180 



THE MICEOSCOPE. 



central with the others : thus, the first condition, that 

 of centricity, will be accomplished. Remove the white 

 paper from the mirror, and also the centring-glass, 

 and replace the cap on the eye-piece, also the object 

 on the stage, of which distinct vision should then be 

 obtained by the rack and pinion, or fine screw adjust- 

 ment, should it have become deranged. 



Beck's New "Wet and Dry" Condenser (fig. 111). 

 In an earlier form of dry condenser (fig. Ilia) Messrs. 

 teck made use of a revolving front, with the intention 



FIG. 112. Powell and Zealand's Condenser. 



of obtaining large angular aperture, and of rotating a 

 series of lenses. They have more recently introduced 

 a new form, and the advantages to be gained are First, 

 That it is available for either dry or immersion object- 

 glasses up to 1'3 numerical aperture on diatoms, &c., or 

 dry ones on histological objects. Secondly, That the 

 spherical form of the front, worked by a milled head, 

 enables a series of lenses to be used, and yet avoids the 

 inconvenience of having the connecting fluid drawn 

 away from the one in use by capillary attraction, as 



