5OO PREPAEATION, 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, and can thus be more easily 

 unrolled. 



The rolling 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 roll, 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 manoeuvres suffice to obtain sufficiently good 

 sections, the object must be 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 must be of a hardness 

 determined by the temperature of the workroom : hard paraffin for a 

 warm room, soft paraffin for a cold room. For the Thoma microtome, 

 a paraffin melting at 45 C. (or 113 F.) gives good results so long as 



1 ' Section- stretchers are instruments consisting essentially of a little metallic roller 

 suspended over the object to be cut in such a way as to rest on its free surface with 

 a pressure that can be delicately regulated so as to be sufficient to keep the section 

 flat without in any way hindering the knife from gliding beneath it.' They are made in 

 various forms, the most convenient being that of 'Mayer, Andres and Giesbi echt, of which 

 a description and figure maybe found \\\i\\e Journal of the Hoy. Microscopical Soc. 

 1883, p. 916. Now that the water flattening process (see below, Flattening) has been 

 perfected, section- stretchers are not so necessary as they were formerly, and for most 

 work may be dispensed with. 



FIG. 410. 



