140 



THE ART OF MAKING MICROSCOPE SLIDES 



S. S. mouse 



ing point. The object is then carefully 

 placed in the box. At this point one may 

 detach the tail from the rod of glass or 

 plastic to which it has been attached. 

 Then wait until the wax in which the 

 specimen is lying commences to solidify 

 and carefully fill the box to the very brim 

 with molten wax. The whole must now be 

 cooled as rapidly as possible. When the 

 block has completely hardened it is slid 

 out of its metal 1)0X and placed in a large 

 jar of water at room temperature, where 

 it may remain overnight or until one is 

 prepared to deal with it. 



In the examples previously given use 

 has been made of the ordinaiy rotary mi- 

 crotome, but such an instrument is useless 

 for the very large sections wliich we are 

 about to cut. No microtome will serve 

 save one of the slider type shown in Fig. 

 55. As these sliding microtomes have no 

 possible justifiable use in the cutting of 

 paraffin sections, save for very large ob- 

 jects, it is curious that so many of them 

 (including the one shown in the illustra- 

 tion) have relatively small object holders 

 provided. The jaws which are nonnally 

 used to hold the metal object holder, how- 

 ever, can be adapted to hold a large piece 

 of wood. The block under discussion had 

 better be attached by melting the wax to 

 a piece of hard wood, previously steeped 

 in paraffin, of a size very little smaller 

 than the block itself. After the block has 

 been attached — it is very dangerous to 

 trj' it before — it must be trimmed to the 

 shape which will be used for actual cut- 

 ting, which differs in every particular from 

 the shape which must be used when cut- 

 ting serial sections of small objects. For 

 ribbon-cutting the block is always rec- 

 tangular and the two sides must be ex- 

 actly parallel. In the case of a very large 

 l)lock from which single sections are to be 

 cut in one of these sliding microtomes, 

 three sides of it may be left more or less 

 rectangular, but the fourth side must come 

 to an aiigl(> pointing to the ))lade of the 

 knife. This angle is not important but 

 should be between 40*^ and 60"^. It does not 

 matter whether the sloping side extends 

 beyond the beginning of the object or not, 

 and it is actually of no ini|)oitance what 

 shape the other sides are i)rovided there 

 is a 40° to 60° angle pointing towards the 



knife. This angle is for the purpose of pre- 

 senting a small area of wax to the first cut. 

 Large sections on a microtome of this type 

 invariably roll themselves up into a cylin- 

 der which is \'ery difficult subsequently to 

 unroll. If, however, there is a sloping angle 

 pointing towards the knife, the flat portion 

 may be held with a brush against the knife 

 and the whole section, therefore, retained 

 more or less flat as it comes off. 



Now take an old microtome knife and 

 cut 25 or 30 micron sections from the top 

 until one gets down t(^ that part of the ob- 

 ject which one wishes to cut. This pre- 

 liminary flattening of the top surface of 

 the block, and cutting away of the un- 

 wanted portions of the specimen, also 

 shows how this particular block is behav- 

 ing in I'elation to the microtome itself. If 

 the sections curl hopelessly, in spite of the 

 point of wax, it is evident that the knife 

 is striking at the specimen too squarely 

 and it should be adjusted to cut at an an- 

 gle more like that of the knife shown in 

 Fig. 83 which is, however, set for celloidin. 

 A certain amount of maneuvering of the 

 knife angle l:)ackward and forward will en- 

 able one to secure a cut in which at least 

 half an inch of the pointed end of the wax 

 remains straight, thus permitting a brush 

 held in the left hand to be pressed down 

 while the right hand completes the move- 

 ment of the knife. Do not imagine that sec- 

 tions of this size will ever come off flat: it is 

 enough if they are reasonably flat. Each 

 section will have to be flattened independ- 

 ently in a bowl of water heated to from 5 

 to 10 degrees below the melting point of 

 the embedding medium employed. The 

 temperature is rather critical, but it may 

 be established by experiments on un- 

 wanted sections, so that when the block 

 has finallj' been trimmed down to the 

 point where the 10 or 15 essential sections 

 can be taken, all difficulties will have been 

 ironed out. It is inevitable, as one cuts 

 farther and farther into the object, that 

 fine readjustments of the orientation will 

 have to he made. These can only be nuule 

 by trial and error, and one should never 

 cut off too many sections until one has 

 finally got the block lying in the exact 

 plane reciuired. It may be said that this 

 l)lane may be determined reasonably when 

 some portion of the vertebrae are being 



