302 H. MORLAND ON DIATOM STRUCTURE. 



may be objected that such a piece does not present the advantages 

 of a section ; perhaps not entirely, but still a great deal can be 

 learnt from such an examination, for, if diatom valves are riddled 

 through and through with perforations as, in general, I maintain 

 they are, it follows that sections would necessarily fall to pieces if 

 they were cut exceedingly thin, and if they were left somewhat 

 thick then the very object of a thin section would be lost, as it is 

 the rows of perforations immediately under the uppermost row 

 under examination which cause all the ambiguity and interference 

 of images. The following sketch will fully explain my meaning: — 



A v w w >w w w w— 



Booooooo 



C o o o o o o 



D _^_^— .^— ^— ^— ^_ ^_ 



Here are four rows of perforations, the upper one, " A," being 

 under examination. If the objective be of wide aperture it is 

 possible that this row alone will be observed, there being in such 

 case, practically speaking, no depth of focus whatever ; but should 

 the objective have less aperture, it is the row "B" which will 

 cause most of the confusion and interference of images, and the 

 further one recedes from the upper row, u A," the less the inter- 

 ference becomes. If my views be correct, why, then, be at the 

 trouble of attempting to cut a section of a diatom unless it can be 

 cut so thin that the row " B " would have no existence whatever ? 

 But this I have no hesitation in saying would be impossible unless 

 the section were ground down on the slip on which it was to be 

 finally mounted. 



Although diatom sections and broken valves afford a great deal 

 of information relative to their structure, yet it must also be 

 allowed that much can be learnt from the examination of the 

 perfect valves themselves, if properly mounted, as well as from de- 

 fective valves. I think I may say that no one valve will afford 

 every information ; it is necessary to examine a number in all 

 sorts of stages and conditions, and to make your own deductions 

 therefrom. 



Proceeding now to the question of the structure of diatom 

 valves, I purchased not long since a copy of Messrs. W. Prinz and 

 Van Ermengem's work on the structure of diatoms contained in the 

 Jutland " Cementstein." I have nothing to add to their observa- 



