SULLIVANT AND WORMLEY, ON NOBERt's TEST-PLATE. 115 



the whole of the 37th band ; but on tlie 28th^ and still more 

 on the 29fch^ they so prevailed, that at no one focal adjust- 

 ment could more than a portion (a third or a fifth part) of 

 the width of these bands be resolved into the true lines. 



The true lines of the 30th band we Avere 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, 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 miero- 

 scopists 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 vray. We are unable to explain it, 

 and can only &a.y that our repeated measiirements of it were 

 very carefully made. 



These experiments, together with those of others before 

 noticed, induce us to believe that the limit of the resolvabi- 

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

 established ; 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 J^th — combin- 

 ing 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 

 separation under the microscope, we leave to those com- 

 petent 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 frustule 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, 

 Avherein (as in the more recent paper of Mr. Sollitt) measure- 

 ments of several diatoms are given shoAving great variable- 

 ness in their striation. To these gentlemen much credit is 

 due for their discovery of high markings, before unsuspected, 

 on certain diatomaceous frustules ; their measurements, how- 

 ever, and the alleged variableness of these markings, we have 

 not been able to verify, as Avill be seen by the folloAving extract 

 from our paper published (this Journal, March, 1859) on the 

 subject ; 



