METALLOGRAPHY 197 



as to project an image of the radiant on to the reflector. 

 The objective acting as a condenser forms a minified 

 image of the condenser aperture on the metal surface. 

 Haze-producing reflections at the various surfaces are 

 among the greatest difficulties encountered, and, as a 

 further help to their elimination, the illuminated area on 

 the specimen should be cut down by means of the iris in 

 front of the condenser until it is just that shown in the 

 eyepiece or on the camera ground glass. An iris is also 

 frequently fitted to the nose-piece. 



It is obvious that this method of illumination, which 

 depends on the projection of a beam of light into a smal] 

 aperture in the tube behind the objective, could only be 

 carried out w r ith great difficulty if the objective were 

 focussed on to the object in the usual way. To avoid 

 this difficulty and consequent loss of time, microscopes 

 for metallography are constructed in such a way that 

 the stage can be moved up and down in the optical axis, 

 in place of, or as well as, the tube. On some stands the 

 stage is fitted with a coarse and a fine adjustment, on 

 others the fine adjustment is fitted to the tube, as the very 

 small motions caused by final focussing with it scarcely 

 necessitate any alteration of the position of the beam of 

 light. The stage itself is frequently made in a special 

 manner, it may be a plane stage without any aperture, 

 and so suited for opaque bodies only, or it may be fitted 

 to hold moderately large pieces of metal between suitable 

 jaws which can be turned about to present the desired 

 surface to the objective. And further, certain stands 

 have no stage at all, but are made to clamp on to any 

 large piece of metal that it is desired to examine. Mirrors 

 are frequently mounted above the stage for reflecting 

 light on to the object. Most makers now make a stand 

 embodying these requirements as an elaboration of their 

 ordinary type, and many of them can be used for any class 

 of work, and are not necessarily restricted to metallography. 





