the Cause of coloured concentric Rings. 
SOI 
XIII. Of the Order of the Colours. 
The arrangement of the colours in each compound ring or 
alternation, seen by reflection is, that the most refrangible rays 
are nearest the center ; and the same order takes place when 
seen by transmission. We have already shown that when a full 
dilution of the colours was obtained their arrangement was 
violet, indigo, blue, green, yellow, orange, and red ; and the 
same order will hold good when the colours are gradually 
concentrated again ; for though some of them should vanish 
before others, those that remain will always be found to agree 
with the same arrangement. 
If the rings should chance to be red and green alternately, 
a doubt might arise which of them is nearest the center; but 
by the method of dilution, a little pressure, or some small in- 
crease of the focal length of the incumbent lens, there will be 
introduced an orange tint between them, which will imme- 
diately ascertain the order of the colours. 
In the second set of rings the same order is still preserved 
as in the first ; and the same arrangement takes place in the 
third set as well as in the fourth. In all of them the most re- 
frangible rays produce the smallest rings. 
XIV. Of the alternate Colour and Size of the Rings belonging 
to the primary and dependent Sets. 
When two sets of rings are seen at once, and the colour of 
the center of the primary set is black, that of the secondary 
will be white; if the former is white, the latter will be blacky 
The same alternation will take place if the colour of the center 
of the primary set should be red or orange ; for then the 
