354 Dr. G. C. Wallich on the Structure of 



we have Mr. Wenham's figure, with the remark that " at the 

 upper part, which represents a portion of the object that was 

 accurately in focus, the hexagonal areas are seen to be light, and 

 the intervening spaces dark ; the reverse being the case with the 

 lower portion which was out of focus ; and a curious transition 

 from one condition to the other being seen in the intermediate 

 part." 



In a short paper by Dr. . Hall, of Sheffield, " On an easy 

 Method of viewing certain of the Diatomacese," published in the 

 Quarterly Journal of Microscopical Science, vol. iv. p. 205 et 

 seq., the structure of P. angulatwn is described and figured as 

 consisting of " dots '" and reference is made, in confirmation of 

 it, to the "hexagonal areolation" supposed to have been de- 

 tected by Mr.Wenham under a power of 15,000 diameters, but 

 evidently under a misconception as to the elevation or depression 

 of the minute siliceous masses composing it, inasmuch as the 

 term " areolation," in this case, evidently refers to depressions 

 with elevated sides, and it is quite clear that such was the gene- 

 rally adopted interpretation of Mr. Wenham's observations, in- 

 asmuch as Dr. Carpenter, till very recently, laid stress on the 

 analogy between the appearance presented by the photographic 

 figure and the plainly discernible depressed areolar spaces of 

 Triceratium and Isthmia. 



The late Rev. W. Smith, in the ' Synopsis of British Diato- 

 macese ; (vol. i. p. 61), expressed himself as having for a time 

 coincided in the opinion of some observers that the appearances 

 of stria? arose from rows of beads or minute elevations, and not 

 from depressions ; but that, with careful manipulation and more 

 perfect optical apparatus, he was led to conclude " that the lines 

 arise from internal hexagonal structure, and that the semblance 

 both of perforations and elevations may be produced in the same 

 object by a slight alteration of focus, such appearances being 

 illusory and due merely to the reflection or refraction of the 

 rays passing through the minute cellular structure of the sili- 

 ceous epiderm." He was also inclined to " attribute the yellow 

 tint seen in the valves of some diatoms to distinct hexagonal 

 structure in their cellular condition, and a purple colour to an 

 absence of such character " (Synopsis, p. 63). 



In the ' Micrographic Dictionary ' (p. 43) we find it stated 

 that " Isthmia requires defining power " (in the lens employed), 

 " whilst Gyrosigma ( = Pleurosigma) requires penetrating power 

 and large angular aperture to exhibit the markings ; and yet the 

 structures differ only in size" And, further, " there can be no 

 doubt that if we could examine the valve of the Gyrosigma under 

 a power as high, relatively to the size of the depressions, as that 

 under which we can examine the Isthmia, the same relations 



