SULLIVAN X AND WOllMLKY, ON NOBERT^S TEST PLATE. 117 



Lastly, Mr. Heudiy states (' Mic. Jour./ July, 1860) that lie 

 lias ^' come to a satisfactoiy conclusion, that it is a sad mis- 

 representation to set down tlie lines so high in the scale as 

 130 in "OOl", and that on a few shells lines may be counted 

 at 12, and many at GO, 70, and 80 in 'OOl".^' A perplexing 

 record, truly ! — reminding one of the celebrated Torbane 

 Hill coal case (^Mic. Jour.,' ii, p. 64). 



It is our impression, notwithstanding these conflicting 

 statements, that the diatom before us presented to all these 

 gentlemen the same appearances, but their interpretation of 

 these appearances have been widely different. 



The testimony of our objectives, as we understand it, seems 

 to indicate that this diatom has a minutely and irregularly 

 broken-up surface, which even on the same valve can be 

 made to show an apparent striation, varying from moderately 

 coarse to extremely fine, according to the obliquity or inten- 

 sity of the illumination, and to the grade, whether low or 

 high, of the objective used, thus proving beyond question 

 that the exhibition is illusory. In numerous trials, par- 

 ticularly on fine English specimens from Hull, sent us by 

 Mr. G. Norman, we have entirely failed, with glasses too of 

 unsurpassed excellence, to bring out I'egular, distinct, and 

 unmistakeable strice such as would be at once so recognised by 

 an eye practised on the striae of other diatoms. 



After all, it is not improbable that true striae, yet unre- 

 solved, may exist on the valves of this species ; and further- 

 more, that the apparent striae of different observers may be 

 similar to the spectral or spurious lines before noted as 

 occurring on the bands of Nobert's test-plate, when 

 examined by an objective incapable of resolving them. 



A summary of the foregoing may be briefly stated thus : — 

 that our experiments lead us to believe — 



1st. That lines on Nobert's test-plate, closer together than 

 about the ^-.\^yth of an inch, cannot be separated by the 

 modern objective. 



2d. That no true striae have yet been seen on the valves of 

 AmpJdpleura pellucida. 



3d. That the alleged variableness in the striation of diatoms 

 among individuals of the same species has been greatly ex- 

 aggerated ; on the contrary, we find a remarkable uniformity, 

 thus sustaining the opinion of Prof. Smith {' Synop. Br. 

 Diat.,' V. 2, Introd., p. 26), that for characterising species 

 " striation is the best guide."- — Columbus Ohio; Nov. 1860. 



