T5S" HERSCHEL ON COLOURED RINGS. 



The same thin^ will happen when four sets are visible; 

 for all the sets that are formed from the primary one will re- 

 semble each other, but will be alternate in the colour and 

 dimensions of their rings with those of the primary set. 



XV. Of the sudden Change of the Size and Colour of the 

 Jiings in different Sets, 



The size and When two sets of rings are viewed, which are dependent 



fin""in 0f thG upon eac}l other > the co,our of tneir centres, and of all the 

 different sets rings in each set, may be made to undergo a sudden change 

 Sly chanced ^ *^ e a PP roacn of the shadow of the point of a penknife or 

 other opaque slender body. To view this phenomenon pro- 

 perly, let a 1 6-inch double convex lens be laid upon a piece 

 of looking glass, and when the contact between them has 

 been made to give the primary set with a black centre, that 

 of the secondary will be white. To keep the lens in this 

 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 colour, 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 expe- 

 riment it happened, that the primary set got an orange co- 

 loured 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 



