ened with 

 emery. 



184, HERSCHEL ON COLOURED RINGS. 



sion, is as follows. Laying down three 54-inch double con- 

 vex lenses, I placed upon the first the plain side of a plano- 

 convex lens of -g. 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 experi- 

 ment 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 bv these three combinations, and found them so per- 

 fectly alike, that it was not possible to perceive any difference 



The first sur- j n t h e size and colour. This shows, that the first surface of 



face simply an . . 



inlet to the *h c incumbent glasses merely acts as an inlet to the rays that 



rays. afterward form the rings. 



Surface roughr To confirm the idea, that the mere admission of light would 

 be sufficient, I 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 ap- 

 pear equally hazy; so that the rings were probably as sharply 

 formed as the letters. 



Having now already great reason to believe, that no modifi- 

 cation, that can be given by the first surface to the incident 

 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 the 

 perfect part of the lens was put a small lump of wax, soft 

 enough to allow a gentle pressure to bring the point of con- 

 tact towards the fractured edge, and to keep it there. In this 

 arrangement it has already been shown, that there are two 

 different ways of seeing two sets of rings: by the rays 1, 2, 

 3, we see a primary set ; and by 1, 2, 4, 5, the secondary set 

 belonging to it : by the rays fj, 7, 2, 3, we see a different pri- 

 mary set ; and by 6', 7, 2, 4, 5, we see its secondary one. 

 That this theory is well founded has already been proved ; 



but 



Experimen- 

 tal!} crucis. 



