ft! i c r o s c o p i c A l Essays. 



121 



To get at the lenfes in the Aider, Fig. 3, Plate V. take out the 

 two fcrews which hold on the cover, 



C, Plate IX. reprefents the general form of the flider-h older. 

 It confifts of a cylindrical tube, in which an inner tube is 

 forced up by a fpring. It is ufed to receive the ivory or any 

 other Aider, in which the tranfparent objects are placed ; thefe 

 are to be Aid between the two upper plates : the hollow part in 

 one of the plates is defigned for the glafs tubes. 



D, the condenfmg lens and it's tube, which fits into the Aider- 

 holder C, and may be moved up and down in it. When this 

 piece is puAied up as far as it will go, it condenfes the light of a 

 candle, which is reflected on it by the plain mirror of the com- 

 pound microfcope, and fpreads it uniformly over the object. ; in 

 this cafe it is befl adapted to the ftialloweft magnifiers. If the 

 deeper lenfes are ufed, it fiiould be drawn down, or rather re- 

 moved further from the object, that it may concentrate the light 

 in a fmall compafs, and thus render it more denfe. The con- 

 denfmg lens is fometimes fitted up differently, but the principle 

 being the fame, it will be eafy to apply it to ufe, notwithstanding 

 fome variations in the mechanifm. 



E, a brafs cone ; it fixes under the Aider-holder. It is ufed to 

 leflen occasionally the quantity of light which comes from the 

 mirror to any object. 



F, a box, with two fiat glafies, which may be placed at different 

 diftances from each other, in order to confine a fmall living infect. 



Q 



