HERCIIRL ON COLOURED RINGS. 185 



but if we should have a doubt remaining, the interposition of 

 any small opaque object upon the looking glass near the frac- 

 ture will instantly stop the latter two sets of rings, ami show 

 the alternate colours of the two sets, that will then be seen by 

 the rays 1, 2, 3, and 1, 2, 4, 5. Remove in the next place 

 the stop from the looking-glass, and bring the second shadow 

 of the penknife over the primary set, and there will then 

 only remain the two sets of rings formed by incident rays 

 which come from rj, and which have never passed through the 

 upper surface of the lens. Now, as both sets of rings in this 

 case are completely formed by rays transmitted upwards from 

 the coated part of the looking-glass without passing through 

 the first surface of the incumbent lens, the proof that the 

 modifying power of that surface is not required to the form- 

 ation of the rings, is established. 



It can hardly be supposed, that the first surface of the lens The upper 

 should have any concern in the formation of the rings, when cessarv for Uvb 

 the rays are reflected from the looking-glass towards the eye; returning ray. 

 but the same experiment, that has proved that this surface 

 was not required to be used with incident rays, will show, that 

 we may do without it when they are on their return. We 

 need only invert the fractured lens, as in figure 12, when ei- 

 ther 1, 2, 4, 5, or 6, 7, 2, 4, 5, will convey the image of the 

 rings, after their formation, to the eye, without passing 

 through any part of the lens. 



XXV. Of the Action of the second Surface* 



As rings are formed when two glasses are laid upon each Action of the 

 other, it is but reasonable to expect, that the two surfaces at second surface, 

 least which are placed together should have an immediate 

 effect upon them ; and so much the more, as it has been ascer- 

 tained, that the first surface assists only by permitting light to 

 pass into the body of the glass. Some of the experiments, 

 that have been instituted for examining the action of the 

 iirst surface, will equally serve for investigating that of the 

 second. 



The lens already used with a strong emery scratch being scratched, 

 again placed on the mirror, but with the injured side down- 

 wards, I found that the rings, when brought under the scratch, 



were 



