213 
the Cause of coloured concentric Rings. 
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 if they could 
be formed without the modification of reflection from the 
upper surface of a subjacent glass or metal, they would still be 
seen when laid on rough surfaces ; and consequently, the effi- 
cient 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 in- 
version of the method of seeing a primary one. For instance, 
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 incum- 
bent lens is therefore the efficient 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. 
XXIIL Of the transmitting Surfaces 
It would seem to be almost self-evident, that when a set of 
rings is seen by transmission, the light which occasions 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 
Ff 
MDCCCVII. 
