146 



THE ART OF MAKING MICROSCOPE SLIDES 



Cutting 



drawn, stood on their ends in a desiccator, 

 and dried. Once prepared, they may be 

 used an indefinite number of times. 



The process of casting the block is not 

 difficult. First of all take a paper collar 

 (ordinary bond paper is excellent) and tie 

 it firmly round the edge of the block so 

 that it projects upwards for a distance of 

 about M inch (Fig. 80). This makes a box 

 the floor of which is the end of the wooden 

 block and the sides of which are of paper. 

 Then pour into the bottom of this box 

 about K of an inch of 16% celloidin (Fig. 

 81) and place it in a desiccator (at the left 

 in Fig. 81) where the alcohol and ether 

 are allowed to evaporate until the surface 

 of the block is firm when touched with a 

 blunt needle. It is not required to be hard; 

 it is only required to be sufficiently firm 

 that an object placed on it will not sink. 

 Remove the block from the desiccator and 

 fill it to the brim with the 16% celloidin 

 containing the object (Fig. 82). The object 

 will sink through the hquid celloidin until 

 it comes to the firm layer underneath. 

 Needles are used to orientate it in the 

 desired position, and it is then either 

 placed in a desiccator to evaporate, or, 

 better, placed in a desiccator in the base 

 of which the desiccant has been replaced 

 by a quantity of chloroform (at the right. 

 Figs. 81 and 82). There is relatively rapid 

 vapor exchange between the chloroform 

 and the alcohol-ether, and the block by 

 this means may be completely hardened 

 overnight. If speed is vital, place the whole 

 block in liquid chloroform as soon as the 

 object has been oriented. By this means 

 the block will be hardened in a few hours, 

 but with some risk that the rapid diffusion 

 currents set up may displace the object. 

 The block should be stored in chloroform 

 until required. 



If the block is to be prepared by the 

 method of evaporation, a box of paper is 

 made and a layer of hardened celloidin set 

 on the bottom. After the object has been 

 oriented in the 16% celloidin, however, 

 the block is placed in a desiccator to evap- 

 orate, and is filled up from time to time 

 with 16% celloidin as it shrinks. Blocks 

 hardened by evaporation are denser and 

 tougher than those hardened with chloro- 



form. The chloroform technique is usually 

 more satisfactory. 



Cutting Sections in Celloidin 



There are as many methods of cutting 

 sections from celloidin blocks as there are 

 workers who have done it, and space does 

 not permit all the variations to be given 

 here. Broadly speaking they fall into two 

 classes: those in which the celloidin is cut 

 dry, and those in which it is cut wet. 

 Celloidin may be cut on any kind of mi- 

 crotome, but unless an attempt is to be 

 made to serialize sections (which is far 

 better done by the double embedding 

 technique described in the next chapter) 

 a shding microtome should be used. 



Celloidin cannot be cut by bringing a 

 square edge of the block against the knife. 

 Not only must the knife be set at an angle 

 of about 30° to the direction of travel 

 (Fig. 83) but the corner of the block must, 

 as shown, be trimmed to an acute angle. 

 The block, therefore, after being trimmed 

 to the shape shown, is clamped by its 

 wooden base in the holder and oriented in 

 the desired position. If the block is being 

 cut dry the knife is now shd forward and 

 the sections removed to a watch glass. Do 

 not worry if they are, as is more than prob- 

 able, considerably curled. When enough 

 sections have been accumulated they may 

 be dealt with in the manner to be de- 

 scribed later. 



The author much prefers to cut his 

 blocks after they have been moistened 

 with oil of cedar. There is a double reason 

 for this. Not only do the sections tend to 

 stay flatter, but if the block is thoroughly 

 impregnated with oil it will become glass- 

 clear so that last-minute adjustments of 

 orientation are easy. By this technique the 

 block, which should have been chloro- 

 form-hardened, is transferred directly to 

 oil of cedarwood and left until it is glass- 

 clear. When it is removed, as much as the 

 cedar oil as possible should be wiped off 

 with a cloth and the block mounted in the 

 appropriate holder. Then take a finger- 

 bowl, or beaker, of oil of cedarwood and, 

 after having adjusted the knife to approxi- 

 mately the correct angle, moisten the 

 blade of the knife with the oil. The knife 

 is then shd forward to remove a section, 



