SUGGESTED IMPROVEMENTS IN THE METALLURGICAL 

 MICROSCOPE 



By Professor H. Le Chatelier (Paris). 



The writer has for some considerable time been endeavouring to 

 extend the use of the Microscope in Metallurgical Works. No one 

 to-day will contest the services that Metallography renders to Industry, 

 and it is possible that the sphere of usefulness of this method of 

 investigation could be still further extended by improvements in 

 detail. 



The object of this brief note is to point out two possible improve- 

 ments. 



In the first case, to obtain good photomicrographs the use of 

 apochromatic objectives is necessary. These are very costly and 

 many workers hesitate to incur the expense of providing them. Would 

 it not be possible to persuade Manufacturers to design Objectives 

 corrected for some single wave-length of the spectrum ? — viz., the 

 blue line of the Mercury Vapour Lamp, which is easily separated from 

 the other rays and which moreover has a considerable actinic effect. 

 Such simple objectives in which it would only be necessary to take 

 into account corrections for spherical aberration could be manufactured 

 as a single lens and would thus be comparatively cheap. 



The second improvement, which it is desirable to introduce into 

 an objective used for the examination of metals is to give to the radius 

 of curvature of the back surface such a value as to prevent concentration 

 of the light reflected from this surface. In all Metallurgical Micro- 

 scopes illumination must necessarily be effected through the objective. 

 This is a new condition and consequently one complication more in 

 the construction of objectives, but perhaps it may not be insuperable. 



From an entirely opposite point of view it would be very useful 

 if a small handbook were drawn up for the use of those who employ 

 the Microscope, as well as for a few of the Manufacturers, such a 

 manual explaining the essential properties of the instrument. Every 

 day the grossest errors are made in this connection. A great number 

 of experimenters imagine that a Microscope Objective can be used 

 like a thin lens. They forget that every objective is constructed to 

 give an image at a fixed point, this being 16 or 25 centimetres according 

 to the country of manufacture. We frequently see photographs 

 taken wuth a Microscope objective, in whicn the adjustment (tube 

 length) is changed so as to project the image a greater or lesser distance 

 according to the magnification it is desired to obtain. Now, on the 

 contrary, the extension of the Microscope should always remain 

 invariable and a projection eye- piece used for taking the photomicro- 

 graph. The distance of the two lenses of this eye-piece should be 

 adjusted according to the magnification desired. 



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