the Valves of Pleurosigma and other Diatoms. 353 



by Mr. Wenham, it is somewhat difficult to gather from the 

 remarks of the Editors in question whether they accepted or 

 rejected my view. It is certain, however, that at the above period, 

 and indeed up to a very recent date, I stood alone in supporting 

 the pyramidal structure by evidence derived from the optical 

 appearances of the valve under high powers of the microscope, 

 and nearly alone as regards the secondary kind of evidence de- 

 rived from the direction of the lines of fracture and effects of 

 moisture. For although it was stated in the admirable work on 

 the microscope by the lamented Prof. Quekett, published in 

 1852, that, under an improved method of illumination and a 

 magnifying power of at least 1200 diameters, Mr. Gillett had 

 succeeded in showing " the lines on P. angulatum to be dots or 

 elevations from the surface," this interpretation of the structure 

 was speedily set aside by subsequent writers, and principally in 

 consequence of the view put forward by Mr. Wenham with regard 

 to the hexagonal structure. Strange to say, Prof. Quekett was 

 himself amongst the first to repudiate Mr. Gillett's observation. 

 In his 'Lectures on Histology^ (vol. ii. p. 59) it is stated that, 

 by careful " management of the light, Mr. Wenham has found 

 that the oblique lines on P. angulatum can be resolved into 

 hexagons, and that he has proved heyond doubt the structure by 

 means of photographs taken under a power of 15,000 diameters, 

 — all the parts accurately in focus exhibiting hexagons with a 

 white centre, but in those out of focus the centre being black." 

 Mr. Wenham's photographic figure is copied by Quekett, as also 

 a figure of the structure in the closely allied variety, P. formo- 

 sum, under a magnifying power of 5500 diameters, the lines 

 being resolved, on the authority of the Rev. Mr. Kingsley of 

 Cambridge, " into dots, studs, or beads ; " but, continues the 

 Professor, " notwithstanding the enormous power employed on 

 this occasion, there are many observers who still regard these 

 markings as depressions, and not as elevations." 



In Carpenter's work " On the Microscope and its Revelations " 

 [not the latest edition), at page 306, it is stated that if we examine 

 P. angulatum with an objective of 5-inch focus and 75° aperture, 

 the valve presents a double series of somewhat interrupted 

 diagonal lines, inclosing between their intersections imperfectly 

 defined lozenge-shaped spaces ; but that if the valve be viewed 

 under an objective- of ^inch focus and an angular aperture of 

 130°, "illuminated by oblique rays, its hexagonal areolation 

 becomes very distinct. And if a photographic representation 

 obtained by such a power be itself enlarged by photography, as 

 has been accomplished by Mr. Wenham, the appearance pre- 

 sented is in all respects comparable with that afforded under a 

 low power by the valve of Triceratium or Isihmia." Here again 



