SURFACE-3IABK1XGS OF BIATOMACE.E. 



279 



Fig. 119. 



under the Binocular Microscope fully confirms the inferences drawn 

 from the phenomena they present to the single eye ; presenting the 

 network in unmistakable relief, and showing the areola? to be really 

 depressions. Moreover, when a valve is broken, the line of fracture 

 corresponds to what, on this view of its structure, is its weakest 

 portion ; since it passes 

 through the areola? instead 

 of through the interven- 

 ing network, which last, 

 instead of forming the 

 thick framework of the 

 valve, would be its weaker 

 portion if the areola? were 

 prominences. But the 

 most satisfactory proof 

 that the areola? are de- 

 pressions is perhaps that 

 which "is afforded by a 

 side-view of them, such as 

 may be obtained by ex- 

 amining the curved edges 

 of the valves in Isthmia ; 

 this, it may be safely 

 affirmed, can leave no 

 doubt in the mind of any 

 competent and unpreju- 

 diced observer as to the 

 nature of the markings in 

 that genus ; and analogy 

 would seem to justify the 

 extension of the same 

 view to the other cases 

 in which the microscopic 

 appearances correspond. * 

 — But it is with regard 



Triceratium fa van ; — a. side view ; 

 b, front view. 



to the more delicate markings on the minuter Diatoms, and 

 especially as to the nature of those on the valves of the various 

 species of Pleurosigma and other forms used as Test-objects (§ 132), 



* When specimens of Diatoms which exhibit this Areolation are ex- 

 amined by Welcker's test of Focal Adjustment (§ 127), it is found that if 

 they are mounted in Canada Balsam, the Optical effects are reversed ; 

 the areola? being made to look bright like elevations) when the distance of 

 the objective is increased, and dark when it is diminished. This, however, 

 is readily explicable by the fact that the refractive power of the Balsam 

 is greater than that of the Siliceous valve ; so that the predominant 

 effect will be produced by the convexities formed in the medium by the 



of Microsc. 



Mr. Rylands 



that the 



