CHEVALIEE S MOUNTING. 



79. When chemical phenomena are submitted to microscopic- 

 examination, and in general when liquids are observed which are 

 liable to evaporation, it is found 



inconvenient to place the stage 

 under the object-glass, inas- 

 much as the vapour proceed- 

 ing from the liquid being 

 more or less condensed upon 

 it, destroys the clearness of 

 the image. 



Acid vapours sometimes rise 

 from the substances under ex- 

 periment, which often tarnish 

 the object-glasses, and almost 

 always corrode the metal of 

 the instrument. 



In such cases, therefore, it 

 is necessary to provide means 

 to place the liquid under ob- 

 servation in a glass capsule 

 (a watch-glass, for example) 

 above the object-glass, which 

 must consequently be directed 

 upwards, the stage supporting 

 the capsule being over it. 



To accomplish this, the rect- 

 angular piece v x is turned 

 within the body upon its bay- 

 onet-joint through half a cir- 

 cumference, so that the object- 

 tube x is presented vertically 



upwards, as shown in fig. 39. The arm e f carrying the stage l r 

 the diaphragm h to limit the illumination, and the illuminating 

 reflector or lens #, is then fixed upon the tube x ; these pieces- 

 being severally moveable on the bar e f in the manner already 

 described. 



This arrangement is also useful when it is required to observe 

 minute bodies which sink to the bottom of liquids, or animalcules 

 which rarely come near the surface. 



In certain cases, also, the circulation of the blood can only be 

 observed with the instrument in this position. 



80. It is sometimes desirable to direct the instrument hori- 

 zontally towards the stage placed vertically. To accomplish 

 this, it is only necessary, after arranging the instrument as 

 shown in fig. 40, to turn the arm E c round through an angle of 



75 



