124 THE AMERICAN MONTHLY (May 
coarser, causes the finer detail to disappear altogether, 
and materially reduces the separating power of the ob- 
jective. With reference to this matter the following ex- 
periment may prove interesting :—Arrange a Cherryfield 
Navicula rhomboides, mounted in a mixture of monobro- 
mide of naphthaline and balsam, under a good semi-apo- 
chromatic +” of N.A. °77, and 27 ocular, so that the valve 
shall lie longitudinally along and on the sharply focuss- 
ed edge of the lamp flame. With slightly under ? cone 
the longitudinal strie will appear conspicuous through- 
out the entire length of the valve, while the closer trans- 
verse stria, although they may be seen to a certain extent, 
are far less satisfactorily defined, no thoroughly clear 
separation being apparent. Now replace the smaller by 
a 5-6ths cone. The coarse strongly-defined longitudinal 
stria disappear, and at the first glance all structure may 
seem to have disappeared with them, but a little careful 
scrutiny will reveal the presence of a faint dotted resolu- 
tion, the transverse divisions of which are as fully and 
cleanly shown as the longitudinal. 
I am aware that the results dealt with in this paper can- 
not meet with general acceptance until they receive con- 
firmation at abler hands than mine, nor indeed would it 
be desirable that they should be so accepted, involving 
as they do important theoretical considerations, until in- 
dependent practical experience shall have placed their 
truth beyond doubt. 
The subjoined notes on some of the forms lately exam- 
ined with the 4mm. may be ofinterest. A very large cen- 
tral solid cone has been invariably employed in conjunc- 
tion with either Gifford’s or the beautiful new acetate of 
copper screen. 
Nitzschia curoula Sm. This diatom is mounted next to 
Amphipleura pellucida on Moller’s dry “Probe-platte.” 
Transverse striw close and delicate, but pagh aaandaitio: re-_ 
solved. 
