128 APPENDIX. 



Specimens should be taken from various parts of the wings 

 of the same insect, as the form of the scales vary according 

 to the position they occupy in the wing. 



SECTIONS or BONE (plate 8, figure 232). All hard and 

 brittle substances from which thin slices cannot be made by 

 a sharp knife, must be reduced to a transparent thinness 

 by the process of grinding down. Having selected the bone 

 from which the section is about to be made, a thin slice 

 should be cut from it with a fine saw. At first the section 

 maybe held by the fingers while grinding down one of its 

 surfaces on a coarse stone ; but when it approaches the thin- 

 ness of a shilling, it must be cemented by some old and tough 

 Canada balsam to a slip of glass. Upon the perfect adhesion 

 of the section to the slide depends in a great measure the 

 success of the operation. Having reduced the thickness of 

 the section by a coarse stone or a tile, transfer it to a hone ; 

 a few turns will obliterate scratches, and produce an even, 

 smooth surface, which may be further polished by rubbing it 

 on a buff-leather strop charged with putty-powder and water. 

 When dry, attach the polished surface to the glass slip : this 

 gives a firm hold of the section, which would otherwise 

 become too thin to be held by the fingers. In rubbing down 

 the unfinished surface, take care that an equal thickness pre- 

 vails throughout the section. As it approaches completion, 

 recourse must be frequently had to the Microscope, in order 

 to determine how much further it is necessary to proceed, 

 a few turns either way at this stage being sufficient to make 

 or mar the specimen. When it has become so transparent 

 that objects may be readily seen through it, remove it from 

 the hone and polish it on the strop. To detach it from the 

 slide when finished, place it in turpentine or ether, both 

 being excellent solvents of balsam. Mount in the dry 

 method, by simply cementing a thin glass cover over it. In 

 recent bone, this method of mounting, though the most 

 difficult, is decidedly the best for displaying its structure. 

 Fossil bone, however, where the interstices are filled with 

 earthy matter, shows best in balsam. 



SPINES OP THE ECHINUS (plate 5, figures 151, 152); SEC- 

 TIONS OP SHELL (plate 6, figures 165 to 169). These are 

 cut and reduced in the same manner as sections of bone ; 

 but they require greater care in grinding, in consequence of 

 being more brittle. The polishing, however, may be dis- 

 pensed with, and the section mounted in balsam. 



STONES OP VARIOUS KINDS OP FRUITS (plate 8, figure 243) 

 will well repay the labour bestowed in producing good sec- 

 tions. The saw, the file, and the hone are the principal 



