190 A Nezv Form of Freezing [Sess. 



used both for specimens frozen in gum and those embedded in 

 paraffin or celloidin. 



The increased size of the screw gives a more steady move- 

 ment than was possessed by the older and smaller microtome, 

 while the greater circumference of the screw-head enables an 

 operator to impart a finer movement to the screw. The re- 

 lation 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. 



It is found that, when the instrument is clamped on both 

 sides, less pressure need be appKed at either side; and the 

 tendency which the instrument had to turn upon the point of 

 clamping, as on pivot, is quite done away with. 



In the original instrument, the plate was supported on two 

 pillars, in order that as little heat as possible might be con- 

 veyed 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 

 ^ai than in the old microtome. The ar- 

 '""^^^^B rangement for cutting embedded sec- 

 tions consists of a tube which fits the 

 principal well of the microtome, and 

 within which fits a hinged part similar 

 to an ordinary vice. With the in- 

 strument are provided the means of 

 preparing paraffin blocks for embedding sections. 



When it is intended to use the microtome for embedding, 

 the 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 with- 

 drawn from the well. The embedding tube 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 backwards. 



The following are ]\Ir Cathcart's directions for freezing : — 



1. Place a few drops of mucilage (1 part gum to 3 parts water) on the 

 zinc plate. 



