the Valves of Pleurosigma and other Diatoms. 355 



being preserved between the angle of aperture of the object-glass 

 and the angular inclination of the refracted rays, the various 

 parts of the depressions and the undepressed portions would be 

 equally recognizable in both cases." Again, in the intro- 

 ductory chapter to the same volume (p. 16), the Editors express 

 their conviction that if Schleiden " were to try to obtain a view 

 of the hexagonal structure of the dots on the valves of a Gyro- 

 sigma with his object-glasses, he would signally fail ; for the ex- 

 hibition of this structure requires a power of about 2000 diame- 

 ters to render it distinct beyond dispute, with the use of stops," 

 &c. &c., ''and the fact of its impressing its own image upon 

 photographic paper at once shows its reality, and that its per- 

 ception is not the result of the imagination/^ 



Dr. Carpenter, in the latest edition of his invaluable treatise 

 on '|The ^licroscope and its Revelations' (1862), whilst seceding 

 from the opinion that any analogy necessarily exists between 

 the structure of the valve of TViceratium and that of Pleuro- 

 sigma, and stating that, although the idea of the valve of the latter 

 being composed of a series of hexagonal depressions at one time 

 received the sanction of Mr. Wenham, the later observations of 

 this gentleman, " with objectives of y- th, and even 4^th of an 

 inch focus, have led him to concur with the view now more 

 generally accepted by microscopists, that the areolae are minute 

 tubercular elevations, the intervening network being formed by 

 the thinner portion of the valve," then goes on to adduce 

 the evidence brought forward some years ago by Mr. Hunt 

 (Quarterly Journal of Microscopical Science, vol. iv. p. 175), 

 which shows that moisture insinuates itself in such a manner 

 between the striae and glass cover as to indicate that the 

 " dots " are elevations, and not depressions. Lastly, as if 

 to add to the apparent confusion of testimony, we find, in 

 the * Micrographic Dictionary* (page xxxiii,, last edition), 

 great stress laid on the fact that the line of " fracture of the 

 broken valves " (of Pleurosigma) " passes through the rows of dots, 

 or the dark lines corresponding to them, showing that they are 

 thinner and weaker than the rest of the substance; for had 

 these dots represented elevations, the valves would have been 

 stronger at these parts;" whilst, in the Philosophical Maga- 

 zine for January 1855, in an abstract of a paper read before 

 the Royal Society "On the Structure of certain Microscopic 

 Test-Objects, and their action on the Transmitted Rays of 

 Light," by Charles Brooke, F.R.S., it is stated with equal 

 certainty and, as I am still prepared to maintain, with perfect 

 correctness, that although "the dots have by some been sup- 

 posed to be depressions, this is clearly not the case, as fracture 

 is invariably observed to take place between the rows of dots, and 



