204 ® r - Herschel’s Experiments for investigating 
have the appearance of a magical delusion, will be found in a 
future article. 
XVI. Of the Course of the Rays by which different Sets of Rings 
are seen. 
In order to determine the course of the rays, which give the 
rings both by reflection and by transmission, we should begin 
from the place whence the light proceeds that forms them. In 
figure 1 , 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 re- 
flected 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 be seen 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 second 
article of this paper, when light is conveyed upwards by reflec- 
tion. In figure 2, consisting of the same arrangement 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 reflected to 12 and transmitted 
