210 Messrs. Sullivant and Wormley on Nobert's Test Plate. 



third or a fifth part) of the width of these bands be resolved 

 into the true hnes. 



The true lines of the 30th band we were unable to see — at 

 least with any degree of certainty; still, from indications, we 

 have no doubt they are ruled as stated by Nobert. 



It will be observed that our measurements of the lines on the 

 1st, 5th, 10th, 15th, and 20th bands vary somewhat from Nobert's 

 registration on the plate as given in the first table above. Such 

 discrepancies are to be expected, and by microscopists familiar 

 with operations of this kind are looked upon as unavoidable ; 

 but that on the 25th band is rather large to be accounted for in 

 this way. We are unable to explain it, and can only say that 

 our repeated measurements of it were very carefully made. 



These experiments, together with those of others before no- 

 ticed, induce us to believe that the limit of the resolvability of 

 lines, in the present state of the objective, is well nigh esta- 

 blished; but that this limit may be carried somewhat higher we 

 are not prepared to doubt, since the handsome advance lately 

 achieved by Mr. Tolles in his ^^^ (combining wide aperture, fine 

 definition, and high amplification) shows that the objective had 

 not, as we were inclined to think, reached the stationary point. 



The theoretical view of this question — that is, what may be 

 the closest approximation of lines consistent with their separa- 

 tion under the microscope — we leave to those competent to the 

 task, by whom, it is to be hoped, we may be favoured with further 

 information on this point. 



With regard to the striation of Diatoms, an opinion generally 

 prevails that the number of striae on a given portion of a frus- 

 tule varies, among individuals of the same species, within wide 

 extremes. This opinion is probably traceable in part to one of 

 the earlier publications on the subject — the paper of Messrs. 

 Harrison and Sollitt before referred to, wherein (as in the more 

 recent paper of Mr. Sollitt) measurements of several Diatoms 

 are given, showing great variableness in their striation. To 

 these gentlemen much credit is due for their discovery of high 

 markings, before unsuspected, on certain diatomaceous frustules; 

 their measurements, however, and the alleged variableness of 

 these markings we have not been able to verify, as will be seen by 

 the following extract from our paper * published on this subject : 



Number of striae in 'OOl". 



Navicula rhomboides. . . 

 Pleurosigma fasciola . . . 

 Pleurosigma strigosum . , 

 Nitzschia sigmoidea . . . 



* Silliman's American Journal, March 1859. 



