the Cause of coloured concentric Rings. 123 



light, which should not be strong, nor should it be very 

 oblique, the reflection from the mirror will thfen exceed that 

 from the surface 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 centre. The rays 1 , 2, 4, passing 

 through the point of contact, will also form a transmitteci 

 set with a white centre, which will be seen when the reflec- 

 tion 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 primary 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 reflection of light which overpowers the feeble illumi-^ 

 nation of the rays 1,2, 3 ; and the real hitherto eclipsed 

 primary set belonging to the secondary one with a white 

 centre, will instantly make its appearance with a black one. 

 We niay alternately withdraw and introduce again the strip 

 of cardj and the centre of the primary set will be as often 

 changed from one colour to its opposite, but the secondary 

 set, not being dependent on the rays 6, 7, will not be in the 

 least affected by the change. 



If the contact should have been such as to give both sets 

 with orange centres, the introduction of the strip of card 

 will prove that the set which is primary to the other has 

 really a green centre. 



Another way of destroying the illusion is to expose the 

 same arrangement to a brighter light, and at the same time 

 to increase the obliquity of the angle of incidence : this will 

 give a sufiicient reflection from the surface of the glass to 

 be no longer subject (o the former deceptive appearance ; 

 for now the centre of the primary set will be black, as it 

 pught to be, 



XXJL Of 



