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O. N. Rood on the investigation of Microscopic forms. 67 
of the shell, and examination by the method here described shows 
that they are convex lenses, 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 oppo- 
site 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 de- 
termine it. 
Although Canada balsam has the same index of refraction as 
quartz, still the valves of the diatoms which are composed of 
Sulica 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 
dinary 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 
Coincidence of refractive indices, but had been anticipated 
eared beautifully, though perversely distinct. A casua 
Temark from Alex. S. son, ., concerning a certain cherai- 
stains of sand on the slide retained their distinctness per- 
tha <tetwards they were visible with tolerable distinctness in 
© same sample of sulphuric acid. 
