208 Dr. Herschei/s Experiments for investigating 
other ; but the strongest of the two, which is generally the direct 
light of figure 7, will prevail. This affords a complete explana- 
tion of all the observed phenomena : by the the rays of figure 
7 the centers will be black and white; by those of figure 8 
they will be white and black ; and by both we shall not see 
the first set so well as when the third shadow being upon it, 
has taken away the rays of figure 8 : indeed we can hardly 
see the secondary set at all, till the shadow of the penknife 
has covered either the rays of figure 7 or of figure 8. 
As soon as we are a little practised in the management of 
the rays, by knowing their course, we may change the colour 
so gradually as to have half the center white while the other 
half shall still remain black ; and the same may be done with 
green and orange, or blue and yellow centers. The rings of 
both sets will also participate in the gradual change ; and thus 
what has been said of the course of rays in the 16th article 
will again be confirmed. 
XIX. Of the Place where the different Sets of Rings are to 
be seen. 
By an application of the same course of the rays, we may 
now also determine the situation of the place where the differ- 
ent sets of rings are seen : for according to what has been 
said in the foregoing article, the situation of the primary set 
should be between the lens and the surface of the looking- 
glass : and the place of the secondary one at the metalline 
coating of the lowest surface. To try whether this be actually 
as represented, let us substitute a metalline mirror 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 
