YouSiS, jviirCS'T ^^^^ Microscopical Society. 11 
spheres are not obscured by the parallel lines of their own shadows. 
When they reach their proper place they seem to start into exist- 
ence, and the degree of elevation is conferred per saltum. By this 
perfect command over its movements, the "diatom-prism" (thus 
named from its first application) will meet every requirement for 
oblique, direct, and dark-ground illumination, while its simplicity and 
independence of harness, in the shape of diaphragms or stops, is a 
chief characteristic. The light being nearly parallel, the prism may 
be moved, by the rackwork adjustment of the sub-stage, to a consi- 
derable distance below the object without materially weakening the 
illumination — and the slight diminution of light thus obtained is 
advantageous when using low powers. 
It is impossible to avoid noticing the remarkable stereoscopic 
effect of this parallel reflected light. On a Barbadoes slide, for 
instance, the objects are seen under an inch power and on a dark 
ground in very striking relief ; and the same effect is remarkably 
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 additional interest. 
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 markings of Triceratium and Istlimia, tliese 
depressions are, as it were, palpably felt. The hexagonal markings 
in Triceratium are of special interest. At every angle of the 
hexagon there is a hemisphere of larger size, and smaller hemi- 
spheres, in contact with each other, form the sides, so that it is 
questionable whether the depression is deeper than the radius of 
the hemispheres themselves. A similar inquiry also presents itself 
when viewing the irregular though somewhat circular markings 
formed by an arrangement of small hemispheres on the surface of 
Istlimia. 
I felt unwilling that the present session should close without 
giving some account of my observations to those who have more 
leisure than myself for pursuing these interesting researches. That 
the hemispheres which Mr. Wenham speaks of generally, and Mr, 
Hogg figures in the case of P. formosum, are such as I have 
described is, I hope, satisfactorily proved. There they are. I can 
number them, I can weigh them, I can measure them — and number, 
measure, and weight may be justly represented as the three rect- 
angular co-ordinates of all accurate knowledge of matter. 
