66 O..N. Rood on the investigation of Microscopic forms. 
in gentle motion. I now adduce one or two applications of this 
method. 
Ezamination of the nature of the markings on the Coscinodiscus 
ricer 9 aC. 
It is well known among microscopists that the controversy re- 
garding the nature of the marking on these shells, after being 
carried on for several years with spirit cannot even yet be con- 
sidered as settled, one party contending that the areole are de- 
pressions, while their antagonists see them as elevations, Com- 
pare Carpenter on the Microscope, page 280, American edition. 
Fine specimens of these shells mounted in water were exam- 
ined by a power of from 600 to 800 diameters; on moving the 
by the wind. The valves when mounted in balsam of tolu, 
which has a still higher index of refraction, gave like results. 
ese experiments, which are not difficult to repeat, prove that 
the areolze are well formed concave lenses. ss 
A similar mode of experimenting, which must be conducted 
on large valves and with some delicacy, shows that the border, 
or setting, so to speak, has the opposite curvature, viz: is con- 
vex ; whether it is convex as a cylinder or beset with several 
convex markings I have not had leisure to determine, though im 
some large specimens the latter seemed to be the : 
case. Indications also were observed in some large 3 ‘| 
ings or to some other cause; thus the small circles at the middle 
and ends of the Pinnularia viridis have been mistaken by somé 
eminent observers for openings. Prof. Bailey proved by the a 
tion of hydrofluoric acia that they are in reality thicker portions 
f 
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