily seen, and they undergo very beautiful modifica- 

 tions in passing from a perpendicular to an oblique incidence. 



There can be little doubt that this variation in the size and 

 number of the rings depends on the thickness of the meniscus 

 of air between the lenses ; but in order to put this to the test of 

 experiment, I separated the two lenses A B, CD, Fig. 5, and I 

 found the rings to increase in number, and diminish in breadth, 

 in proportion to the distance of the two lenses. Hence it fol- 

 lows, that, in all those object-glasses where the inner surfaces 

 are coincident, or are cemented by mastic or other varnishes, no 

 rings will be produced, and that the number of the rings fur- 

 nish us with a measure of the difference of curvature of the inner 

 surfaces of the combined lenses. 



In some of the oblique systems of rings which I have observed, 

 the outer fringe n of one of the central systems approached so 

 near the outer fringe m of one of the external systems, that the 

 space between them was straw-yellow, in place of white ; and in 

 one case, the four bounding fringes united, and formed a black 

 cross, as shown in Fig. 4. 



In a large double object-glass, made by GILBERT, 3.8 inches 

 in diameter, and in a similar one by DOLLOND, 2.75 inches in 

 diameter, the rings could only be seen by looking through the 

 convex side A 1 B, Fig. 5. In the first of these lenses there were 

 only two fringes in the near central system of rings, so that the 

 inner surfaces must have been nearly coincident. 



If we separate the lenses a little at A, Fig. 1, and Fig. 5, the 

 system of rings approaches the edge B, and become more nu- 

 merous and more close to each other. The other systems close, 

 and become concentric to them, and the whole become an ellip- 

 tical system. 



When the lenses are separated a little at B, Fig. 1, and Fig. 5, 

 the system enlarges, and the rings grow more numerous, the other 

 systems becoming concentric with them, and forming a close 

 system. 



