DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 143 



distinct vision, let a drop of spirits of wine, or ether, 

 fall upon the infusoria, and observe the effect. As the 

 evaporation proceeds, the shells of the creatures will be 

 lifted up, presenting to the eye alternately their edges 

 and sides, whilst they are kept in motion. When all 

 the spirit has evaporated, excepting so much as is con- 

 tained within the shells, they will lie on their sides, and 

 the fluid within them will flow towards the opening. 

 Thus a correct idea is obtained, not only of their external 

 but also of their internal forms. 



The evaporation will be retarded by the addition of a 

 little water to the spirits of wine, and accelerated by the 

 use of ether. 



SECT. 20. On viewing the Currents in Fluids during their 

 Evaporation. For conducting this experiment, the fol- 

 lowing method is to be pursued. Place the microscope 

 in a vertical position ; select shallow or medium-power 

 object-glasses, which with the body shall magnify from 

 about 40 to 100 diameters; or single lenses from a 

 quarter to the tenth of an inch focus. Take an aquatic 

 live-box of moderate size, and place upon the centre of 

 it a drop of the oil of turpentine, or spirits of wine, &c. 

 Then cover it with the thin glass cover, taking care that 

 the thickness of the fluid be not more than about l-15th 

 of an inch. 



If this fluid be quite pure, the motion of its particles 

 will not be perceptible : but Mr. Varley, who introduced 

 the experiment to the Society of Arts, grinds a small 



