EERORS OF INTERPRETATION. 183 



Microscopists. With lenses of high power, and especially with 

 those of large angular aperture, it very seldom happens that all 

 the parts of an object, however small and flat it may be, can be 

 in focus together ; and hence the focal adjustment being exactly 

 made for one part, everything that is not in exact focus is not 

 only more or less indistinct, but is often wrongly represented. 

 The indistinctness of outline will sometimes present the appear- 

 ance of. a pellucid border, which, like the ditfraction-band, may 

 be mistaken for actual substance. But the most common error 

 is that which is produced by the reversal of the lights and 

 shadows, resulting from the refractive powers of the object itself; 

 thus, the bi-concavity of the blood-disks of Human (and other 

 Mammalian) blood, occasions their centres to appear dark,- when 

 in the focus of the Microscope, through the dispersion of the 

 light which it occasions ; but when they are brought a little 

 within the focus, by a slight approximation of the object-glass, 

 the centres appear brighter than the peripheral parts of the disks 

 (Fig. 315). The same reversal presents itself in the case of the 

 markings of the Diatomacese ; for these, when the surface is ex- 

 actly in focus, are seen as light hexagonal spaces, separated by 

 dark partitions ; and yet, when the surface is slightly beyond the 

 focus, the hexagonal areas are dark, and the intervening parti- 

 tions light (Fig. 80). The best means of avoiding errors of in- 

 terpretation arising from this source, lies in the employment of 

 the lowest powers with which the particular structures can be 

 distinguished ; since, if the different parts of the surface and 

 margin of the object can be simultaneously brought so nearly 

 into focus that a distinct view may be gained of all of them at 

 once, no false appearances will be produced, and everything will 

 be seen in its real aspect. 



98. 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 

 form or constitution of objects, upon the rays of light trans- 

 mitted 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. A very characteristic illustration of the fallacy resulting 

 from external configuration, is furnished by the notion which 

 long prevailed among Microscopic observers, and which still 

 lingers in the public mind, of the tubular structure of the Human 

 hair. This notion has no other foundation, than the existence 

 of a bright band down the axis of the hair, which is due to the 

 convergence of the rays of light occasioned by the convexity of 

 its surface, and which is equally' shown by any other transparent 

 cylinder; and it is unmistakably disproved by the appearances 

 presented by thin transverse sections of Hair, which show that 

 it is not only filled up to its centre with a medullary substance, 

 but that its centre is sometimes ev#n darker than the surrounding 



