118 Beck, on a Live-Trap and Parabolic Reflector. 
To throw the illumination, therefore, more perpendicularly, 
he attached a small flat mirror (Fig. VIII, m) immediately 
in front of the object-glass, and covering half of its aperture, 
at the same time stopping off, by a semicylindrical tube [oc), 
all illumination from the parabolic reflector ; by this arrange- 
ment (for the flat mirror is mounted so as to be easily turned 
aside by the small milled head w) Mr. Sorby obtains in an 
instant two difierent illuminations, and he finds the reverse 
appearances they give are valuable aids in analysing the true 
condition of the object. 
The difi'erence between the illuminations may be clearly 
seen by reference to the following diagrams (Figs. IX and X) , 
Supposing a to be the object-glass, and b an object with a 
perfectly reflecting surface at right angles to the axis of the 
microscope, it is evident that a ray of light [d) will be re- 
flected by the parabolic reflector (Fig. IX,/), and then by the 
object, to a point [e) entirely beyond the object-glass ; 
whereas in Fig. X the light is thrown by the flat mirror [g) 
almost perpendicularly upon the object, which consequently 
returns it to a point within the aperture of the object-glass. 
