142 JlETtSClIEL ON COLOURED RINGS. 



primary centre must now be white, and the secondary one 

 black. 



Without being well acquainted with this double course of 

 rays, we shall be liable to frequent mistakes in our estimation 

 of the colour of the centres of two sets of fittgs ; for by a cer- 

 tain position of the Itgnt, or of the eye, we may see one set 

 by one light, and the other set by the other. 



XVIII. Of the Cause of the sudden Change of Colours. 



Cause of the Having thus accounted for the alternation of the central 



sudden change co ] ourS) we mav easily conceive, that the interposition of 

 of colour. . , 



the penknife must have an instantaneous effect upon them. 



When it stops the rays of figure 7, which will happen when 

 its second shadow falls upon the primary set, the rings will 

 then be seen by the rays I, 2, 3, 4, and 1, c 2, 3, 5, 6, of figure 

 8. When it stops the rays of figure 8, which must happen 

 when the third shadow falls upon the primary set, we then 

 see both sets by the rays 1, 2, 3, and 1, 2, 4, 5, of figure 7. 

 When the penknife is quite removed, both sets of rays will 

 come to the point of contact, and in some respects interfere 

 with each other ; but the strongest of the two, which is gene- 

 rally the direct light of figure 7, will prevail. This affords a 

 complete explanation of all the observed phenomena: by the 

 rays of figure 7 the centres will be black and white ; by those 

 of figure 8 they will be white and black ; and by both we 

 shall not see the first set so well as when the third shadow, be- 

 ing upon it, has taken away the rays of figure 8: indeed we 

 can hardly see the secondary set at all, till the shadow of the 

 penknife has covered either the rays of figure 7 or of figure 8. 

 Both thecen- As soon as we are a little practised m the management of 

 trcs and rings t ] ]e rd y Sf by knowing their course, we may change the colour 

 ^partially, so gradually as to have half the centre white, while the other 

 half shall still remain black ; and the same may be done with 

 green and orange, or blue and yellow centres. The rings of 

 Loth sets will also participate in the gradual change; and 

 thus what has been said of the course of rays in the l6th ar- 

 ticle will again be confirmed. 



To he concluded in our next, 



VIII. 



