*ruru'aljiuefS'] of Eiffh-pou'er Definition. 307 



The linear Ehomboides' beading was estimated at frds closer than 

 the rest. I had not time at my disposal to measure them, but 

 estimate them at about 120,000 to the inch, reckoning the Ehom- 

 boides 80,000, which vary very much in different si^ecimens. Robert's 

 XlXth band would thus appear less difficult of resolution than the 

 central rouleaus of the Khomboidean median lines, in which sixteen 

 beads appear to occupy the same space as about ten striae ; and as it is the 

 crescentic notch which alone renders the beading visible, which notch 

 is much more minute than the interval between the lines, it may be 

 assumed that an assemblage of minute beading, closely placed in 

 contact, 120,000 to the inch, is more difficult to be seen than parallel 

 lines wider apart ; 112,000 to the inch, as in Band XIX. 



I now propose to allude to some points that have been made out 

 by the able observers on the other side of the ocean. We are greatly 

 indebted to Mr, Charles Stodder, who, in the 'American Naturalist' 

 for April, 1868, contributes an interesting and elaborate paper on 

 the resolution of Nobert's lines, reflecting the highest credit upon 

 the writer, and which does not appear to have attracted the attention 

 which it deserves. Not many of Nobert's j)lates have been sold in 

 England ; their great cost and difficulty of access, and the fact that 

 many diatoms are known with finer provisional hnes than Nobert's, 

 may probably account for this. But still it is scarcely apphcable, now, 

 what Mr. Stodder quotes from English works (15 years in print) : — 



" Mr. Boss found it impossible to ascertain the position of a 

 line nearer than the -go^oirth of an inch." (1855.) 



" Dr. Carpenter (2nd edition, 1859) repeated the same remarks, 

 substituting s^siooth for Towoth. There is good reason to believe 

 that the limit of perfection has been nearly reached." 



*' On the other side (of the water) the late Professor T. W. 

 Bailey claimed to have seen lines xowooth to the inch, and Messrs. 

 Harrison and Sollit the lines of Ampliipleura pellucida " — now a 

 favourite test-object with Messrs. Powell and Lealand — " 120,000 

 and 130,000 to the inch, and expressed an opmion that lines as fine 

 as 175,000 to the inch might be seen." 



" Experiments induced Messrs. Sullivant and Wortly to believe 

 that Nobert's 27th band of lines 81,213 to the inch* gave the 

 limit of resolvability." The objectives used were " Tolles' ^^rth, 

 aperture 160°. Besides other objectives, iVth and iVth of eminent 

 opticians." 



Mr. Stodder then says : — " Dr. Woodward has resolved finer 

 lines than any other observer has yet seen, so far as report gives us 

 any information.t With Tolles' immersion ith, aperture 170°, B 

 eye-piece, power 550, Mr. Greenleaf and myself )3oth saw the 19th 

 band satisfactorily, thus being probably the first ever to see lines 

 112,000 to the inch. . . . Mr. Eulenstein states that Nachet claims 

 * A former test-plate. t Old series of bands. 



X 2 



