the Cause of co toured concentric Rings . 207 
is represented in figure 7, it is evident that we see the primary 
set with its center at 2 by reflection, and the secondary one 
at 4 by transmission. Hence it follows, in consequence of the 
admitted principle, that if the contact is. such as to give us the 
primary set with a black center, the secondary set must have 
a white one ; and thus the reason of the alternation is ex- 
plained. 
But if the rays come as represented in figure 8, we see the 
primary set by transmission, and the secondary one by reflec- 
tion ; therefore, with an equal pressure of the glasses, the 
primary center must now be white, and the secondary one 
black.. 
Without being well acquainted with this double course of 
rays, we shall be liable to frequent mistakes in our estimation 
of the colour of the centers of two sets of rings ; for by a cer- 
tain position of the light, or of the eye, we may see one set 
by one light and the other set by the other. 
XVIII. Of the Cause of the sudden Change of the Colours . 
Having thus accounted for the alternation of the central co- 
lours, we may easily conceive that the interposition of the 
penknife must have an instantaneous effect upon them. When 
it stops the rays of figure 7, which will happen when its second 
shadow falls upon the primary set, the rings will then be seen 
by the rays 1, 2, 3, 4, and 1, 2, 3, 5, 6 , of figure 8. When it 
stops the rays of figure 8, which must happen when the third 
shadow falls upon the primary set, we then see both sets 
by the rays 1, 2, 3, and 1, 2, 4, 5, of figure 7. When the 
penknife is quite removed both sets of rays will come to the 
point of contact, and in some respects interfere with each 
E e 2 
