66 



THE ART OF MAKING MICROSCOPE SLIDES 



Alga 



until the excess blue has been removed 

 from them. The object of this acid is not 

 to remove blue from tissues which have 

 already taken it, which it will not do, but 

 to rinse off the outside and thus leave 

 the red nuclei bright and clear. If, through 

 any accident, the material has been left 

 in the blue stain too long, it should be 

 transferred to a large volume of 95 % alco- 

 hol and left there until the whole of the 

 blue color has been removed. This will 

 also remove most of the red from the nu- 

 clei, and one must, therefore, start the 

 whole process over again. 



After the specimens have been stained 

 they must be put into Venice turpentine. 

 The reason for the selection of this mate- 

 rial is that they may be passed directly to 

 it from alcohol without the intervention 

 of any clearing agent which would cause 

 them to collapse. They cannot, of course, 

 be passed directly from the alcohol to full- 

 strength balsam but must be got into it 

 by a process of evaporation. The exact 

 strength to which one transfers them is 

 immaterial, but the strongest which can 

 be safely used is a mixture of ten parts of 

 95% alcohol to one part of the balsam. 

 No difficulty at all will be occasioned in 

 evaporating off the alcohol, provided it is 

 done in an absolutely dry place. A large 

 desiccator is, therefore, charged with silica 

 gel. This material was not available at the 

 time of the descriptions already cited but 

 it provides what the earlier workers lacked 

 — a material to be used in a desiccator 

 which both provides an adequately dry 

 atmosphere and absorbs a certain amount 

 of alcohol vapor. The weak Venice turpen- 

 tine containing the specimens is, therefore, 

 placed in an evaporating basin, or crystal- 

 lizing dish, and placed in a desiccator con- 

 taining siUca gel. As the alcohol is ab- 

 sorbed relatively slowly by the silica gel, 

 it is preferable to have two desiccators 

 and to transfer the dish from one to the 

 other daily, thus avoiding saturating the 

 atmosphere with alcohol. The evaporation 

 of the alcohol should l^e continued until 

 the Venice turpentine is as thick as the 

 original specimen; and it may be neces- 

 sary, after about two or three days have 

 been spent in the evaporation, to transfer 

 it to a desiccator containing a fresh batch 



of siUca gel in an oven at 30° to 40°C. 

 This also renders the Venice turpentine 

 more fluid and permits the mount to be 

 more easily made. 



When the Venice turpentine is suffi- 

 ciently thick, a perfectly clean shde is 

 taken and a small drop of Venice turpen- 

 tine is placed on it. A needle is then dipped 

 into the Venice turpentine and, with an- 

 other needle, one of the short filaments of 

 alga maneuvered alongside it; the first 

 needle is then drawn slowly at an angle 

 about 45 degrees from the Venice turpen- 

 tine, so that the filament will remain lying 

 along the side of it. The needle is then 

 laid flat on the Venice turpentine on the 

 slide and, by a roUing movement, the alga 

 is transferred to the drop. As many fila- 

 ments as are required should be trans- 

 ferred and the slide containing the speci- 

 mens in Venice turpentine should then be 

 placed in a desiccator. 



One of the easiest errors to make in this 

 technique is to leave a small quantity of 

 alcohol in the Venice turpentine before 

 starting to mount. If this is done, it is al- 

 most impossible to prevent the moisture in 

 the breath from clouding the resin and 

 ruining the whole long complex operation 

 that has already been undertaken. When 

 as many mounts have been prepared as are 

 required, they are removed one at a time 

 from the desiccator, a little fresh Venice 

 turpentine placed on top of each and the 

 coverslip apphed. They may then be 

 placed on a warm stage at a temperature 

 of 30° to 40°C. and permitted to harden 

 for a week or two. 



It is difficult to clean a Venice turpen- 

 tine mount for if one tries to dissolve off 

 the excess Venice turpentine with alcohol, 

 the covershp may be dislodged. It is better 

 to use only as much Venice turpentine as 

 will exactly come to the edge of the cover- 

 shp to avoid having to clean at all. Vos- 

 seler 1889 (23632, 6:292) recommends 

 that the mount, as soon as it is made, 

 should be ringed (as described in Chapter 

 2) with a solution of Canada balsam 

 in xylene, and that the Canada balsam 

 should then be hardened. This gives a very 

 attractive and clean mount and is strongly 

 to be recommended. 



