122 Exper'wientsfor investigating 



sets af rings upon their primary one may be given ; — I shaU 

 onlv mention a few. 



When two sets of rings are seen by a lens placed upon a 

 looking-glass, the centre of the secondary set will always 

 remain in the same plane with the incident and reflected 

 rays passing through the centre of the primary one. If the 

 point of contact, by tilting is changed, the secondary set 

 will follow the motion of the primary set ; and if the look- 

 ing-glass is turned about, the secondary will be made to 

 describe a ciroie upon that part of the looking-glass which 

 surrounds the primary one as a centre. If there is a defect 

 in the centre or in the rings of the primary set, there will be 

 exactly the same defect in the secondary one ; and if the 

 rays that cause the primary set are eclipsed, both sets will 

 be lost together. If the colour of the primary one is changed, 

 that of the secondary will also undergo its alternate change, 

 and the same thing will hold good of all the dependent rings 

 when three or four sets of them are seen that have the same 

 primary one. 



The dependency of all the sets on their primary one may 

 also be perceived when we change the obliquity of the inci- 

 dent light ; for the centres of the rings will recede from one 

 another when that is increased and draw together when we 

 lessen it, which may go so far that by an incidence nearly 

 perpendicular we shall bring the dependent sets of rings 

 almost under the primary one. 



XXI. To account for the Appearance of several Sets of Rings 

 with the same coloured Centres, 



It has often happened that the colour of the centres of 

 different sets was not what the theory of the alternation of 

 the central colours would have induced me to expect ; I have 

 seen two, three, and even four sets of rings, all of which 

 had a white centre. We are however now sufticiently pre- 

 pared to account for every appearance relating to the colour 

 of rings and their centres. 



Let an arrangement of glasses be as in figure 9. When 

 this is laid down so as to receive an illumination of day^ 



li^ht. 



