the Cause of coloured concentric Rings* 
si i 
XXL To account for the Appearance of several Sets of Kings 
with the same coloured Centers. 
It has often happened that the colour of the centers of 
different sets was not what the theory of the alternation of 
the central colours would have induced me to expect : I have 
seen two, three, and even four sets of rings, all of which had 
a white center. We are however now sufficiently prepared 
to account for every appearance relating to the colour of rings 
and their centers. 
Let an arrangement of glasses be as in figure 9. When 
this is laid down so as to receive an illumination of day light, 
which should not be strong, nor should it be very oblique, the 
reflection from the mirror will then exceed that from the sur- 
face of glass; therefore the primary set will be seen by the 
rays 6, 7, coming to the mirror at 7, and going through the 
point of contact in the direction 7, 2, 3, which proves it to be 
a set that is seen by transmission, and it will therefore have a 
white center. The rays 1, 2, 4, passing through the point of 
contact, will also form a transmitted set with a white center, 
which will be seen when the reflection from 4 to 5 conveys it to 
the eye. But these two sets have no connection with each other; 
and as primary sets are independent of all other sets, I have 
only to prove that this secondary set belongs not to the pri- 
mary one which is seen, but to another invisible one. This 
may be done as follows. 
Introduce the black strip of card that has been mentioned 
before, till it covers the mirror at 7; this will take away the 
strong ref?e don of light which overpowers the feeble illumi- 
nation on 1, 2, 3 ; and the real hitherto eclipsed pri- 
