MICROSCOPIC TEST OBJECTS. 
145 
DIATOM STRUCTURE. 
The typical structure of the Naviculce , as made out by this 
method of illumination, with even low powers as well as with a 
-i-th, is a series of silicious hemispheres inclined at varying 
angles to each other and to the median division of the valve, or 
rather (as a section of Pleurosigma quadratum reveals the 
fact) a series of perfect spheres of silica set equatorially in the 
silicious tissue of the valve. Mr. Wenham informs us that he 
has determined the markings of some diatoms to be spherical 
by the examination of fractured specimens. In one of the 
fragments of quadratum a line of spherules was detached like 
a row of beads, and single spherules were sufficiently detached to 
be examined in an isolated state under a ^th of his own work- 
manship. Mr. Wenham therefore stated that I had confirmed 
his views in a remarkable manner, 
POLVCYSTINJl* ETC, . 
It is impossible to avoid noticing the stereoscopic effect of 
the parallel reflected light of the prism.. On a Barbadoes slide 
of Polycystince , for instance, the objects are seen, under an inch 
power and on a dark ground, in very striking relief, their 
varied forms and characteristics being well exhibited ; and the 
same effect is visible when viewing the proboscis of the blow-fly 
on a light ground. The peculiar character of muscular fibre is 
also well displayed, new beauty is seen in the Podura scale, and 
infusoria and portions of insects may be examined with addi- 
tional interest. 
TRICERATIUM AND ISTIIMIA. 
It seems to be owing to this stereoscopic effect of parallel 
light and natural shadows, that the hemispheres of diatom- 
valves are seen beyond all doubt as elevations. We seem to be 
looking at an opaque body illuminated from above, and the 
appearance in the microscope is exactly similar to a model, 
made to scale, in plaster of Paris. On the other hand, when 
we have anything approaching to depressions , as in the mark- 
ings of Triceratium and Isthmia , these depressions are, as it 
were, palpably felt. The true hexagonal markings in Tricera 
tium are of special interest. At every angle of the hexagon 
there is a hemisphere of larger size, and smaller hemispheres, in 
contact with each other, form the sides, so that it is question- 
able whether the depression is deeper than the radius of the 
hemispheres themselves. A similar condition also presents 
itself when viewing the irregular though somewhat circular 
markings formed by an arrangement of small hemispheres 
on the surface of Isthmia . 
VOL. IX. - NO. XXXV. L 
