192 DEVELOPMENT OF THE METALLURGICAL 



to the objective, the more even is the illumination obtained. In 

 the method of plane glass reflection, the rays of illumination and 

 of the image penetrate the entire objective simultaneously; the final 

 image suffers somewhat thereby, and does not appear as sharp as 

 with the prism illuminator. This disadvantage can be remedied 

 somewhat by decreasing the opening angle of the objective. The 

 important advantage of the 45 degree plane glass illumination is 

 that the light strikes the etched surface at exactly 90 degrees, ro 

 that with the highest magnifications and in working with very fine, 

 slightly etched specimens images richer in detail and free from 

 spectral colours are obtained; the rays of light are, in this case, 

 uniformly distributed over the entire field covered by the objective. 

 In applying the Le Chatelier prism illuminator, one half of the 

 objective serves for illuminating the specimen, the other half for 

 producing the image. This arrangement offers the advantage thit 

 by dividing the function of the objective the formation of reflexes 

 is reduced and the full angle of opening of the objective is utilised. 

 The images thus obtained are clearer and sharper, of special advan- 

 tage in photography. On the other hand, fine details of structuT-e 

 may be lost through the one-sided illumination striking the etch<3d 

 surface at an angle. Since both forms of vertical illuminators possess 

 certain distinct advantages and disadvantages, it will be found 

 very convenient to be able to change quickly from one to the 

 other, and to select the one which will give the more satisfactory 

 image, depending upon the nature of the microstructure under 

 investigation and upon those points that the metallographer desires 

 to bring out more prominently in his micrographs. The latest 

 metallographic outfit brought on the American market by my firm 

 possesses this important feature of *' selective " vertical illumination. 



Many of the steel works' metallographers prefer now the inverted 

 form of microscope, first designed by Le Chatelier and first made 

 by Pellin. I desire to mention here that the original Le Chatelier- 

 Pellin outfit carried a stage supported on one point only, which 

 was easily bent out of focus, and did not possess sufficient rigidity. 

 Le Chatelier designed in 1911 another and very much improved inverted 

 stand, also made by Pellin, which carries a firmly supported stage 

 and which was imitated by German and Austrian manufacturers. 

 Nevertheless, the largest number of German steel works, amongst 

 them the Krupp works, preferred the new Le Chatelier-Pellin stand 

 which was marketed in 1912 and 1913 with much success in Ger- 

 many by Dujardin, who imported the microscopes from Paris and 

 fitted them with Zeiss apochromatic objectives, thus combining best 

 mechanical design with good optical equipment. 



Returning to the question of metallurgical microscope stand 

 design, I want to say that the popular form of inverted stand really 

 has only the one advantas^e of eliminating the necessity of levelling 

 the specimen, and this advantage disappears mostly in using an oil 

 immersion lens. The disadvantages of the inverted stand are the 

 limited field which can be observed, the large leverage of the stage 

 resulting in magnification of vibrations, and the impossibility of 

 working with daylii^ht. Microscope stands have been successfully 

 designed (Felix Robin's outfit, formerly made by Collot, Paris), 

 which combine the advantage of horizontal camera with firmly sup- 



