August, 1911. 



KNOWLEDGE. 



291 



when it will be seen how 

 accuratcK- the broken parts 

 tit into each other. One 

 layer was on the slip and 

 the other tight against the 

 cover glass. 



He cannot claim to have 

 been the first, then, to 

 broach the subject of a 

 second layer in the tiner 

 forms. \et he thinks he 

 was the first to bring it 

 within the borders of fact, 

 b}- definition. The work 

 presented here, except in 

 the instances mentioned, 

 was all done with the two 

 millimetres apochromatic of 

 Zeiss, of 1-40 X..\. Not 

 that such an objective is 

 necessarv in order to see 

 the structure figured, nor 

 even to photograph the 

 same. It can all be seen 

 with an ordinary cheap oil 

 immersion of 1 -30. He has 

 such a lens, of Swift and 

 Son, costing onl_\- £5 5s., 

 and it is astonishing to find 

 how nearl)- it works up to 

 the Zeiss, costing £"20. This 

 for the comfort of micro- 

 scopists with only moderate 

 means — by far the greater 

 number. Every leading 

 maker now produces a 

 similar lens at about the 

 same price. 



The points desired to be 

 driven home in the present 

 article are that diatom 

 structure consists of neither 

 beads nor perforations, as 

 commonly understood. The 

 conventional presentment of 

 the Pleiirosignia everv micro- 

 scopist knows. The figures 

 given here will differ from 

 it in many particulars, as 

 one would expect to follow 

 from everv increase of aper- 

 ture in the micro-objective. 

 A dry lens of anv kind will 

 always produce the same 

 appearance from every unit 

 of a particular species : the 

 same can be said of an oil 

 immersion when the valves 

 are mounted in a medium, 

 but when mounted dry, and 

 an oil immersion used, it 

 will be found that thev 



-^•^8 



FiGUKE 5. The outer side of PUurosigma 



formoium, showing ihe structure torn and 



opticallv separated from that below, X 1750. 



-V 



Fini^Kii 6. 1 !. I I 111 Hgute 5, 



taken witli a ai\ ^ix iiiiimiitiit;, ien^, by Zeiss; 



the upper structure is invisil)le owing to a too great 



depth of focus in the lens. X 720 



FiGL'Ki: 7. Tlie wiiter's "running down" case, X 1750, 

 anil further enlargeifto 2500. 



differ both visually and in 

 cur\e. Figures 3, ia and 4, 

 from two valves of P. formo- 

 siiiii. will illustrate this fact. 

 I'igure 3 shows the outer 

 side of the valve, 3a the 

 structure immediately under- 

 neath, and Figure 4 the 

 inner side from another 

 valve. 



Now the difficult\- attend- 

 ing upon the working of an 

 oil immersion under these 

 conditions, is that the object 

 to be examined must be in 

 optical contact with the 

 cover glass. \\'hen against 

 the slip, or even a little way 

 off the cover, the lens is no 

 longer at its fullest available 

 aperture, indeed, it is but 

 little better, if at all. than 

 a drv glass. The same nia\' 

 be said when the object has 

 lUDie than one layer of 

 structure, no matter how 

 slightly the\' may be separ- 

 ated. There is the fatal film 

 (if air between to prevent 

 that below being seen at its 

 best. This is wh\' nothing 

 will be said here as to a 

 third, intermediate, structure 

 in the finer and thinner 

 diatom valves. Analog}- with 

 the larger and thicker forms 

 may seem to point in this 

 direction, \et it has never 

 been established by demon- 

 stration. Dr. \'an Heurck 

 asserts it in his more recent 

 work, \et gi\es no figures as 

 [1 roofs. 



It nia\" be asked, \\h\' 

 not examine the objects 

 mounted in a medium ? 

 This, certainlw is hv far 

 the better waw when thick 

 enough. On the other hand 

 with \'er\' thin examples, 

 upon removing one difficultv. 

 one of another kind steps 

 in, equalh' fatal. Though 

 homogeneous, then. b\' 

 lessening the angle of refrac- 

 tion, due to the medium, 

 the depth of focus of the 

 objective is increased, with 

 the result that in such 

 objects as a Pleumsigma 

 valve, it pierces through 

 all the layers, confusing the 



