WALTER ROSENHAIN 129 



difficulty might perhajDs be overcome in another way by adopting 

 geometrical contacts instead of plain sliding contacts. The advan- 

 tage of this system has long been recognised in theory, but instrument 

 makers do not appear to have seen their way to its adoption 

 on any large scale. 



The illuminator and its adjustments deserve a little further con- 

 sideration. Both for visual and photographic purposes the author 

 has found it a very great advantage to have an illuminator whose 

 position is capable of a very considerable range of adjustment. 

 Whatever form of reflector be employed, it is always an advantage 

 to be able to adjust its position not merely by rotation but by lateral 

 and longitudinal movement in the tube. This is important, not only 

 for the purpose of securing illumination at the precise incidence best 

 suited for showing any particular feature, but also for the purpose 

 of eliminating that most fruitful source of difficulties — internal 

 reflections from the lenses of the objective. 



Two further features of the mechanical design are of some impor- 

 tance. The first of these is the provision for a large working distance 

 between stage and objectives. This is necessary not only to provide 

 for the examination of thick specimens, but also because for many 

 purposes the use of long focus objectives is necessary. This latter 

 aspect of metallurgical work is assuming increasing importance at 

 the present time owing to the fact that the study of macro-structures 

 is now demanding much greater attention. In many cases these 

 macro-structures are large enough to be photographed with an 

 ordinary camera or even to be reproduced by means of direct contact 

 printing. There are, however, many conditions in which the macro- 

 structure is still sufficiently small to require magnifications of from 

 2 to 10 diameters, and it is very convenient for those who are not 

 in a position to set up a separate apparatus for this purpose if their 

 metallurgical microscope is capable of being used with long focus 

 objectives working either with or without an eye-piece. 



Another matter of some convenience in the metallurgical micro- 

 scope is the provision of a complete rotation of the stage together 

 with a simple centering device attached either to the stage or to the 

 nose-end of the body tube. Rotation of the specimen is important 

 for two reasons: — In the first place under oblique illumination the 

 aspect of an .etched surface varies in a most instructive manner with 

 varying incidence of the light, and it is sometimes convenient to 

 apply coloured illumination from two or more directions, and to 

 be able to rotate the specimens under such illumination. In the 

 second place, when a vertical illuminator is used which covers one- 

 half of the aperture of the objectives, the resolving power is much 

 greater in the direction parallel to the edge of the illuminator than 

 in the direction at right angles to it. 



Consequently in examining such a structure as finely laminated 

 pearlite, this may appear uniform or " sorbitic " when viewed in 

 the one position, while it becomes clearly resolved into laminae 

 when turned through a right angle. This, of course, applies mainly 

 to work at high magnifications under lenses of large resolving power. 



Turning to the optical equipment of the metallurgical microscope, 

 there can be no question that the requirements of metallurgy demand 

 the best and even more than the best that optical achievements can 



