Monthly Mlcroscoplcan 
Journal, Feb. 1, 1869. J 
Boyal Microscojpieal Society, 
91 
into thirty-seven lines, farther on he says that forty is the real 
numher in that band. 
It cannot be supposed that all the corresponding bands on 
different plates agree precisely in difficulty to resolve ; it follows 
that a comparison of results obtained by two objectives, each tried 
on a different plate, may be very deceptive ; though when tried on 
the same plate their relative separating power is at once decided. 
It was only after frequent trials that I could be assured of dis- 
tinguishing readily between the appearance of the true consecutive 
lines and those woolly or wavy-looking lines, which are shown either 
by defective illumination or by the want of power in the objective, 
but which are sometimes believed to be imperfectly ruled lines. 
With the ith and the -^^th. by Eoss, and the -^th. by Smith, 
in the possession of this Society, and with a ^th, a t\^\ ^ xV^h, 
and a o^V^^ Powell and Lealand, all dry objectives (not to 
mention others which gave similar or inferior results), on a new 
nineteen-band plate, all the bands beyond the twelfth seemed im- 
perfect — the lines were not separated. 
But with a -^th and a -y^th by Hartnack, of Paris, a -^ih by 
Merz, of Munich, and a V^th by Nobert, all immersion objectives, 
straight and well-defined lines were separated as far as the fifteenth 
band inclusive. In the last four bands true consecutive lines were 
seen ; but they are so extremely slightly ruled, that the eye fails to 
appreciate their increased fineness. I should expect that, if the 
lines in the twelfth band were as slightly ruled as those on the 
last four bands, there would be nearly the same difficulty in seeing 
them, notwithstanding their wider separation. If the lines were 
exposed to the incident light, without the interposition of any 
reflecting surface or refracting medium, they would, doubtless, be 
seen with much greater ease. 
These results were the best I obtained by the dry and immersion 
objectives with the same method of artificial illumination. Several 
trials with sun-light proved that the lines were thus rendered more 
visible ; but the immersion objectives maintained their superiority 
by all methods. 
I can find no evidence whatever for the statement that the 
resolving of Test-diatoms and Nobert's Test-lines is merely a question 
of angular aperture. Boss's -y^th. Smith's ^V^^j Powell and 
Lealand's -r^th., and -rwth. dry objectives were all of larger aperture 
than Merz's -p^th Immersion, yet neither gave as good results on 
Surirella gemma, Frustulia Saxonica, Grammatopliora Suhtillis- 
sima, P. Angulatum, P. Attenuatum, P. Macrum, &c., nor on 
Nobert's plate, as Merz's objective. Hartnack asks the question, 
"Are large apertures an advantage to the microscopist or to the 
optician ? " and he frankly says that " the disadvantages are for 
the latter only." 
