212 Prof.O.N. Rood on the Investigation of Microscopic Forms 
badly defined, the focal adjustment is best made while the rod is 
in gentle motion. I now adduce one or two applications of this 
method. 
Examination of the nature of the Markings on the Coscinodiscus, 
Triceratium, Sc. 
It is well known among microscopists that the controversy 
regarding the nature of the marking on these shells, after bemg 
carried on for several years with spirit, cannot even yet be con- 
sidered as settled,—one party contending that the areolx are 
depressions, while their antagonists sce them as elevations. 
Compare Carpenter ‘On the Microscope,’ page 280, American 
edition. : 
Fine specimens of these shells mounted in water were ex- 
amined by a power of from 600 to 800 diameters; on moving 
the compound body within the focus, each hexagon was found 
to contain a small distinct image of the flame; the motion of 
the rod showed that the images were inverted, and consequently 
formed by concave lenses. As the index of the refraction of 
water is much less than that of silica, its effect is merely to 
diminish the action of the curved surfaces, but in no case to re- 
verse it. These shells were now mounted in Canada balsam and 
observed. As the index of refraction of the balsam is some- 
what greater than that of silica, it was to be expected that in 
the compound lenses of silica and balsam the latter would pre- 
dominate, and reverse the action, so as to present effects due to 
convex lenses. This was found to be the case, and in some of 
the valves the eye could readily follow, in a hundred areolz at a 
time, each flickering motion of the flame as it was stirred by the 
wind. The valves, when mounted in balsam of tolu, which has 
a still higher index of refraction, gave like results. These ex- 
periments, which are not difficult to repeat, prove that the 
areolz are well-formed concave lenses. 
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 in 
some large specimens the latter seemed to be the 
case. Indications also were observed in some large 
specimens that would lead to the deduction of a 
form optically equivalent to that seen in fig. 1; and = 
certain allied forms readily furnished the curve seen | a /| 
in fig. 2, the small depressions being pits. 
This mode of experimenting often furnishes us the means of 
determining whether certain appearances are really due to open- 
1. 
