COLLECTION, EXAMINATION, ETC. 315 



In 1861 Dr. Wallich described the following very 

 ingenious way of obtaining slices of minute objects, 

 as the shells of the Foraminifera ; the chief point in 

 his method being the plan of turning a number of 

 specimens over together, so as to expose the alternate 

 sides for grinding. The specimens are cemented 

 with Canada balsam in the first instance to a thin 

 film of mica, which is then attached to a glass slide 

 by the same means ; when they have been ground 

 down as far as may be desired (upon the snakestone) 

 the slide is gradually heated, just sufficiently to 

 allow of the detachment of the mica-film and the 

 specimens it carries, and a thin slide with a clean 

 layer of hardened balsam having been prepared, the 

 mica-film is transferred to it with the ground surface 

 downwards. When its adhesion is complete the 

 grinding may be proceeded with, and as the mica- 

 film will yield to the stone without the least diffi- 

 culty, the specimen, now reversed in position, may 

 be reduced to the requisite thinness. 



In the preparations of sections of Foraminifera by 

 this method it is necessary, in the case of shells with 

 inflated chambers, to take care that the whole test is 

 permeated with the fluid Canada balsam, and in 

 order to do this the shells must be placed in the 

 balsam before the latter is quite tough, so that 

 during the few seconds of prolonged heating which 

 will be required to bring the cement up to the proper 

 degree of toughness, the balsam will have the oppor- 

 tunity of penetrating to the innermost recesses of 

 the shell. Instead of grinding off the mica-film, as 



