the C&use of coloured concentric Rings, I2f 



rence in their size or appearance could be perceived. This 

 seems to prove that no modification of the first surface in 

 which the angle of incidence is concerned, such as refrac- 

 tion and dispersion, has any share in the production of the 

 rings, and that it acts merely by the intromission of light ; 

 and though even this is not without being influenced by a 

 change of the angle, it can only produce a small difference 

 in the brightness of the rings, 



A more forcible argument, that leads to the same con- 

 clusion, is as follows : Laying down three 54-inch double 

 convex lenses, I placed upon the first the plain side of a 

 plano-convex lens of f inch focus ; upon the second, a 

 plain slip of glass ; and upon the third, the plain side of a 

 plano-concave lens also |- inch focus. I had before tried 

 the same experiment with glasses of a greater focal length, 

 but selected these to strengthen the argument. Then, as 

 nothing could be more different than the refraction of the 

 upper surfaces of these glasses, I examined the three sets of 

 rings that were formed by these three combinations, and 

 found them so perfectly alike that it was not possible to per- 

 ceive any difference in their size and colour. This shows 

 that the first surface of the incumbent glasses merely acts as 

 an inlet to the rays that afterwards form the rings. 



To confirm the idea that the mere admission of light 

 would be sufficient, T used a slip of glass polished on one 

 side but roughened with emery on the other : this being laid 

 upon a 21 -feet object-glass, I saw a set of rings through the 

 rough surface ; and though they appeared hazy,, they were 

 otherwise complete in figure and colour. The slip of glass 

 when laid in the same manner upon the letters of a book 

 made them appear equally hazy; so that the rings were 

 probably as sharply formed as the letters. 



Having now already great reason to believe that no modi- 

 fication, that can be given by the first surface to the inci- 

 dent rays of light, is essential to the formation of the rings, 

 I made the following decisive experiment : 



Upon a small piece of looking-glass I laid half a double 

 convex lens of l6-inches focus, with the fracture exposed 

 to the light, as represented in figure 11. Under the edge of 



tke 



