26 Eemarhs on the Structure [ jSil ^lyTff* 
of our object-glasses, made within the last fifteen years, the struc- 
ture of many of the Diatoms, such as P. angulatum, Formosum, 
Balticum, &c., seems to have been decided, and their nature, as 
nodules of silex, generally admitted as facts, from the palpable 
method of examining broken edges of the scale and detached frag- 
ments. 
Though the Podura may be well defined with objectives, which 
fail to afford even indications of markings on these diatomaceous 
tests, yet the structure of this scale is far from being satisfactorily 
determined. To bring out to the best advantage the opaque-looking 
spines, or "note of exclamation" markings (which is their un- 
doubted form), it is generally admitted that the direct light from 
an achromatic condenser is the best mode of illumination, as it is 
also most rehable and free from error, and is the one adopted by 
the constructor of object-glasses for the final examination of his 
workmanship, the test marks being this recognized form. Obhquity 
of illumination would blend and confuse these markings, and produce 
an appearance much similar to that of spherical aberration in the 
object-glass itself, and might be mistaken for such. 
Let a strongly-marked and suitable Podura be now examined 
with the highest powers, say ^ or using the deepest eye-piece, 
and even lengthening the body by the draw-tube (and many of our 
recent object-glasses will bear this admirably), the illumination 
being that of the achromatic condenser with adjustable apertures. 
Under these circumstances the scale will be so enormously magnified, 
that only a small portion of it will occupy the whole field of view, 
supposing the adjustment for spherical aberration or thickness of 
glass-cover also to be exactly corrected. Under this excessive 
amplitude, each individual marking still retains its characteristic 
form ; but though it is a body evidently having .some bulk, not the 
most careful focussing can determine that it stands above the surface 
of the scale. On the contrary, there is a slight and peculiar shading- 
off of an apparent intervening membrane next the markings, giving 
rise to the idea of a kind of rising between them, so that I have 
known several that have seen the Podura under these conditions 
declare that the markings are actual degressions, or dark pigment 
cells. 
Let us next work through this subject with the Binocular 
microscope. Though it must be admitted that the appHcation of 
this to difficult tests is not very satisfactory, yet in a coarse Podura 
the markings are sufficiently well defined and still retain their 
characteristic form, but it utterly fails in throwing the spines up 
in relief as from an underlying surface, and affords no additional 
knowledge of the structure ; but at the same time, if the entire scale is 
hollow or distorted, this is immediately detected by the aid of double 
vision. Further, let the test be examined by some approximate 
mode of opaque illumination : by this we are limited to much lower 
