180 METALLURGICAL MICROSCOPE 



design rendered necessary owing to the new type of construction) 

 from the corresponding parts of other similar apparatus. 



As is clearly shown in the illustration, in the new instrument I 

 have endeavoured to eliminate the first of the two disadvantages 

 mentioned, by supporting the stage by a bar fixed to it at two opposite 

 points. The bar is, in its turn, supported by the rack, the axis of 

 which coincides with the perpendicular of the stage, carried through 

 the centre of the stage itself. It is evident that in this way the defects 

 due to the sagging of the rack are eliminated, provided care is taken 

 in centreing the objects to be examined in the middle of the stage. 



In the instrument constructed by Messrs. Reichert it is possible 

 to support on the stage specimens weighing several kilogrammes, 

 without any appreciable deviation from the normal between the 

 optical axis and the plane of the polished surface resting on the stage. 



Owing to the frequency of cases in which in practice it is necessary 

 to place very heavy objects on to the stage, I have thought it necessary 

 to take the weight of the object off the fine focussing micrometer 

 screw, by fixing — as will be seen in the photograph — the screw itself 

 to the slide which carries the tubes of the visual and projecting eye- 

 pieces. The result is that the coarse movements and approximate 

 focussing are effected by moving the stage, while the comparatively 

 delicate movements required for very fine focussing are made by 

 manipulating the eyepiece tubes. It will be recognised, owing to the 

 smallness of the movements necessary to bring the objects into correct 

 focus, that such a modification does not detract from the proper 

 illumination of the object. 



I have overcome the second disadvantage mentioned by adding 

 to the microscope a proper revolving holder for 4 objectives. The 

 use of the revolving holder offers some difficulties in this case ; both 

 owing to the necessity for fitting it in such a way as not to hamper 

 the various functions of other components of the instrument, and 

 with a view to preventing any modification in the characteristic 

 dimensions of the objectives by deviating from those w^hich give the 

 best results in the examination of opaque metallic specimens illumin- 

 ated by reflected light. The first difficulty has been overcome by 

 replacing obliquely the bar which supports the stage, in the manner 

 shown in the photograph. The second difficulty has been eliminated 

 by giving to the revolving objective holder the special shape repre- 

 sented in the same illustration. 



The above description refers to the design of instrument for visual 

 observation. The complete apparatus as used for Photomicrography 

 is showTi in Fig. 2. 



In addition to the advantages mentioned above, the new Microscope 

 offers still another — not indispensable — of permitting the oblique 

 illumination of the specimen examined with the low power objectives. 

 The adjustment for oblique lighting, which already existed in the 

 Metallurgical Microscope designed by Martin, had to be abandoned, 

 from considerations of manufacture, in that of Le Chatelier, but it 

 has been satisfactorily applied in the]_new instrument, thanks to the 

 special design of its essential parts. 



For the reasons already referred to, it is unnecessary for me to 

 describe the new instrument in more detail. I would only add that in 

 practical application the features of design which I have briefly 

 outlined above have shown themselves to be extremely useful. 



