178 Ill.RSCIIEL ON COLOURED RINGS. 



with a slip 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 au 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 Pl.V, 

 fig. 9> where both sets of rays are delineated. Then i-f we in-? 

 terpose a narrow tapering strip of card, discoloured with ja- 

 pan 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 si- 

 tuated 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. situation 

 was on the face of the mirror. 

 Eye-glass re- When several sets of rings are to be perceived, by the same 



quires a differ- eve _cri ass an d they are placed at different distances, a parti-* 

 entadjustment J ° ,.' % . r ., v . . . _ ■ 1 . , 



for each set. cular adjustment of it will be required tor 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 adjustment to the 



distance of the set intended to be viewed, we shall be seldom 



successful; and this is therefore a corroborating proof of the 



situation, that has been assigned to different sets; of rings. 



XX. Of the Connection between different Sets of Rings. 



Connexion be- It will now be easy to explain in what manner different 

 i ween different setg of ^^ are connected> and w \ iy taey 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 the pri- 

 mary one: when this is formed, some of the rays are conti- 

 nued by transmission or reflection, but modified so as to con- 

 vey an image of the primary set with opposite colours for- 

 ward through any number of successive transmissions or re- 

 flections; whenever this image comes to the eye, a set of 

 lings will again be seen, which is a dependent one. Many 

 proofs of the dependency of the second, third, and fourth sets 

 of rings upon their primary one may be given; I shall only 

 mention a few. 

 P;oofs that all When two sets of rings are seen by a lens placed upon a 



looking- 



