198 USB OF THE MICEOSCOPE. 



the small condenser, before described at page 123 ; this, gene- 

 rally speaking, is a double convex lens of two or three inches 



Fig. 126. 



focus, either supported on a separate foot, or else attached 

 to some Immovable part of the stand of the microscope, and 

 should be so placed that the rays of light from the large 

 condenser may fall upon it. These rays being slightly con- 

 vergent, and falling upon a plano-convex lens, will be ren- 

 dered more convergent, and brought to a focus, F, upon the 

 object, C, as seen in fig. 126, where L represents the lamp, 

 D E the large condenser, and A B the smaller. The small 

 condenser answers uncommonly well for daylight, for, in this 

 case, the rays being parallel, they will be made to converge 

 and be brought into focus upon the object. This mode of 

 illuminating an opaque object appears to have been first 

 contrived by Hooke, in 1675, for, in his Micrographia, 

 plate 1, the same arrangement is represented as the one 

 above described, but, instead of a large condenser, a globe 

 of glass full of water was used by him. 



The other mode of illuminating an opaque object by oblique 

 light is that by the side reflector, the invention of Mr. Ross : 

 it consists of a concave oblong silver cup, described at page 

 127, which is attached to some immovable part of the stage or 

 stand of the instrument, and is supplied with a sliding arm 

 and a ball and socket joint for adjustment, so that it may be 

 used for an opaque object either placed on the stage, or held 

 by the forceps ; the mode is as foUows : — 



