DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 1-13 



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 1 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 I -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 



