ETHER FREEZING MICROTOMES 475 



The increased size of the screw gives a more steady movement 

 than was possessed by the older and smaller microtome, while the 

 greater circumference of the screw-head enables an operator to im- 

 part a finer movement to the screw. The relation between the pitch 

 of the screw and the circumference of its head is such that if the 

 edge be moved forward a quarter of an inch, an object will be raised 

 one-thousandth of an inch ; and if it be moved an eighth of an inch, 

 the object will be raised a two-thousandth of an inch. 



In the original instrument the plate was supported on two 

 pillars, in order that as little heat as possible might be conveyed to 

 the freezing- plate from the body of the instrument. In the new 

 instrument the size of the three supporting pillars and screws is so 

 much reduced that the conducting surface is not greater than in the 

 old microtome. The arrangement for cutting imbedded sections 

 consists of a tube w r hich fits the principal well of the microtome, and 

 wdthin which fits a hinged part similar to an ordinary vice. With 

 the instrument are provided the means of preparing paraffin blocks 

 for imbedding sections. 



When it is intended to use the microtome for imbedding, the 



FIG. 402. Holder for Cathcart's microtome. FIG. 403. Dropping-bottle. 



ether spray, spray-bellows, and ether-bottle should be removed, and 

 the freezing-tube, having been raised as far as possible by means of 

 the principal screw, should then be withdrawn from the well. The 

 imbedding tube, fig. 402, is now placed in the well, and, having been 

 pushed down until it rests upon the point of the large screw, it may 

 be lowered to a convenient height by working the large screw back- 

 wards. 



Mr. Cathcart recommends in freezing with this instrument that 

 a few drops of mucilage (1 part gum to 3 parts water) be placed on 

 the zinc plate, and that a piece of the tissue be cut, of about a quarter 

 of an inch in thickness, and pressed into the gum ; the ether-bottle, 

 filled with anhydrous methylated ether, is taken and the spray points 

 pushed into their socket. All spirit must of course have been pre- 

 viously removed by soaking for a night in water. The tissue should 

 afterwards be soaked in gum for a like time before being cut. The 

 operator must now work the spray- bellows briskly until the gum 

 begins to freeze ; after this, work more gently. Raise the tissue by 

 turning the milled head, and cut by sliding the knife along the glass 

 plates. 



