116 £xperime?itsjbr investigating 



contact, a pretty heavy plate of lead with a circular hole in 

 it of nearly the diameter of the lens should be laid upon it. 

 The margin of the hole must be tapering, that no obstruc- 

 tion may be made to either the incident or reflected light. 

 When this is properly arranged, bring the third shadow of 

 the penknife upon the 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 centres, 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 coloqr, will be of a different 

 size from the former black ones. 



When the weight is taken from the lens the black con- 

 tact will be changed into some other. In the present ex- 

 periment it happened that the primary set got an orange- 

 coloured centre, and the secondary a green one. The same 

 way of proceeding with the direction of the shadow being 

 then pursued, the orange centre was instantly changed to a 

 green one, while at the same moment the green centre was 

 turned into orange. With a different contact I have had the 

 primary set with a blue centre and the secondary with a deep 

 yellow one; and by bringing the second and third shadows 

 alternately over the primary set, the blue centre was changed 

 to a yellow, and the yellow centre 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 centre, 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 

 1 have 



