HERSCIIEL ON COLOURED RINGS. 179 



looking-glass, the centre of the secondary set will always re- the ot * ier sets 



main in the same plane with the incident and reflected rays primary TneV* 



passing- through the centre of the primary one. If the point 



of contact is changed by tilting, the secondary set will follow 



the motion of the primary set; and if the looking-glass is 



turned about, the secondary will be made to describe a circle 



upon that part of tlie 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 delect 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 al- 

 most 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 Why several 



difterents sets was not what the theory of the alternation of sets hav( r the , 

 i 11 i i i • n i -r i same coloured 



the central colours would have induced me to expect : I have centre. 



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

 a white centre. We are however now sufficiently prepared, 

 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 light, 

 which should not be strong, nor should it be very oblique, the 

 reflection from the mirror will then exceed that from the sur- 

 face 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 

 N 2 a set 



