THE SILVER SIDE- REFLECTOR. 



189 



he found that, owing to the obliquity of illumination, 

 the more brilliantly polished parts 

 reflected the light beyond the aper- 

 ture of the objective, and he could 

 not therefore distinguish them from 

 those parts which merely absorbed 

 the light. To throw the illumination 

 more perpendicularly upon the speci- 

 men, he was obliged to place a small 

 flat mirror immediately in front of 

 the objective, and cover half its aper- 

 ture, and at the same time stop-off, 

 by means of a semi -cylindrical tube, 

 the light from the parabolic reflector. 

 J>y such an arrangement, the light 

 produces the reverse appearances of 

 the former mode of illumination, and 

 is a valuable aid in determining the 

 true condition of the object. 



The Bull's-eye Condenser. The 



Fio. 123. Beck's Silver 



FIG. 123a. Beck's Pmraljolic Reflector. 



bull's-eye condensing lens (fig. 124) is used for con- 

 verging rays from a lamp upon the mirror; or for 

 reducing the diverging rays of the lamp to parallelism, 

 for use either with the parabolic illuminator, or silver 

 side-reflector. A plano-convex lens of about three 

 inches focal length, is the form generally adopted ; it 

 is borne upon a swivel-joint, which allows of its being 

 turned in any direction, and placed at any angle ; the 

 tube is double, and thus admits of being lengthened or 

 shortened. When used by daylight, its plane side 

 should be turned towards the object, and the same 

 position should be given when used for converging the 

 rays from a lamp ; but when used with the parabolic 



