184 MANAGEMENT OF THE MICBOBCOPE. 



part (Fig. 311). Of tlie fallacy whicli may sometimes arise from 

 diversities in the refractive power of the internal parts of an ob- 

 ject, we have an equally "pregnant instance" in the misinter- 

 pretation of the nature of the lacunoe and canaliculi of Eone 

 (Fig. 300), 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 pas- 

 sages, 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, 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 

 effects of the infiltration of Canada balsam through the osseous 

 substance ; for when this fills up the excavations, — being nearly 

 of the same refractive power with the bone itself, and being also 

 quite transparent, and (in thin laminse) quite colorless,— it ob- 

 literates them altogether. So, again, if a person who is unac- 

 customed to the use of the microscope should chance to have his 

 attention directed to a preparation mounted in liquid or in bal- 

 sam, that might chance to contain air-bubbles, he will be almost 

 certain to be so much more strongly impressed by the appear- 

 ance 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. 

 99. No experienced Microscopist could now be led astray by 

 such obvious fallacies as those alluded to ; but it is necessary to. 

 dwell upon 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 

 and a drop of water together on a slip of glass, laying a thin glass 

 cover upon them, and then moving the cover several times back- 

 wards and forwards upon the slide.' JSTow when such a mixture 

 is examined with a suflBciently high magnifying power, all the 

 globules present nearly the same appearance, namely, dark mar- 

 gins with bright centres ; but when the test of alteration of the 

 focus is applied to them, the difiTerence is at once revealed; for 

 whilst the globules of Oil surrounded by water become darker as 

 the object-glass is depressed, and lighter as it is raised, those of 



' If this latter mode be adopted, it is preferable, as suggested by the authors of the 

 " JMicrographio Dictionary" (Introduction, p. xxxii), to color the oil of turpentine with 

 alUanet, or some similar substance, for itsifnore ready distinction. 



