SIDE-EEFLECTOR FOR OPAQUE OBJECTS. 



145 



Fig. 46. 



of using the light of a lamp for this purpose, is afforded by the 

 Side-B^ector contrived by Mr. Ross. This is a highly polished 

 concave speculum (Fig. 46), which can be placed above and to 

 one side of the object; and which is so mounted as to be capable 

 of being placed in every kind of position, according to the place 

 of the lamp, and the degree of obliquity 

 of the illumination required. The squared 

 stem, with which the speculum is con- 

 nected by several intermediate joints, may 

 be fitted to a socket, either in the stage or 

 in some part of the microscope-stand, like 

 that of the smaller condensing lens. The 

 light reflected by the speculum upon the 

 object, may be either that which falls on 

 it direct from the lamp, or may come to it 

 through the intervention of the bull's-eye, 

 arranged so as to throw parallel rays upon 

 the speculum (§ 64). The prisms already 

 described as in use for the illumination of 

 transparent objects by the reflection of 

 light from beneath, may also be employed, 

 by an inversion of their position, for the 

 illumination of opaque objects from above. 

 In Continental Microscopes, the prism is 

 frequently attached to the lower end of the 

 body ; but this is an undesirable mode of 

 supporting it, since the illumination is dis- 

 turbed by every alteration in the distance 

 between the body and the object. This 

 seems to be provided against by the mount- 

 ing of the prism in Mr. Grubb's micro- 

 scope (§ 60), which allows it to be used at 

 any angle either above or below the stage. 

 A mode of illuminating opaque objects by 

 a small concave speculum reflecting the 

 light directly down upon it, was formerly 

 much in use, but is now comparatively 

 seldom employed. This concave speculum, termed a " Lieber- 

 kiihn" from the celebrated Microscopist who invented it, is made 

 to fit upon the end of the objective, having a perforation in the 

 centre for the passage of the rays from the object to the lens ; 

 and it receives its light from the mirror beneath, the object being 

 so mounted as only to stop out the central portion of the rays 

 that are reflected upwards. The curvature of the speculum is so 

 adapted to the focus of the object-glass, that, when the latter is 

 duly adjusted, the rays reflected up to it from the mirror shall 

 be made to converge strongly upon the part of the object that 

 is in focus ; consequently, unless (as is sometimes done) the spe- 

 culum should be mounted on a tube sliding over the "nose" of 



10 



side-Reflector. 



