243 
the twelfth, as I have seen them in the microscope, there is 
great difficulty to decide on the first and last lines. Dr. 
Woodward seems not to have been sure of the accuracy of 
the count he made on his photograph ; for although in one 
part of his paper in the current (October) number of the 
Journal of this Society, he says the photograph shows the 
twelfth band as resolved into thirty-seven lines, farther on he 
says that forty is the real number 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 objec- 
tives, 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 
distinguishing 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 -^th and the -pb-th by Ross, and the -g^th by Smith, 
in the possession of this Society, and with a ^th, a -f^th, 
a T^th, and a ^thby 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 imperfect — the lines were not separated. 
But with a Toth and a -p^th by Hartnack, of Paris, a -p^tli 
by Merz, of Munich, and a Toth of 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 x-uled, 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 im- 
mersion objectives with the same method of artificial illumi- 
nation. Several trials with sunlight proved that the lines 
were thus rendered more visible ; but the immersion objec- 
tives 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 
