seen in Achromatic Object-Glasses. 193 



do the same as shewn at bb, cc, Fig. 2., and at a greater inci- 

 dence, the dark ring nn, 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 enclosed a smaller but similar system of rings, which 

 are shewn at n' n' n' n', and may be called the near central system. 

 While these changes are going on, the rings without nn, Fig. 1, 

 are undergoing analogous, though opposite, inflexions. The 

 outermost ddd, Fig. 1, 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, m, form- 

 ing 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 #, #, to which they are 

 all related, that it is extremely difficult to perceive them. By 

 increasing the incidence still farther, the rings close in upon the 

 centres on, tf, and become exceedingly close and numerous. The 

 points x, x approach to the circumference of the lens, and the 

 rings become more luminous from the increase in the reflected 

 light, at increased obliquities of incidence. 



In some object-glasses the rings are exceedingly numerous 

 and close, whether seen as in Fig. 1, or as in Fig. 3 ; and when 

 this is the case, the black rings m f n, and the centres ae / x, are 

 near the circumference. In other object-glasses, particularly in 

 a large one of TULLEY'S, in the Calton Hill Observatory, the 

 rings are very few in number, and the dark fringes m / n, and 

 the centres x t x, are advanced considerably from the circum- 

 ference towards the centre of the lens. In this case the rings 



