1G8 ERRORS OF INTERPRETATION. 



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 refrac- 

 tive power with the bone itself, it obliterates them altogether. So, 

 again, if a person who is unaccustomed to the use of the Microscope 

 should chance to have his attention directed to a preparation mounted 

 in liquid or in balsam that might chance to' v 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. 



129. 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 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 backwards and forwards 

 upon the slide.* Now when such a mixture is examined with a 

 sufficiently high magnifying power, all the globules present nearly 

 the same appearance, namely, dark margins with bright centres ; 

 but when the test of alteration of the focus is applied to them, the 

 difference is at once revealed ; for whilst the globules of Oil sur- 

 rounded by water become darker as the object-glass is depressed 

 and lighter as it is raised, those of Water surrounded by oil become 

 more luminous as the object-glass is depressed and darker as it is 

 raised. The reason of this lies in the fact that the high refracting 

 power of the oil causes each of its globules to act like a double 

 convex lens of very short focus ; and as this will bring the rays which 

 pass through it into convergence abore the globule (i.e., between 

 the globule and the Objective), its brightest image is given when 

 the object-glass is removed somewhat further from it than the 

 exact focal distance of the object. On the other hand, the globule 

 of water in pil, or the minute bubble of air in water or balsam, 

 acts, in virtue of its inferior refractive power, like a double-con- 

 cave lens ; and as the rays of this diverge from a virtual focus 

 below the globule (i. e. , between the globule and the Mirror), the 



* If this latter mode be adopted, it is preferable, as suggested by the 

 Authors of the " Micrographic Dictionary" (Introduction, p. xxxii.), to 

 colour the oil of turpentine with alkanet, or some similar substance, for 

 its more ready distinction. 



