20 Sir D. Brewster on a new Species of Coloured Fringes, 



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 shown in Fig. 1, approaching 

 closer and closer to each other as they advance from the centre 

 to the circumference of the lens. Two of these rings, mm mm, 

 nnn7i, having an intermediate position in the system, are di- 

 stinguished from the rest by their darkness, and by the white- 

 ness of the hght between them ; and they enjoy the remarkable 

 property of becoming the bounding lines o^ 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 further from the eye than 

 B, and so that the eye receives the rays that are reflected ob- 

 liquely 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 bi'oader and 

 wider on the side B, having intermediate breadths and di- 

 stances at intermediate points of the circumference between A 

 and B. By increasing the incidence, the inner ring a a. Fig. 1, 

 contracts into a sort of irregular ci-escent a a, Fig. 2. The 

 second and third rings, bb, cc. Fig. 2, do the same as shown 

 a.tbb,c c, Fig. 2, and at a greater incidence, the dark ring ««, 

 Fig. 1, assumes a similar form nnnn. Fig. 2, and forms the 

 boundary of the remote central system ncbaabcn. In like 

 manner, the lower part of the ring nn. Fig. 1, has inclosed a 

 smaller but similar system of rings, which are shown at n!nhhi', 

 and may be called the 7iear central system. While these 

 changes are going on, the rings without n n. Fig. 1, are under- 

 going analogous, though opposite, inflections. The outermost 

 ddd. Fig. i, divides itself into two unequal portions, which 

 run out into the circumference at the points d d d' d', Fig. 2. 

 Then the next ring, viz. the dark one, m m m ?h, forming the 

 boundary of the remote external system m m m. A, and of the 

 near central system m' m' m' B. 



The four groups of rings thus developed, assume, at greater 

 incidences, the character shown in Fig. 3, but they are not 

 seen all at once ; and in tracing their form, it is necessary to 

 cause the image on which they are produced, to be reflected 

 successively from different parts of the lens. The rings are 

 so closely packed together at a distance from the centres x, x, 

 to which they are ail related, that it is extremely difficult to 

 perceive them. By increasing the incidence still further, the 

 rings close in upon the centres x, x, and become exceedingly 

 close and numerous. The points x^x approach to tlie circum- 

 ference of the lens, and the rings become more luminous from 



