Professor Smith's Conspectus of the Diatomacese. 165 



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

 leaving. 



This memoir is of necessity incomplete, for want of definite 

 information concerning the optical properties of various kinds of 

 glass. Data obtained from working them into small lenses furnish 

 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 appearance 

 of more or less of a secondary spectrum. 



Nothing of importance has been published since Fraunhofer's 

 Table, containing the refractive indices for each of the seven 

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

 advance has been effected since that date in the manufacture of opti- 

 cal glass, a most complete collection of which of every variety has 

 been made by the Bosses up to the present date. Selected speci- 

 mens 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 subject of a future communication. — 

 Proceedings of the Boy al Society, No. 141, 1873. 



V. — Professor Smith's Conspectus of the Diatomacese. 

 By F. Kitton, Norwich. 



Captain F. H. Lang will probably excuse the following remarks 

 on his critique upon the above-named Conspectus. The late Dr. 

 Arnott, whose knowledge of the Diatoniaceae was perhaps greater 

 than any other diatomist, always contended that the stipitate, con^ 

 catenate, or frondose states were not of any generic or specific 

 value. 



Professor Smith, in placing Arachnoidiscus in the same tribe as 

 Melosira, has surely brought together forms more nearly allied than 

 Kiitzing has in his arrangement. He places the Melosireae in the 

 same tribe as Eunotieas, Surirellese, and Naviculese. Professor 

 Smith does not refer all the Triceratia to Biddulphia. Some are 

 referred to Ditylum, another to Eucampia, others to Eupodiscus, 

 and some to Liradiscus and Stictodiscus. 



Captain Lang says he has never seen Amphitetras or Triceratium 

 in zigzag chains, and fancies they do not occur in that state. It is 

 the usual state of Amphitetras, and two species of Triceratium have 

 been found in that condition, viz. Triceratium arcticum, described 

 and figured by Mr. Koper in ' Trans. Mic. Soc.,' vol. viii., p. 55, 



