MEMORANDA. 61 
A proof amongst others of the efficacy of this ilumination 
has been to resolve by it very easily the thirty groups of 
Nobert’s test-lines, which Mr. Norman of Hull has been so 
kind as to lend to me, which experiment has been witnessed, 
amongst others, by the well-known director of the Astrono- 
mical Observatory of the Roman College, P. Angelo Secchi. 
“A considerable augmentation of power in the microscope 
for the use of such an illumination would afford an easy way 
to decide the question which micrographers are still debating 
about the true form of the minute structural details of 
diatoms; first among them is the Pleurosigma angulatum, 
which earliest microscopists have deservedly chosen to test 
the power of their instruments. It is true that the material 
improvement which the microscope has obtained these last 
fifteen years has superseded this old test object. Still we may 
be allowed to observe that the assumed easiness in resolving 
the details of pleurosigma must be understood for the oblique, 
not for the direct and central illumination, especially when 
the preparation is made in Canada balsam. 
** W hatever the direction of the light may be, and notwith- 
standing the great improvements which the microscopical 
objectives have received, the mode of explaining those very 
minute forms which adorn the surfaces of diatoms is still at 
variance. Schiff, Schultze, Schact, and Hartnach, amid the 
German—Wallich, Wenham, and Carpenter amongst the 
English, do not agree with themselves on this subject. They 
began noticing on pleurosigma some very minute striz which 
present themselves in a particular direction under the in- 
fluence of an oblique illumination; then, changing the course 
of the light, they observed another system of striz. Then the 
opinion of some writers who, having noticed successively three 
different systems of strie, two oblique and one direct, con- 
cluded they ought to be disposed in different planes. But 
more perfect objectives by connection and immersion, showing 
the three systems of striz simultaneously, caused the first judg- 
ment to be rejected, and acknowledged that they were placed 
on the same level. The greatest difficulty they met with was 
to determine the shape of the areola limited by the different 
directions of the striez. Some believed they were square, 
assuming that the transversal system of striae was nothing 
but an illusion caused by aberration of spheriority. Schact 
considers them as hexagons, each side of them being the basis 
of a small equilateral triangle. Quekett, following Wenham, 
who succeeded in obtaining an image of P. angulatum, 
increased to 15,000, and another of P. formosum to 35,000 
diameters, recognised and described the structure of these 
