382 INVERTED MICROSCOPE. 



sented in fig. 3, over the top of which is stretched a piece of 

 sheet india-rubber, and by pressing and relaxing it the liquid 

 is- drawn in, and by pressing the same the smallest possible 

 quantity can be discharged on the object subject to examination. 

 The tube also serves as a stopper to the vial, for the length 

 of the capillary extremity is such that it reaches nearly to the 

 bottom. 



The acids and ammonia used are always diluted to about 

 one half their ordinary strength, to prevent any unnecessary 

 disengagement of vapors. 



A movable stage, under many circumstances, is very con- 

 venient, and I have adopted one of a very simple character, 

 and quite equal to any of those where the motion is produced 

 by screws or pinions. It is a circular plate of metal or glass, 

 about three fourths of an inch less in diameter than the fixed 

 stage of the instrument, and an eighth of an inch thick, with 

 a hole in the center of nearly an inch diameter. This is laid 

 in the stage of the instrument, the glass sustaining the object 

 placed on it, and when required the former is moved by the 

 fingers, which can readily impart to it the most delicate motion, 

 as they are in part supported by the edge of the fixed stage. 

 For this suggestion I am indebted to Prof. Eiddell, and both 

 he and myself, after much experience, feel convinced of its 

 usefulness. 



In observing with high powers, as the object-glass is beneath 

 the glass supporting the object, and as this glass is usually of a 

 certain thickness, we have to change our method of observa- 

 tion — for all powers resorted to in chemical examination this 

 difficulty never occurs, and in using high powers it is easily 

 obviated. Where the object is already mounted and dry, the 

 thin glass can be readily turned downward; but where it is 

 moist — as, for instance, in examining fresh Desmidice and Dia- 

 tomacice— the following plan is resorted to, namely, to use a cell 

 made of a thin piece of brass or glass, perforated with a hole 

 about half an inch in diameter ; it is best to give the hole a 

 considerable bevel in one direction, as it facilitates the cleaning 

 of it; over the small end of the hole a piece of thin glass is 

 stuck with balsam or other cement. When used the object to 

 be examined is placed within, and a cover of thin glass placed 

 above. When brass is used to make the cell it may be as thin 



