332 



KNOWLEDGE. 



September, 1911. 



Figure 15. Inciiuiur sidu ul I'lcurosigma fonuosiiiu 



where the valve has been acted upon in parts, leaving 



bare the fibrils or grating, X 1750. 



images thrown from 



the other side of the 



vahe, as on a screen. 



The structure does 



not seem to differ 



much in cliaracter 



from that of the 



outside, except in . • 



being more robust. - 



See Figure 15 for i 



this, taken also from , 



the inner side. 



Comparing the 



o()tical results from 



the same diatom, in 



the photographs 



taken both b\' the 



oil immersion and 



the dry lens, is there 



any reason for r-,^.,,,,. ic tt * i 



. ■, . , Figure 16. Fragments ot 



doubtmg the utter one showing the outer and 



futilitS' of attempting 



to elucidate further the structure of P. fttninisiim 

 with an objective of only 1 -0 N.A.? Yet the Tnjus- 

 dcfiuiis of flic Royal Microscopical Socicfy show that 

 this feat has been attempted by one of its Fellows 

 within the last two or three years. The chief, 

 the almost sole, advantage of the oil immersion 

 apochromatic lens is that bv its refinements 

 of correction it can stand more light, and have 

 more of the available aperture utilised. The 

 almost possible limit of resohing power was 

 reached by the old glasses with an aperture pushed 

 to l-5(). It was proposed to carry resolution still 

 further by an objective of only 1 •(), with a top-stop 

 as an accessory. The results are recorded and 

 figured in the Traiisacfioiis — let the readers of 

 " Knowledgk "■ compare them. 



torn structure to confirm the practical identit\- of the 

 two. In P. fonnosiim we can obtain it in long chains, 

 as it were, here we shall find only links. Yet to those 

 who know a chain when thev see it, identical links 

 should be sufficient for evidence. Indeed, from the 

 \arious processes to which the valves are subjected 

 It is only to be e.xpected that the more fragile forms 

 will get the most damage, and the\- do. The 

 jihotographs of this species have been enlarged 

 twice, for the better comparison : figure 16 shows 

 the fragments of two valves, each with the opposite 

 face outwards, forming a striking contrast. The 

 torn structure seen just abo\e the median line is 

 from the outer side, and exhibits similar fibrils, with 

 the same arrangement as in P. formosuin — more 

 especially seen at the left-hand bottom corner of the 

 picture, where it is marked with a X — exhibits the 

 same focal itnages, projected from the other side of 

 the valve. Above this again it will be seen that 

 we get the normal aiipearance of the o//^er side when 



sound, and, in the 

 uppermost division in 

 strong contrast the 

 characteristic hexa- 

 gonal structure of the 

 inner side. Figure 

 17 is another ex- 

 ample from the outer 

 side of still more 

 torn structure; 

 -\ further, it is put here 

 also to i)oint a moral, 

 if not to adorn a tale: 

 for there is a tale. 



Throughout the 

 greater part of the 

 period over which 

 the history of the 

 achromatic micro- 

 scope extends, this 

 species has been 

 emplo}'ed as a 



\ 



'>« 



the \ai\e ot P. angiilcitiiin, 

 one the inner side, X 3500. 



When we come to P. an^iilatiiiii we are dealing 

 with structure of just double the fineness, where it 

 is not so easy to get such striking examples of the 



Figure 17. The outer side of the same, X 2500. 



