ladd's microscope. 83 



stops wlien central. The stage is of an entirely new con- 

 struction, having vertical, horizontal, and circular move- 

 ments, and graduated for the purpose of registering objects 

 so as to be found at pleasure ; and in order to do this 

 eifectually a clamping piece is provided against which the 

 object sli)^e rests, and the circular motion of the stage is 

 stopped. It is an exceedingly effectual method of finding 

 a favourite object. The stage is remarkably strong, and 

 at the same time so thin, that the utmost obliquity of 

 illumination is attainable, the under portion being entirely 

 turned out : it has a dove-tailed sliding bar moveable 

 by rack and pinion; into this bar slides the under stage, 

 having vertical and horizontal motions for centering, and 

 also a circular motion ; into the stage are affixed the 

 various appliances for underneath illumination, removed 

 in our woodcut. Their achromatic condenser is of 170" 

 aperture, with nine openings and five central stops. The 

 openings and stops having independent movements, the 

 mani^Julator can regulate these at will, which is admitted 

 to be an improvement. There is an especial appliance 

 provided for the use of a set of dark- wells, with or with- 

 out the condenser. The mirror is attached to a quadrant 

 of brass and two arms, in order to obtain greater obliquity 

 of illumination ; the whole fits into a short piece of tube 

 made to slide either up or down the long tube attached 

 to the bottom of the stage by which the mirror is con- 

 nected with the other part of the stand ; the reflectors 

 are both plane and concave, as in other instruments. 



Powell and Lealand have another pattern, somewhat 

 resembling the instrument just described, but larger and 

 more massive in its general arrangements. The construc- 

 tion of the stage and sub- stage differ entirely : both of 

 these rest on a large solid brass ring, firmly attached to 

 the stem of the instrument. The upper side of this ring 

 bears a sort of carriage that supports the stage, and to 

 this carriage a rotatory motion is given by a milled-head, 

 the amount of the movement which may be carried tlirough 

 an entire revolution being exactly measured by the gradua- 

 tion of a circle of gun-metal, which is borne on the upper 

 surface of the ring. The rotatory action of the stage being 

 thus effected beneath the traversing movement, the center- 



a2 



