OBJECTS FOR POLARIZED LI&HT. 451 



pose are mentioned. So little being yet known as to what 

 effect is produced by polarized light upon many of the most 

 delicate organic structures, it should be laid down as a rule 

 that every new variety of tissue be submitted to its action, 

 and doubtless many important results would follow. 



Currents in Fluids observed during their Evaporation. — Mr. 

 Varley, in the fiftieth volume of the Transactions of the Society 

 of Arts, first introduced a series of experiments on this subject 

 to the attention of microscopists. The plan recommended is as 

 follows : — Take an animalctde cage of moderate size, and upon 

 the tablet place a drop of turpentine or spirits of wine, &c., 

 then slide over it the thin glass cover, but do not compress 

 the fluid very much; the microscope being placed in the 

 vertical position, and provided with a magnifying power from 

 forty to one hundred diameters, the contents of the cage are 

 to be examined in the same way as if animalcules were 

 contained in it; as the evaporation of either of these fluids 

 takes place, numerous currents and vortices will be seen, 

 especially if a small quantity of flnely powdered coal be 

 ground into them ; the particles of coal being very light, are 

 held in suspension whilst the evaporation is going on, and by 

 the currents are whirled about in different directions. The 

 following fluids Mr. Varley has given as the best for the illus- 

 tration of the currents : — 



" 1. A drop of spirit of wine, or of naptha, exhibits two, 

 three, or four vortices or centres of circulation, according to 

 the size of the drop ; and if these vortices are viewed laterally, 

 the lines of particles will be seen forming oblique curves from 

 top to bottom of the drop. 



" 2. Oil of turpentine shows a rapid circulation in two con- 

 tinuous spirals, one to the right, the other to the left, around 

 the drop. These meet in the opposite diameter, from which 

 the particles are carried slowly across the diameter to the place 

 of starting, and this continues while there is fluid enough to 

 let it be seen. 



" 3. If, however, the drop does not exceed one-tenth of an 

 inch in diameter, it presents the appearance of particles con- 

 29* 



