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the Cause of coloured conce?itric Rings. 
primary set, which is that towards the light. The real colours 
of this and the secondary set will then be seen to the greatest 
advantage. When the third shadow is advanced till it covers 
the second set, the second shadow will at the same time fall 
upon the first set, and the colour of the centers, and of all the 
rings in both sets, will undergo a sudden transformation from 
black to white and white to black. 
The alternation of the colour is accompanied with a change 
of size, for as the white rings before the change were of a 
different diameter from the black ones, these latter, having 
now assumed a black colour, will be of a different size from 
the former black ones. 
When the weight is taken from the lens the black contact 
will be changed into some other. In the present experiment 
it happened that the primary set got an orange coloured center 
and the secondary a green one. The same way of proceeding 
with the direction of the shadow being then pursued, the orange 
center was instantly changed to a green one, while at the same 
moment the green center was turned into orange. With a 
different contact I have had the primary set with a blue center 
and the secondary with a deep yellow one ; and by bringing 
the second and third shadows alternately over the primary set, 
the blue center was changed to a yellow, and the yellow center 
to a blue one ; and all the rings of both sets had their share 
in the transformation of colour and size. 
If there are three sets of rings, and the primary set has a 
black center, the other two will have a white one ; and when 
the lowest shadow is made to fall on the third set, the central 
colour of all the three sets will be suddenly changed, the first 
from black to white, the other two from white to black. 
A full explanation of these changes, which at first sight 
