358 Dr. G. CWaWich oh the Structure of 



transitional series of concentric rings is invariably engendered 

 as the focal distance is altered — showing clearly that these rings 

 are not the exponents of successive laminae of silex, or due to 

 the " sutural and median siliceous ring " adduced as character- 

 istic of that variety in the ' Synopsis of British Diatomacese"; ' 

 but that they are simply indicative of the successive horizons 

 (so to speak) of the conical-shaped valve, on reaching which the 

 rays of light are refracted in their passage to the eye of the 

 observer, and during the changes in the focal distance *. 



Professor Schultze points out that the alternately luminous 

 and darkened aspect of minute elevations and depressions is 

 reversed according as the object is immersed in a medium of 

 higher or lower refractive power than itself. I may add two 

 still more perplexing difficulties which tend to render Welcker's 

 system nugatory, namely, the impracticability of determining 

 at all times whether a Diatom-valve is placed with its external 

 or its internal surface towards the observer, and of ascertaining 

 when that precise amount of definition has been arrived at under 

 which the luminous or dark spots are to be regarded as charac- 

 teristic. In endeavouring to resolve the marking on PleurO' 

 sigma angulatum by Welcker's method, Schultze allows that he 

 signally failed, and that " the indistinct bright points " which 

 precede the dark points brought into view upon the accurate 

 focusing arrived at by the lowering of the tube " do not coin- 

 cide exactly with them in position, but may rather be said to be 

 contiguous to them, and to represent, consequently, the borders of 

 the depressions," — the general conclusion adopted by him being, 

 ''that neither spherical, conical, nor pyramidal elevations are 

 the cause of the punctated appearance on the surface of the 

 above-named species of Pleurosigma, although the decussating 

 sets of ridges, at the points of intersection, afford an appearance 

 resembling that of tubercular elevations ) " and further, " in 

 cases where two sets of lines intersect each other at a right 

 angle, as in P. balticum, hippocampus, &c., the disposition of the 

 ridges at once suffices to account for the arrangement of the 

 quadrangular interspaces. But it is not so easy, he adds, to ex- 

 plain the disposition of the hexagons produced by the three sets of 

 ridges intersecting each other at an angle of 60°, which exist in 

 P. angulatum and its allies. It is most natural to suppose that they 

 would be arranged like the cells of a honeycomb, as figured in 

 the plate [appended to the abstract] . An arrangement of this 

 kind is figured, amongst others, by Carpenter and Mr. Charles 

 Hall." * * * " That the hexagons are arranged as figured by 



* In specimens of this Diatom collected near Guernsey, the raised ring* 

 described in the Synopsis are extremely faint, the outline accordingly 

 presenting an unbroken series of curves. 



