There are no set of objects which have given rise to more discussion as to their precise character than the scales of the podura (_Lepidocyrtus cervicollis_), to the intimate structure of which Mr. Smith turned his attention, and succeeded, I am inclined to think, in his attempt to settle the structure of these very minute scales, and which heretofore have been described as “notes of exclamation.” By the aid of the same power as that employed in the examination of the _pleurosigma formosum_, the old conventional markings have disappeared, and well-defined “_featherlets_” have taken their place. By careful focussing up and down, a series of whitish pin-like bodies is to be seen, with an intervening secondary structure. A micro-photograph of a portion of a scale taken by Mr. Smith shows that these pin-like bodies are inserted in a fold of the basement membrane, which, in his opinion, furnish unmistakable evidence of the fact that these projecting bodies are real, and must no longer be looked upon as mere _ghosts_. Quite recently, a micro-photograph of a portion of a podura scale was placed in my hands, taken by Mr. J. W. Gifford with a Swift’s 1/12-inch apochromatic objective, of numerical aperture 1·40, and a deep eye-piece, having a combined magnifying power of 3,827 diameters. Fig. 206 shows a portion of the photograph which, it will be admitted, supports Mr. Smith’s view of the structure of the podura scale.
[Illustration: Fig. 206.--Podura Scale, taken with a 1/12 Swift’s Immersion × 3,827.]
Many other errors of interpretation are not unknown to the experienced operator with the microscope, arising, for the most part, from an influence exerted by peculiarities in the internal structure of certain objects; for example, that offered by the human hair, and which, when viewed by transmitted light, presents the appearance of a flattened-out band, with a darkish centre, due to the refractive influence of the rays of light transmitted through the hair. That it is a solid or tubular structure is proved by making a transverse section of the hair-shaft, when it is seen filled up by medullary matter, the centre being somewhat darker than the outer part. It is, in fact, a spiral outgrowth of the epithelial scales, overlapping each other, imparting a striated appearance to the surface. A cylindrical thread of glass in balsam appears as a flattened, band-like streak, of little brilliancy. Another instance of fallacy arising from diversity in the refractive power of the internal parts of an object is furnished by the mistakes formerly made with regard to the true character of the _lacunæ_ and _canaliculi_ of bone structure. These were long supposed to be solid corpuscles, with radiating opaque filaments proceeding from a dense centre; on the contrary, they are minute chambers, with diverging passages--excavations in the solid osseous structure. That such is the case is shown by the effects of Canada balsam, which infiltrates the osseous substance.