Geological Society. 231 



set off in the diagram behind, and neither of them in front of the 

 ray in course of projection ; this leaves the way clear, with the least 

 confusion of lines. 



At the same time a second or counterpart diagram should be at 

 hand, to which the rays only are transferred as soon as their direc- 

 tion is ascertained ; with these precautions a mistake is scarcely 

 possible. 



Now it is hoped that some improvements may be effected by 

 this investigation, on account of the simplicity attained in the com- 

 bination, in which we have two single lenses of crown, whose foci 

 bear a definite proportion to each other ; while all the corrections 

 are performed by one concave of dense flint, the acting condition of 

 which is not altered by the influence of any other concaves acting 

 in the combination, and hitherto .taking a share of the duty. This 

 one flint is now to be considered singly as the heart and centre 

 of the system in reference to the correction of the rays entering 

 and leaving. 



This memoir is of necessity incomplete, for want of definite in- 

 formation concerning the optical properties of various kinds of 

 glass. Data obtained from working them into small lenses fur- 

 nish only a rough approximation to the mean dispersive power of 

 the combined flint and crown having the best apparent effect. 

 Of the intermediate rays, little can be known beyond the mere ap- 

 pearance of more or less of a secondary spectrum. 



Nothing of importance has been published since Eraunhofer's 

 Table, containing the refractive indices for each of the seven pri- 

 mary colour-lines of the spectrum for ten kinds of glass : great 

 advance has been effected since that date in the manufacture of 

 optical glass, a most complete collection of which of every variety 

 has been made by the Rosses up to the present date. Selected spe- 

 cimens from this will be worked into prisms, and the relative 

 spectra mapped out by the Fraunhofer lines, leading, it is hoped, 

 to the discovery of a combination of crown and flint glass which 

 shall be free from secondary spectrum or absolutely achromatic. 

 The result of this investigation will be the subject of a future 

 communication. 



GEOLOGICAL SOCIETY. 

 [Continued from p. 152.] 

 June 19, 1872.— Prof. Eamsay, V.P.G.S., in the Chair. 

 The following communications were read : — 

 1. " On Trochocyatlius anglicus, a new species of Madreporaria 

 from the Eed Crag." By P. Martin Duncan, M.B., F.E.S., V.P.G.S., 

 Professor of Geology in King's College, London. 



The author described a Coral of which a single specimen had been 

 found in the Eed Crag, in the grounds of Great Bealings Eectory, 

 Norfolk. He stated that it belonged to the genus Trochocyatlius, 



