290 



KNOWLEDGE. 



Arc.rsT. 1911. 



FiGLRE 3. The outer side of Plcitr(isij>ina 

 funitusKiii when the valve is sound, x 1 750. 



When it \s'as recognised 

 that e\cr\' increase of aper- 

 ture in the objective revealed 

 "markings"' on forms deemed 

 smooth before, there natur- 

 ally arose from the students 

 of diatoms a demand for 

 more, and still more, light. 

 It is customar\' with biolo- 

 gists to sneer at such as 

 mere '" diatom dotters." yet 

 nearK' all the optical im- 

 provements of the microscope 

 have been due to their claims. 

 Onlv by the consequent res- 

 ponse of opticians could the 

 science of bacteriology have 

 become possible, which now 

 plays so important a part in 

 their (the biologists") o\\ n 

 laboiu's. From 1824 up to 

 witliin the last twent\" \ears there has been a 

 regular progression of the powers of the objective 

 culminating in the apochromatic oil iiiiincrsion 

 Unfortunately, at this point we seem to stick 

 though the cr\- is no less urgent for de\'elopment 



The right interpretation of the "' markings " 

 turned mostlv upon whether they were raised 

 "beads"" upon the surface, or holes through the 

 substance of the valve. In the larger discoid and 

 other forms it was admitted that the so-called 

 cellules were excavations. Even here, however, 

 Mr. Slack, in 1873. maintained that the he.\agonal 

 framework enclosing the cellules was made up of 

 rows of beads. To the imagination of Mr. Slack, 

 indeed, diatom structure began, continued and 

 ended in beads. 



The writer also must confess to a f(U"mer belief in 

 " beads."" In a slide of P. foniiosiiin mounted in 

 balsam were some isolated ones, even under the 



Figure 3 A. The same \ahe as seen in 

 Figure 3, showing the structure imniedi- 

 atelv below that seen in Figure 3, X 1750. 



widest aperture of an oil immersion. Now. except 

 one possesses tiie lively imagination of the Irishman, 

 who explained the making of a cannon by saying 

 " that they took a big long hole and poured molten 

 metal around it."" isolated perforations are unthink- 

 able. One can only explain the puzzle bv assuming 

 that tile structure around them, mounted in a 

 mediimi of the same refractive index, was con- 

 sequenth' invisible. 



From analogy, the two or three layers of structure 

 in the finer valves were merelv assumed. It could 

 scarcely be otherwise before the ad\-ent of the oil 

 immersion. R\lands. in an article in Tlie Uiiaiierlv 

 Journal of Microscopical Science for October 1859. 

 on ■■ the markings of Diatomaceae,"" savs of P. 

 angiilatinu : " Individual specimens of this species 

 are far from uncommon in which the outer 

 ' aerolated " layer is partiallv removed, leaving the 

 inner laver entire. Isolated portions of the outer 

 la\ er may he found upon the valve, but I have never 



seen them separated, the 

 force which removes them 

 being apparentiv only suffi- 

 cient to break them up into 

 single ' aerolations"; the term 

 in this case is unfortunate, 

 for they are. in fact, hemi- 

 spherical elevations."" 



Figures 1 and 2 prove 

 that the valves of P. angii- 

 lafiini can separate entirelv. 

 It seems strange that to 

 demonstrate this should be 

 left to the present writer, 

 nearly tiftv years after 

 the discoverv of this 

 form. The two pictures 

 will well repay studv with 

 a single lens, 

 some five or 



magnitymg 

 six times, 



^^^:55?5 



i^^i!^::!!"'- •'-'-' -2^ 



FiGURF. 4. The inner side of Plcurosi^iiia foriiiosiim. 

 The ordinary appearance of a sound vaKe, X 1750. 



