a a TO a es a ae 
W. S. Sullivant and T. G.Wormley on Nobert’s Test Plate. 15 
and very nearly on the whole of the 27th band; but on the 28th, ” 
and still more on the 29th, they so prevailed, that at no one focal 
adjustment 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 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 
Ist, 5th, 10th, 15th, 20th, bands vary somewhat from Nobert’s reg- 
crepancies 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. Weare 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 estab- 
lished; 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, 
ne 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 favored with fur- 
ther information on this point. 
With regard to the striation of Diatoms, an opinion generally 
prevails that the number of striz 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 
the earlier publications on the subject, the paper of Messrs. 
arrison and Sollitt before referred to, wherein (as in the more 
recent paper of Mr. Sollitt) measurements of sever iatoms 
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 (this Jour., March, — 
1859,) on this subject. ees : 
Number of striz in 001” eee 
‘Hand 8. Sm. Syn. 8. an 
Navicula rhomboides Oto tt cee 
igma fasciola 50 to 90 64 52to56 
Niteschis sigmoidea | 105 we 70 
