chevalier’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 g , 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 
