ERRORS OF INTERPRETATION. 



167 



Hexagonal areolationof Pleurosigma angulatum, 

 as seen in a Photograph magnified to 15,000 

 diameters. 



of the object-glass, the centres appear brighter than the peripheral 

 parts of the disks (Fig. 371). An opposite reversal presents itself 

 in the case of the markings of the Diatomacece ; for these, when 

 the surface is exactly in focus, are seen as light hexagonal 

 areolse separated by dark partitions (Fig. 90, a) ; and yet, when the 

 surface is slightly beyond the focus, the hexagonal areolae are dark, 

 and the intervening partitions light (Fig. 90, b). — The experienced 

 Microscopist, on the other hand, will find in the Optical effects 

 produced by variations of Focal adjustment the most certain indi- 

 cations in regard to 



the nature of such B FlG - 90 - A 



inequalities of sur- 

 face as are too minute 

 to be made appa- 

 rent by the use of 

 the Stereoscopic Bi- 

 nocular. For, as 

 Welcker has pointed 

 out, * superficial ele- 

 vations must neces- 

 sarily appear bright- 

 est when the dis- 

 tance between the 



Objective and the object is increased, whilst depressions must appear 

 brightest when that distance is diminished. And it is the applica- 

 tion of this test to the minute markings of Diatom-valves, which 

 most certainly indicates that they are due to hexagonal elevations. 

 128. A very important and very frequent source of error, which 

 sometimes operates even on experienced Microscopists, lies in the 

 refractive influence exerted by certain peculiarities in the internal 

 structure of objects upon the rays of light transmitted through them ; 

 this influence being of a nature to give rise to appearances in the 

 image, which suggest to the observer an idea of their cause that may 

 be altogether different from the reality. Of this fallacy we have 

 ' pregnant instance ' in the misinterpretation of the nature of the 

 lacuna and canalicidi of Bone (Fig. 356), which were long supposed 

 to be solid corpuscles with radiating filaments of peculiar opacity, 

 instead of being, as is now universally admitted, minute chambers 

 with diverging passages, excavated in the solid osseous substance. 

 For just as the convexity of its surfaces will cause a transparent 

 cylinder to show a bright axial band, f so will the concavity of the 

 internal surfaces of the cavities or tubes hollowed-out in the midst 

 of highly-refracting substances occasion a divergence of the rays 

 passing through them, and consequently render them so dark that 



* See "Quart. Journ. of Microsc. Science," Vol. vii. (lSo!^, p. 240, and 

 Vol. viii. (1860), p. 52. 



t This was the appearance which gave rise to the erroneous notion 

 that long prevailed amongst Microscopic observers, and still lingers in 

 the Public mind, of the tubular structure of the Human Hair. 



