152 THE MICROSCOPE AND ITS REVELATIONS. 



brightest when the distance between the Objective and the object is in- 

 creased, whilst depressions must appear brightest when that distance i& 

 diminished. The Student should be warned against supposing that in 

 all cases, the most positive and striking appearance is the truest; for 

 this is often not the case. Mr. Slack's optical illusion, or silica-crack 

 slide, 1 illustrates -an error of this description. A drop of water holding 

 colloid silica in solution is allowed to evaporate on a glass slide, and, 

 when quite dry, is covered with thin glass to keep it clean. The silica 

 deposited in this way is curiously cracked; and the finest of these cracks 

 can be made to present a very positive and de/eptive appearance of being 

 raised bodies like glass threads. It is also easy to obtain diffraction-lines 

 at their edges, giving an appearance of duplicity to that which is really 

 single. 



153. A very important and very frequent source of error, which some- 

 times operates even on experienced Microscopists, lies in the refractive 

 influence exerted by certain peculiarities in the internal structure of ob- 

 jects 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 a ' pregnant instance ' in the 

 misinterpretation of the nature of the lacuncB and canaliculi of Bone, 

 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 surface will cause a transpa- 

 rent cylinder to show a bright axial band, 3 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 they are 

 easily mistaken for opaque solids. That such is the case with the so-called 

 ' bone corpuscles,' is shown by the effect of the infiltration of Canada 

 balsam through the osseous substance; for when this fills up the excava- 

 tions, being nearly of the same refractive power with the bone itself, it 

 obliterates them altogether. So, again, if a person who is unaccustomed 

 to the use of the Microscope should have his attention directed to a 

 preparation mounted in liquid or in balsam that might chance to contain 

 air-bubbles, he will be almost certain to be so much more strongly im- 

 pressed by the appearances of these than by that of the object, that his. 

 first remark will be upon the number of strange-looking black rings 

 which he sees, and his first inquiry will be in regard to their meaning. 



154. Although no experienced Microscopist could now be led astray 

 by such obvious fallacies as those alluded to, it is necessary to notice them 

 as warnings to those who have still to go through the same education. 

 The best method of learning to appreciate the class of appearances in 

 question, is the comparison of the aspect of globules of Oil in water, with 

 that of globules of Water in oil, or of bubbles of Air in water or Canada 

 balsam. This comparison may be very readily made by shaking up some 

 oil with water to which a little gum has been added, so as to form an 

 emulsion; or by simply placing a drop of oil of turpentine (colored by 

 magenta or^ carmine) and a drop of water together on a slip of glass, lay- 



1 " Monthly MicroscopicalJournal," Vol. v. (1872), p. 14. 



2 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. 



