196 Dr BREWSTER on a new species of Coloured Fringes. 



so the interval of retardation, so that the orders of the rings de- 

 scend, as in Fig. 1. But there is a particular point between the 

 rings m and n, where the interval of retardation is nothing, or 

 where the lengths of the paths of the two interfering pencils 

 are equal, so that we have a white ring at that place. Beyond 

 this, the interval of retardation becomes perceptible, and another 

 system of rings commences, rising to their highest order at the 

 very circumference of the object-glass. 



When the eye and the flame are in the axis of the object-glass, 

 the isochromatic lines are circles ; but at oblique incidences they 

 have the singular forms shewn in Figs. 2 and 3, the line where 

 there is no interval of retardation being the boundary of the 

 four different systems of fringes shewn in these figures. 



As the paths of the interfering pencils are performed in three 

 media, crown-glass, flint-glass, and air, and as their lengths vary 

 very quickly and irregularly, as the angle of incidence varies, 

 and as the point of incidence changes its position, the analytical 

 expression of the interval of retardation will be very complex. 



