ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 433 



wards. These bars corresiDond to the cutting plate in the usual form 

 of microtome, and upon them, as will be seen at A in the drawing, a 

 flat brass frame carrying a knife is made to glide. The knife is kept 

 firmly in position on tbis framework by means of the binding screw C, 

 the end of which, terminating in a square clamp, presses against the 

 back of the blade. The face of this clamp is grooved in different 

 directions in such a manner that, according as tbe back of the blade is 

 received into one or another of these grooves it is pushed from or 

 drawn towards the level of the framework, thus affording a means by 

 which the edge of the knife may be set at varying angles to the tissue 

 to be cut. In front of the iron stand will be seen an angular ujjright 

 pillar carrying in front of it a short length of sprung brass tube B, 

 into which any of the apparatus presently to be described may be 

 firmly fixed by a clamping screw. By means of a micrometer-screw 

 E fixed at the base of the angular pillar, the sprung tube, and of 

 course whatever it may carry, can be acted upon so as to raise or 

 lower it at pleasure. The amount of movement thus effected is 

 registered by the milled head of the screw, for which purpose three 

 concentric circles have been drawn upon its face, each of which is 

 so graduated that, as the face rotates from mark to mark, the distance 

 traversed by the screw, and which of course determines the thickness 

 of the section, will in the case of the outer circle be 1000th, in that 

 of the middle 500th, and in the inner one 400th of an inch. The 

 index by which these measurements are recorded consists of a spring 

 catch so fitted that, as the milled head rotates, it drops into the 

 divisions of the circles, into either of which it can be shifted at 

 pleasure, or if desired can be thrown out of gear altogether. When 

 it is intended to use the microtome for freezing with ether, the 

 chamber provided for that purjiose, and which in the engraving is 

 shown in position, must be emploj-^ed. This chamber is like the one 

 already described when sjieaking of the Groves- Williams microtome, 

 and consists of a reservoir for containing the ether and an upright 

 cylinder leading from it, and terminating in a flat plate, upon which 

 the object to be frozen lies. To use the machine, remove the cup D, 

 fill the chamber with ether, then fix the cylinder in the clamp B, 

 when the bellows F being worked the ether will project through the 

 tubes in the interior of the chamber (which were described at p. 431), 

 upon the plate holding the tissue, with the effect of speedily freezing 

 it. When, under the action of the micrometer-screw, the object to 

 be cut has moved upwards between the cutting bars sufficiently high 

 for the purpose, sections are to be obtained by simply pushing the 

 frame carrying the knife obliquely across the bars and through the 

 tissue. For freezing purposes common methylated ether of a density 

 of • 720 answers perfectly well. In winter when ice is plentiful, and 

 where only a very small piece of tissue requires to be frozen, the 

 freezing may be effected without the employment of ether. For this 

 purpose it will be necessary to use Dr. Pritchard's solid freezer. 

 Fig. 85. As will be seen, it consists of a solid metal block, having 

 its upper surface, upon which the tissue to be frozen lies, roughened 

 so as to prevent the specimen from slipping during section. For 



