by means of their Images of External Objects. 213 
ings or to some other cause. Thus the small circles at the middle 
and ends of the Pinnularia viridis have been mistaken by some 
eminent observers for openings. Prof. Bailey proved, by the ac- 
tion of hydrofluoric acid, that they are in reality thicker portions 
of the shell; and examination by the method here described 
shows that they are convex lenscs, giving often very well-defined 
- images of the flame. The dots characterizing the Conifers fur- 
nish images of the flame indicating two or more curvatures ; the 
ribs of the Pinnularia and the spaces between them have opposite 
curvatures, &c. But the examples already given may be sufficient 
to show the usefulness of the proposed method. 
Index of Refraction of the Silica composing the Valves of the 
Diatoms, 
This point is closely connected with the foregoing; and it 
may not be amiss to detail a few experiments that were made to 
determine it. 
Although Canada balsam has the same index of refraction as 
quartz, still the valves of the Diatoms which are composed of 
silica are seen almost as distinctly in balsam as when mounted 
in water. 
To ascertain the relation between the index of refraction of 
quartz and Canada balsam independently of optical tables or 
laborious experiment, I combined a convex quartz lens, of one- 
inch focus, cut at right angles to the optic axis, with unheated 
fluid balsam placed on a glass slide; the two opposite refrac- 
tions balanced each other so perfectly, that the combination 
acted like a plate of glass with plane parallel sides, and, with 
ordinary means, I was at a loss to discover any tendency to con- 
vexity or concavity. Balsam which had been heated was now 
combined with the quartz lens in the same manner: the balsam 
proved to have gained in refractive power, so that the combina- 
tion now acted distinctly as a concave lens of weak curvature. 
Diatoms were then mounted in this unheated fluid balsam, 
in which properly they should have been invisible, owing to the 
coincidence of refractive indices; but, as had been anticipated, 
they appeared beautifully, though perversely, distinct. A casual 
remark from Alex. 8. Johnson, Esq., concerning a certain che- 
mical difference he had often noticed between ordinary silica and 
that composing the Diatom-valve, again turned my attention to 
this point. Experiments were made upon a sample of the 
Rappahannock infusorial earth, which had been given to me by 
Prof. Wm. B. Rogers, in its natural state. By immersing the 
valves in various liquids, I finally ascertained that in strong 
sulphuric acid they became either iavisible or very nearly so, 
while the grains of sand on the slide retained their distinctness 
