386 THE MICROSCOPE. 



dom happens that the bone-cells are visible in it ; but by a fragment 

 from about the middle of the laminse of the shaft the characteristic 

 bone-cells can at once be recognised. But in the , comparison of the 

 bone of a mammal with that of a bird, from the peculiarity in the 

 arrangement of the Haversian canals in the latter class, it is highly 

 important always to bear in mind that the specimens used for compari- 

 son should be cut in one and the same direction; for as it has been 

 stated that the bone-cells, on which we are to rely for our determina- 

 tion, are always longest in the direction of the shaft of the bone, it 

 would follow, that if one section were transverse and the other longitu- 

 dinal, there might be a vast difference in the measurement of the bone- 

 cells, in consequence of their long diameter being seen in the one case, 

 and their short diameter in the other : hence the caution of having all 

 the sections made in one direction. In all doubtful cases, the better 

 plan is to examine a number of fragments, both transverse and longi- 

 tudinal, taken from the same bone, and to form an opinion from the 

 shape of bone-cell which most commonly prevails. 



Prepa/ring Sections- of Bone. — Sections of bone or teeth may be 

 cut with a fine saw, such as is used for cutting metal ; then filed down 

 with a flat safe-edged fine file, and afterwards polished between two hones 

 of the Water of Ayr stone, or two pieces of very smooth boxwood; 

 finally polish off on a strop of buff-leather charged with putty powder. 



Should the specimen be small or brittle, after it has been filed and 

 rubbed down on the hone, and polished on one side, dry it, and cement 

 it — polished side downwards — with Canada balsam to a slip of glass ; 

 when the balsam is dry and hard, proceed to file and polish the other 

 side J the section must be examined from time to time under the 

 microscope, and when found to be thin enough, it can be easily re- 

 moved from the glass by steeping it for a short time in ether. After 

 the section has been dried, it is ready for permanent mounting : this 

 may be done by immersing it in a thin cell of fluid, or in Canada 

 balsam ; if in the latter, it should be imbedded in it, and heat em- 

 ployed to expel all air-bubbles, and fill up the lacunae and canals in the 

 bone. 



When we wish to examine the bone-cells of fossil bone, chippings 

 only are required ; these may be procured by striking the bone with 

 the sharp edge of a small hammer used by mineralogists : carefully 

 select the thinnest of the chips, and mount them at once without grind- 

 ing in Canada balsam. 



All sections of recent and greasy bones should be soaked in ether 

 for some time, and afterwards dried in the air, before they are fit for the 



