Professor Smith's Conspectus of the Diatomacese. 219 



inent, in case a point does not fall in the centre of each eye-piece. 

 If the bodies are set at any other angle, the prisms are tilted to cor- 

 respond. The achromatic separating prism, b, is only ■ 08 in thick- 

 ness from apex to base, and is turned to a circle • 26 in diameter ; 

 it is set in the end of a thin sliding tube, that can be slid down, 

 more or less, within the body of an object-glass as circumstances 

 allow. The small range of motion thus given is not found appre- 

 ciably to disturb the central position of an object in the eye-pieces. 

 For optical effect, it is of no consequence at what distance the 

 right-angled or erecting prisms are placed ; they may be half-way 

 up the bodies and the bend made there, or even employed in 

 diagonal eye-pieces; these being swerved round, would enable two 

 observers to see the same object with any form of binocular. 



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



By Professor H. L. Smith. 



I notice in the March number of the e M. M. J.,' which I have 

 just received, the criticism of my " Conspectus," by my friend, 

 Captain Lang. He had already notified me of his intention. 

 Although he deprecates it as an artificial, instead of a natural 

 classification, my own opinion is, that the Diatoniaceae are grouped 

 in a much more natural manner by it than by W. Smith's 

 method, in the ' Synopsis of the British Diatoniaceae,' and which 

 the Captain seems to think more natural ! It is quite true I do 

 not call my " Conspectus " by any higher name than an " Artificial 

 Key," for I am very far from presuming, in the present state of 

 our knowledge, to say that I know enough about our little friends, 

 to construct anything better; I fancy, however, that W. Smith's 

 classification will be found quite as artificial, and even less natural. 

 In having regard, for this purpose, to the "general build," 

 or structure, I am quite sure it is what a student in any other 

 department of natural history would do, sooner than depend upon 

 circumstances or conditions, which the slightest experience would 

 show to be variable; and so Captain Lang will find out one of 

 these days, " when knowledge shall be increased," and he tries the 

 system, that stipitate, tubular, frondose, persistent and non- 

 persistent membrane, are characters absolutely of no value in a 

 natural classification. He knows very well that Staunner's acuta, 

 e.g., grows in filaments ; shall we therefore put this species, follow- 

 ing W. Smith's more natural classification, along with Melosira in 

 Sub-Tribe V., keeping the others of the genus in Sub-Tribe I. ? 

 It seems to me that this is worse than associating Arachnoidiscus 

 with Melosira. But let us see where this so-called natural classifi- 



