32 On the Use of the Noberfs Plate. 



of lines. If this has been anywhere stated, I should he glad to 

 learn it, hut my counts were all made in perfect ignorance of any 

 enumeration hut my own. Nohert, to whom I sent the photographs 

 with my count, acknowledged the resolution in handsome terms. 

 If there are any persons who still remain unsatisfied, I much hope 

 that a better acquaintance with the subject will lead them to modify 

 their opinion. 



I pass by here the question of intermediate spurious lines 

 between the real ones, since this concerns chiefly the lower bands of 

 the plate. 



It has, however, been suggested that if the criterion I have pro- 

 posed be accepted, it ought to be applied also to the diatoms, and 

 that it would invalidate all claims as to the resolution of these which 

 are unaccompanied by a count and actual measurement. As to this, 

 I would say that the optical conditions in the case of the diatoms 

 are so different from what we have to deal with on the plate, that I 

 cannot see that the one conclusion follows from the other. 



The lines of the plate are minute grooves on the under-surface of 

 the thin glass cover, and the j)oint is to distinguish them from the 

 spurious images to which they give rise. The striae of the diatoms 

 are the optical expression of sculj)turing8 on frustules of silica. The 

 appearance of lines is now generally conceded to be an illusion. 

 What seem to be such are generally the optical expression of minute 

 elevations, most probably hemispherical in shape, though the ques- 

 tion of their form cannot be regarded as settled. These elevations 

 are arranged in rows, to which the apparent striae correspond. 

 False lines of greater or less number are occasionally produced, and 

 have in some instances been described as real ones, but this does not 

 occur with facility on the frustules of most species, and on many 

 does not occur at all. The question of the resolution of the diatoms, 

 however, is too complex for further discussion in this place. 



In closing this paper, I trust I may' be pardoned a few remarks 

 which appear to me to be warranted by the tone of Mr. Stodder's 

 reclamations. He is quite right in his allegation that I have done 

 " something more " than ignore his claims. I controvert them. I 

 do so, first, because I have carefuUy tried a number of objectives 

 made by Tolles, and have been unable to see with them any but 

 spurious lines in the nineteenth baud. Among those which I have 

 tried is the much-talked-of xoth and the new yVth belonging to Dr. 

 Josiah Curtis. Secondly, because Mr. Stodder has never yet oflered 

 any sufficient evidence that the lines he saw were not spurious also. 

 He rests on a simple supposition, and in his recent paper supports 

 this supposition by the mere opinion of several gentlemen to whom 

 he has shown lines in the nineteenth band, but who, like himself, 

 have taken no precautions to determine whether the lines seen were 

 spurious or real. 



