IIERfiCHEL ON COLOURED RINGS. 189 



that ftrch a surface is not essential to the formation of rings, 

 but as not only in direct transmission, but also when two sets 

 of rings are to be seen, one of which may be formed by trans- 

 mission, this fourth surface must be introduced ; I have ascer- 

 tained by the following experiments how far the same has any 

 share in the formation of rings. 



- In direct transmission, where the light comes from below, 

 the fourth surface will take the part which is acted by the 

 first, when rings are seen reflected from a metalline mirror. 

 Its office therefore will be merely to afford an entrance to the 

 rays of light into the substance of the subjacent glass ; but 

 when that light is admitted through the first, second, and 

 third surfaces, the fourth takes the office of a reflector, and 

 sends it back towards the point of contact. It will not be re- 

 quired to examine this reflection, since the light thus turned 

 back again is, with respect to the point of contact, in the 

 same situation in which it was after its entrance through the 

 first surface, when it proceeded to the same point ; but when 

 two sets of rings are to be formed by rays, either coming 

 through this point directly towards the fourth surface, or by 

 reflection from the same point towards the place where the 

 secondary rings are to be seen, it will then be necessary to 

 examine, whether this surface has any share in their forma- 

 tion, or whether these rings, being already completely form- 

 ed, are only reflected by it to the eye. With a view to Experiment 



this, I selected a certain polished defect in the surface of a Y^ ha P oll f l ! e * 

 1 # # defect in tlus 



piece of coach-glass, and when a 26-inch lens was laid upon surface. 

 it, the rings of the set it produced were much distorted. 

 The lens was then put upon a perfect slip of glass, and both 

 together were then laid upon the defective place of the coach- 

 glass. The rings of the secondary set reflected by it were 

 nevertheless as perfect as those of the primary set. It oc- 

 curred to me, that these rings might possibly be reflected 

 from the lowest surface of the perfect slip of glass, espe- 

 cially as by lifting it up from the coach-glass I still continued 

 to see both sets. To clear up this point, therefore, I took 

 away the slip, and turning the defective place of the coach- 

 glass downwards, produced a set of perfect rings between 

 the lens and the upper surface of the coach-glass, and 

 brought it into such a situation, that a secondare set must 



be 



