120 lExperiments for investigating 



colours, we may easily conceive that the interposition of 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 1, 2, 3, 4, and 1, 2, 3, 3, 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, 3, qf 



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 generally the direct light of figure 7, will prevail. 



This affords a complete explanation of all the observed phge- 



nomena : by the rays of figure 7 the centres will be black 



and white ; by those of figure 8 they will be white and black ; 



jmd by both we shall not see the first set so well as when the 



third shadow being 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. ,,, 



As soon as we are a little practised in the managemeht of 

 the rays, by knowing their course, we may change the co- 

 lour 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 both sets will also participate in the gradual 

 change ; and thus what has been said of the course of rays 

 in the l6th article will again be confirmed. 



XIX. Of the Place where the different Sets of Rings are to 



le seen. 

 By an application of the same course of the rays, we may 

 now also 'determine the situation of the place where the dif- 

 ferent sets of rings are seen : for according to what has been 

 said in the foregoing article, the situation of the primary set 

 should be between the lens and the surface of the looking- 

 glass : and the place of the secondary one at the metalline 

 coaling of the lowest surface. To try whether this be ac- 

 tually as represented, let us substitute a metalline mirror 



with 



