184 Experiments for investigating^ 



XXII. Of the reflecting Surfaces, 

 The rays of light that form rings between glasses must 

 undergo certain modifications by some of the surfaces 

 through which they pass, or from which they are reflected j 

 and to find out the nature of these modifications, it will be 

 necessary to examine which surfaces are efllicient. As we 

 see rings by reflection and also by transmission, I shall be- 

 gin with the most simple, and show experimentally the 

 situation of the surface that reflects, not only the primary, 

 but also the secondary sets of rings. 



Upon a slip of glass, the lowest surface of which was de- 

 prived of its polish by emery, I laid an object-glass of 21 feet 

 local length, and saw a very complete set of rings. I then 

 put the same glass upon a plain metalline mirror, and saw 

 likewise a set of them. They were consequently not reflect- 

 ed from the lowest surface of the subjacent glass or metal. 



It will easily be understood, that were we to lay the same 

 object-glass upon a slip of glass emeried on both sides, or 

 upon an unpolished metal, no rings would be seen. It is 

 therefore neither from the first surface of the incumbent ob- 

 ject-glass, nor from its lowest, that they are reflected ; for 

 if they could be formed without the modification of reflec- 

 tion from the upper surface of a subjacent glass or metal, 

 they would still be seen when laid on rough surfaces ; and 

 consequently, the efficient reflecting surface, by which we 

 see primary sets of rings, is that which is immediately un- 

 der the point of contact. 



To see a secondary set of rings by reflection, is only att 

 inversion of the method of seeing a primary one. For in- 

 stance, when a lens is laid upon a looking-glass, the course 

 of the rays. represented in figure 8, will show that the rays 

 1, 2, 3, 5, 6, by which a secondary set is seen, are reflected 

 about the point of contact at 3, and that the lowest surface 

 of the incumbent lens is therefore the efficient reflecting 

 one ; and thus it is proved, that in either case of seeing re- 

 flected rings, one of the surfaces that arc joined at the point 

 of contact contributes to their formation by a certain modi- 

 fication of reflection. 



xxiir. Of 



