On the Investigation of Microscope Forms. 223 



this method that the image of the flame is not sufficiently perfect 

 to decide whether it is erect or inverted ; the motion of the rod 

 then furnishes us with a certain means of deciding this point ; if 

 its motion is natural the image is erect and the curvature convex, 

 &c. After some practice it becomes easy to obtain the best focal 

 adjustment for distinct vision of the rod, and in extreme cases, 

 where the image is very 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 Marhings on the Coseinodiseus 

 triceratium, &c. 



It is well known among microscopists that the controversy 

 regarding the nature of the marking on these shells, after being 

 carried on for several years with spirit, cannot even yet be consi- 

 dered as settled, one party contending that the areolae are depres- 

 sions, while their antagonists see them as elevations. Compare 

 ' Carpenter on the Microscope,' page 280, American edition. 



Fine specimens of these shells mounted in water were examined 

 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 sur- 

 faces, but in no case to reverse it. These shells were now mounted 

 in Canada balsam and observed. As the index of refraction of the 

 balsam is somewhat greater than that of silica, it was to be expected 

 that in the compound lenses of silica and balsam the latter would 

 predominate 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 areolae 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 experiments, which 

 are not difficult to repeat, prove that the areolae 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 convex; 

 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 



