THE MICROSCOPE. 87 



with a power of from two to three diameters, and absolutely 

 achromatic. Specimens to be studied with a view to determining 

 their internal structures should be surfaced in a planer, and 

 smoothed by draw-filing in the direction of the fibre. The surface 

 thus obtained is treated with slightly diluted nitric acid, which gives 

 a rapid and wide development of the structure, which may be studied 

 with advantage while it lasts, and will prepare the student for finer 

 work. For fine development more care and time are needed. After 

 planing, the surface of the metal is ground with fine emery, or under 

 a metallic mirror-grinder. It is then treated with acid, Mr. Bayles 

 describing the manner in great detail. A thorough development 

 with weak acid requires from twenty-four hours to six days, accord- 

 ing to the composition of the metal. Small specimens are prepared 

 by planing down from the back to a thickness of -£% to ^ of an inch. 

 The planed face is then ground and surfaced on a fine whetstone, 

 developed with weak acid, and mounted between glasses with 

 Canada balsam. In selecting a microscope, care should be taken 

 that the lenses give a good definition, that there is no 'shake' or 

 or lateral motion in the adjustments for focus, and then the table 

 should admit of inclination at any angle found most convenient for 

 observation. Concerning the results to be expected from the micro- 

 scopic analysis of metals Mr. Bayles expressed the belief that it 

 opens a vast field of knowledge not yet reached by either chemical 

 analysis or physical test. There are many conditions, the result of 

 changes produced by mechanical treatment, to which chemical 

 analysis gives no clew, and which are detected, but not explained, 

 by the tests of the physical laboratory. The microscope will, no 

 doubt, explain many of the mysterious changes which occur in 

 metals of given chemical composition under different conditions, 

 and will give the metallurgist an opportunity of studying ,the 

 anatomy and physiology of iron and steel, which, in a most import- 

 ant sense, will supplement analysis and mechanical test, which have 

 thus far, to some extent, run in parallel lines. When, between the 

 report of analysis and the fracture of the broken test-piece, we can 

 place a polished longitudinal or cross-section of the material, its in- 

 ternal structure developed by acid, and admitting of careful micro- 

 scopic study we are furnished with the missing link in the chain of 

 evidence required for a correct conclusion as to the nature of the 

 material under investigation. 



