Beck^ on a Live-Trap and Parabolic Reflector. 117 
verse the direction of the light when carefully examining an 
opaque object. My attention was more particularly drawn 
to the subject by Mr. Sorby, who required some arrange- 
ment of reflector in the examination of his specimens of iron 
and steely and I applied half of a silvered paraboloid at the back 
of the object-glass^ but attached to its front tube, as shown in 
Fig. VII; this arrangement is very satisfactory^ and all the 
adjustments are easily made with the |rd and lower powers; 
its focus is about -^th of an inch from its lower edge^ and it 
must, of course, receive parallel rays, so that by lamplight a 
condenser must be placed at the distance of its focus before 
the light. 
To this reflector Mr. Sorby has made an addition, which 
shows how necessary it is to study the character of an object 
when determining the structure under any kind of illumina- 
tion, for he found, on examining his specimens of iron and 
steel, that, owing to the obliquity of the illumination, the 
brilliantly polished parts reflected the light beyond the aper- 
ture of the object-glass, and could not be distinguished from 
other parts which merely absorbed the light. 
Fig. VIII. 
