500 PREPARATION. MOUNTING, AND COLLECTION OF OBJECTS 



edge enters it at one angle and leaves it at another angle (in fig. 410, 

 the knife enters at a and leaves at c). The prism should be so cut 

 as to leave the imbedded object near to the side which is furthest 

 from the angle a which is first touched by the knife. Then if the 

 section should roll, at all events the section of the object will come 

 to lie in the most open spire of the coil, arid can thus lie more easily 

 unrolled. 



The rollhty of sections above referred to is an annoying 

 phenomenon of very frequent occurrence. Its most usual cause is 

 over-hardness of the paraffin, but it is favoured by excessive 

 obliquity of the knife, and other circumstances. With large sections 



it is not difficult to catch them by the edge 

 as they begin to mil. and hold them down 

 with a camel's-hair brush. Or a section- 

 stretcher may be used. 1 



If the paraffin be too soft, the sections 

 will not roll, but will become creased. 



Either of these defects may be dimi- 

 nished, sometimes even totally cured, by 

 simple means. Firstly, due attention must 

 be paid to the position of the knife ; not 

 only to its obliquity, but also to its tilt, as 

 explained above 



Secondly, if the paraffin should be too 

 hard, it may be softened by setting up a 

 lamp near it, or even by closing the win- 

 dow, if this should happen to be open, or 

 by carrying the microtome to a warmer 

 place, or by any device that will have the 

 effect of exposing the paraffin block to an 

 increase of temperature. An incredibly 

 slight increase will sometimes suffice. 

 Thirdly, if it should be too soft, an opposite 



treatment must be tried. The microtome is removed to a cooler 

 place, or the window is opened, or the like. 



If none of these maniruvres suffice to obtain sufficiently good 

 sections, the object must lie re-imbedded in a harder or softer 

 paraffin. But it will generally be possible to save the sections by 

 flattening them out by the water method, to be presently described. 

 The paraffin employed for imbedding tnvxl In' of n hardness 

 determined by the temperature <>/' iln- workroom: hard paraffin for a 

 I'-ni-ni room, soft paraffin for a coldroom. For the Thoma microtome, 

 \ paraffin melt in- .-i t I ."i ( '. (or 11:1 F.) gives good results so long as 



'Section stretchers are instruments coiisi-,thi";e^-.euti:ill\ of a little metallic roller 

 suspended over the -.lijcct tn ! i-nt in -urli a \\ay MS to rest on its free surface with 



;i pressure lli.i! can lie delieately ivj'nlaled SO as 1" l>e siil'lieienl !o keep (lie se, 



flatwithoul in an\ \va\ hindering ilie knife from gliding beneath it.' They are made in 



i MI the most convenient being that of 'Mayer, Andres and Giesbiecht, of which 



cription :nnl li-jin-e m;iyl>e found i&tiie Journal oftheRoy. Microscopical Soc. 



ISN::. p. :iii;. \,, u that the water flattening process (see below, Flattening) has been 



ion stretchers are no1 so necessarj as they were formerly, and for most 



in. iy lie di*.peii-ed with. 



FIG. 410. 



