336 NOTES AND MEMORANDA. 



permeated, it may then be poured into a paper trough, 

 watch glass, or other vessel and allowed to cool. 



Cutting. — There are two forms of microtome suitable for 

 cutting sections in series. In one the tissue is raised directly 

 by a fine screw and the sections cut with an ordinary razor : 

 in the other form, now so largely used, the tissue in its 

 holder is moved up an inclined plane and the sections cut 

 with a large knife which works backwards and forwards in 

 a horizontal slot running parallel to the inclined plane. In 

 both cases the machine is fixed to the table or heavy 

 enough to remain steady, so that while the razor is worked 

 with one hand the other hand is at liberty to hold a little 

 paper spatula — a small piece of paper run on at the end of 

 a small scalpel — to prevent the sections curling. The 

 paraffin block is pared down to the smallest size possible, 

 and, as the razor is drawn along, the edge, which commences 

 to curl, is caught by the paper and prevented from so doing ; 

 the section is then transferred to the slide. 



Preparation of the slide. — The slide is smeared with a 

 strong solution of shellac in anhydrous creosote. Care must 

 be taken to have as little as possible on the slide. By this 

 method the sections are stuck to the slide, thereby saving 

 the most delicate objects from falling to pieces after the 

 paraffin is removed and enabling one to mount numerous 

 section on one slide. The importance and value of this 

 treatment cannot be over-estimated. It enables one to 

 mount with absolute certainty whole sections of the most 

 friable objects, such as an insect, without a single fragment 

 of the section becoming displaced. 



Mounting. — The slide bearing the sections is now placed 

 in a water-oven or on a tin box containing water at a tem- 

 perature two or three degrees above the melting point of 

 the paraffin used. The slide is left here for at least half an 

 hour. The object of this warming is twofold, to evaporate 

 the creosote and to melt the paraffin. 



The slide is now taken up, and while the paraffin is still 

 molten is flooded with turpentine dropped from a small 

 pipette. This dissolves melted paraffin instantaneously, and 

 precipitates the shellac fastening the sections to the slide. 

 The turpentine is allowed to flow off and replaced by new 

 until all the paraffin is removed. The slide is then allowed 

 to drain, the edges wiped, and the cover glass put on. The 

 Canada balsam, which should be very fluid, is placed on the 

 under surface of the cover glass ; this is turned over and 

 quickly lowered. The balsam dissolves the shellac, and if the 



