o 



09 



AN ATTEMPT TO RESOLVE AND PHOTOGRAPH 



PINNULARIA NOB I US. 



By T. A. O'Doxohoe. 



(Read January 21th, 1914.) 



Most microscopists are acquainted with the little diatom called 

 Pinnidaria nobilis, which, on the test slides of twenty diatoms 

 mounted by Moller and Thum, takes the second lowest place, 

 with striae numbering from 11,000 to 12,000 to the inch. 

 It is just because it occupies such a lowly place that it is 

 passed over with contempt as being worthy of the notice only 

 of the babes and sucklings of microscopy who find themselves 

 in possession of a 2-inch or 1-inch objective. 



Such was my own feeling towards it until quite recently, 

 when Mr. Akehurst showed me by resolution into dots that it 

 deserved a better fate, and invited me to resolve and photograph 

 it, if I could, and for this purpose he, at the same time, lent 

 me a realgar mount and the reflecting dark-ground condenser 

 of Leitz. I have since learnt that an objective and illumination 

 which, without any manipulation, showed me at once the striae of 

 Nitzschia linearis, Frustulia saxonica, and Amphiplewa pellucida, 

 and the very distinct black dots of all the other diatoms on 

 Thum's test plate of 30 forms, failed completely in inducing the 

 Pinnidaria nobilis on the same slide to yield up its secrets. So 

 that the diatom to which almost the lowest place is assigned 

 by the mounter is, in fact, by far the most difficult to resolve. 

 Examined with a drv lens of N.A. - 85 and direct cone of 

 light, we get an image in which on each side of the raphe 

 are seen two zig-zag lines running from end to end and dividing 

 the linger-like bands into three series or each band into three 

 compartments. This is all that can be seen with a dry lens. 



Now using a Zeiss 2-mm. apochromat N.A. 1*3, and Watson's 

 Holoscopic immersion condenser, and finding a central cone of 

 light unavailing, I inserted the crescent stop in the condenser, 

 and proceeding as if I were resolving the striae of Amphiphura 

 pellucida, I succeeded in getting an image which shows what one 



