Markings of Navicula Bhomhoides. Bij Dr. Woodward. 211 



the field to print. Among these objects are several other diatoms, 

 of which the one indicated with a cross (thus x ) is described by 

 Moller, in the catalogue accompanying the Museum plate, as 

 " Navicida crassinervis " ; in its size, the character of its markings, 

 and all essential points, it resembles the specimens of Frustidia 

 Saxonica, which are represented as seen with a higher power in 

 the photographs accompanying my former paper. A comparison of 

 these pictures with those is therefore respectfully solicited. 



Thus far to convince those who examine these pictures that I 

 have really before me substantially what Horr Seibert's pictures 

 represent. But I have next to observe that, even with the Conti- 

 nental lenses which I suppose that gentleman to have used, he 

 ought to have been able both to see and to photograph the hemi- 

 spherical heads which are the true markings of this diatom. I 

 retract the error into which I fell when, misled by the imperfect 

 descriptions of Mr. Hickie, I supposed the longitudinal lines photo- 

 graphed by Herr Seibert to be diffraction phenomena. They are 

 merely the result of imperfect definition. I send herewith a photo- 

 graph of the same frustule, marked C, magnified 870 diameters by 

 the same Hartnack immersion 9. I have simply increased the 

 distance from the object to the screen to gain the increased power, 

 and taken a little more care with the adjustments than in the 

 former picture. The true markings are, however, much more bril- 

 liantly shown by a good English or American immersion objective. 

 I send herewith a photograph (marked D) of a part of the same 

 frustule, magnified 1550 diam. by Powell and Lealand's immer- 

 sion Ttr^^h, without eye-piece ; and another (marked E) of the same, 

 magnified 2700 diameters by the same objective with eye-piece, 

 which may serve to demonstrate the truth of this statement. 



In conclusion, I may remark that while Mr. Hickie argues that 

 there are valid distinctions between F. Saxonica and N. cras- 

 sinervis, several gentlemen with whom I am acquainted go to the 

 other extreme, and hold not only that there are none between these 

 two, but even that there is none between them and Navicula Bhom- 

 hoides. I prefer not to discuss either of these points at present ; 

 but may, perhaps, do so after I have read Mr. Hickie's paper. 



Q 2 



