. The microscope and its revelations. nce of two distinct layers : an external,of a brownish-yellow colour; and an int<rn«l, which lias a pearlyor • nacreous aspect, and is commonly of a lighter hue. The structure of the outer layer may be conveniently studied inthe shell of riiiixt., in which it commonly projects beyond the inner,and there often forms lamina? sufficiently thin and transparent toexhibit its general characters without any artificial reduction. If asmall portion of such a lamina be examined with a low magnifyingpower by transmitted light, each of its surfaces will present ver
. The microscope and its revelations. nce of two distinct layers : an external,of a brownish-yellow colour; and an int<rn«l, which lias a pearlyor • nacreous aspect, and is commonly of a lighter hue. The structure of the outer layer may be conveniently studied inthe shell of riiiixt., in which it commonly projects beyond the inner,and there often forms lamina? sufficiently thin and transparent toexhibit its general characters without any artificial reduction. If asmall portion of such a lamina be examined with a low magnifyingpower by transmitted light, each of its surfaces will present verymuch the appearance of a honeycomb ; whilst its broken edge exhibitsan aspect which is evidently fibrous to the eye, but which, whenexamined under the microscope with reflected light, resembles thatf an assemblage of segments of basaltic columns (fig. 696). Thisouter layer is thus seen to be composed of a vast number of prisms,having a tolerably uniform size, and usually presenting an approach 920 MOLLUSCA AND BEACHIOPODA. FIG. 693.—Section of shell of Pinna, takentransversely to the direction of its prism. to the hexagonal shape. These are arranged perpendicularly (ornearly so) to the surface of the lamina of the shell; so that its thick-ness is formed by their length, and its two surfaces by their extremi-ties. A more satisfactory view of these prisms is obtained by grindingdown a lamina until it possesses a high degree of transparence, the prisms being then seen () to be themselves com-posed of a very homogeneoussubstance, but to lie sepa-rated by definite andstrongly marked lines ofdivision. When such alamina is submitted to theaction of dilute acid, so asto dissolve away the car-bonate of lime, a tolerablyfirm and consistent mem-brane is left, which exhibitsthe prismatic structure justas perfectly as did theoriginal shell (fig. 694), its hexagonal divisions bearing a strong resemblance to tlie walls ofthe cells of the pith or bark of a plant. By making a secti
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmicrosc, bookyear1901
