192 Dr BREWSTER on a new Species of Coloured Fringes 



glass AD, and interposing a small screen G between the flame 

 and the eye at E, I held the eye as close to S as possible, and 

 varied the distance of the object-glass till the inverted greenish- 

 coloured flame* reflected interiorly from the concave surface 

 A 1 B seemed to cover the whole area of the object-glass. When 

 this is accomplished, the rings may, by a slight change in the 

 position of the object-glass, or by screening the image formed 

 by one reflection from A 1 B, be distinctly seen over the ex- 

 panded but enfeebled image formed by a second reflection from 

 the same surface. 



When the flame is very small, and the eye sees it projected 

 against the centre of the object-glass, the rings are grouped into 

 a concentric system, as shewn in Fig. 1, approaching closer and 

 closer to each other as they advance from the centre to the cir- 

 cumference of the lens. Two of these rings, mmmm, nnnn, 

 having an intermediate position in the system, are distinguished 

 from the rest by their darkness, and by the whiteness of the light 

 between them ; and they enjoy the remarkable property of be- 

 coming the bounding lines of four systems of fringes, into which 

 the general system is subdivided by oblique reflection. 



In order to observe this interesting change, incline the object- 

 glass so that the point A is farther from the eye than B, and so 

 that the eye receives the rays that are reflected obliquely from 

 every point of the surface A 1 B. At a very slight deviation from 

 a perpendicular incidence, the rings will become smaller and closer 

 on the side A, and broader and wider on the side B, having in- 

 termediate breadths and distances at intermediate points of the 

 circumference between A and B. By increasing the incidence, 

 the inner ring aa, Fig. 1, contracts into a sort of irregular cres- 

 cent a a, Fig. 2. The second and third rings, bb, cc, Fig. 2. 



* This flame has a greenish-colour, in consequence of the rays which form it hav- 

 ing passed through twice the thickness of the crown-glass lens A B. 



