428 MANIPULATION ANL> PRESERVATION OF THE MICROSCOPE 



more or less indistinct, but is often wrongly represented. The in- 

 distinctness of outline will sometimes present the appearance of a 

 pellucid border, which, like the diffraction-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 biconcavity of the 

 blood-discs of human (and other mammalian) blood causes their 

 centres to appear dark when in the focus of the microscope, through 

 the divergence of the rays 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 discs. 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 011 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 deceptive 

 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. 



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 interim 1 

 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 a ' pregnant instance ' in the misinterpretation of the nature 

 of the lacunae 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. 

 When Canada balsam fills up the excavations, being nearly of the 

 same refractive power as 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 

 impressed 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. 



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 

 1 Monthly Microscopical Journal, vol. v. 1872, p. 14. 



