HKRSCIIEL ON COLOURED RINGS. ]gl 



of the suiace 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 focal 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 

 reflected 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 

 object-glass, nor from its lowest, that they are reflected; for The . reflecting 

 if they could be formed without the modification of reflec- under the point 

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

 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 under 

 the point of contact. 



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

 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, pi. IV, 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 effi- 

 cient reflecting one; and thus it is proved, that in either case 

 of seeing reflected rings, one of the surfaces that are joined 

 at the point of contact contributes to their formation by a 

 certain modification of reflection. 



XXII T. Of the transmitting Surfaces, 



It would seem to be almost self-evident, that, when a set Transmitting 

 of rings is seen by transmission, the light which occasions sur aces ' 

 them must come through all the four surfaces of the two 

 glasses which are employed; and yet it may be shown, that 

 this is not necessary. We may, for instance, convey light into 

 the body of the subjacent glass through its first surface, and 

 fet it be reflected within the glass at a proper angle, so that 



it 



