the Cause of coloured concentric Rings, 1 1 7 



have the appearance of a magical delusion^ will be found in 

 a future article. 



XVI. Of the Course of the Raj/s hj which different Sets of 

 , Rijigs are seen. 



In order to determine the course of the rays, which giv-e 

 the rings both by reflection and by transmission, we should 

 begin from the place whence the light proceeds that forms 

 them. In figure I, (PI. VI.) we have a plano-convex lens laid 

 upon three slips of glass, under which a metalline mirror is 

 placed. An incident ray, 1,2, is transmitted through the 

 first and second surface of the lens, and comes to the point 

 of contact at 3. Here the rings are formed, and are both 

 reflected and transmitted : they are reflected from the upper 

 surface of the first slip, and pass from 3 to the eye at 4 : they 

 are also transmitted through the first slip of glass from 3 

 to 5 ,• and at 5 they are again both reflected and transmitted ; 

 reflected from 5 to 6. and transmitted from 5 to 7 ; from 7 

 they are reflected to 8, and transmitted to 9 ; and lastly 

 they are reflected from 9 to 10. And thus four complete 

 sets of rings will beseen at 4, 6, 8, and 10. 



The most convenient way of viewing the same rings by 

 transmission, is that which has been mentioned in the se- 

 cond article of this paper, when light is conveyed upwards 

 by reflection. In figure 2, consisting of the same ar^range- 

 ment of glasses as before, the light by which the rings are to 

 be seen comes either from 1, 2, or 3, or from all these places 

 together, and being reflected at 4, 5, and 6, rises up by 

 transmission to the point of contact at 7,' where the rings 

 are formed. Here they are both transmitted up to the eye 

 at 8, and reflected down to 9 ; from 9 they are reflected up 

 to 10, and transmitted down to 11; from 11 they are re- 

 flected to 12, and transmitted to 13; and lastly, from 13 they 

 are reflected to 14 ; so that again four. sets of rings will be 

 seen at 8, 10, 12, and 14. 



This being a theoretical v/ay of conceiving how the rays 

 of light may produce the effects, it will be required to show 

 by experiments that this is the actual progress of the rays, 



H 3 and 



