the Cause of coloured concentric Rings, 121 



with a s-lip of glass laid upon it in the room of the piece of 

 looking-glass ; and let there be interposed a short bit of 

 wood, one-tenth of an inch thick, between the slip of glass 

 and the mirror, so as to keep up that end of the slip which is 

 towards the light. . This arrangement is represented in fi- 

 gure 9, where both sets of rays are delineated. Then if we 

 interpose a narrow tapering strip of card, discoloured with 

 japan ink, between the slip of glass and the mirror, so as to 

 cover it at 7, we do not only still perceive the primary set, 

 but see it better than before : which proves that, being situ*- 

 ated above the slip of glass, the card below cannot cover it. 

 If on the contrary we insert the strip of card far enough, 

 that it may at the same time cover the mirror both at 4 and 

 at 7, we shall lose the secondary set; which proves that its 

 aituation was on the face of the mirror. 



When several sets of rings are to be perceived by the same 

 eye-glass, and they are placed at different distances, a par^ 

 ticular adjustment of it will be required for each set, in order 

 to see it well defined. This will be very sensible when we 

 attempt to see three or four sets, each of them situated 

 lower than the preceding ; for without a previous adjust- 

 ment to the distance of the set intended to be viewed we 

 shall be seldom successful : and this is therefore a corrobo- 

 rating proof of the situation that has been assigned to differ-* 

 pit sets of rings. 



XX. Of the Connection between different Sets of Rings ^ 

 It will now be easy to explain in what manner different 

 gets of rings are connected, and why they have been called 

 primary and dependent. When the incident rays come to 

 the point of contact and form a set of rings, I call it a pri- 

 mary one : whenT tl\is is formed some of the same rays are 

 continued by transmission or reflection, but modified so as 

 to convey an image of the primary set with opposite colours 

 forward through any number of successive transmissions or 

 reflections : whenever this image comes to the eye, a set 

 of rings will again be seen, which is a dependent one, 

 Many proofs of the dependency of second^ third, and fourth 



sets 



