S. W. SMITH ON THE MICROSTRUCTURE OF METALS AND ALLOYS. 129 



possesses the disadvantage of involving considerable loss of 

 intensity by the passage of a lai-ge proportion of the light straight 

 through without undergoing reflection. 



Photographing the Structure. — In obtaining permanent records 

 of the structures seen under the microscope, the usual methods of 

 photo-micrography are adopted, the chief point on which a 

 difference of opinion exists being the relative merits of the 

 horizontal and vertical forms of enlarging camera. The question 

 of illumination is of considerable importance. Wherever possible 

 the arc light should be used, although good work can be accom- 

 plished with less brilliant illuminants, but these, of course, 

 necessitate somewhat long exposures. The Welsbach lamp is 

 very satisfactory, except that some difficulty may be experienced 

 in getting rid of the " gratings " caused by the structure of the 

 mantle. In any case the light is concentrated on the Beck 

 illuminator (or its equivalent) by a condenser. 



To those whose interest in this work may have been aroused, it 

 may be said that the alloys of tin, lead, antimony, bismuth and 

 zinc, offer ample scope for interesting work, and are, moreover, 

 easy to make, on account of their comparatively low melting- 

 points, while the study of steel under varying conditions of thermal 

 and mechanical treatment opens up unlimited possibilities, although 

 the various structures in this case are perhaps less striking, and 

 are often only visible under high powers. 



For further details respecting the application of the microscope 

 to the study of metals and alloys, reference should be made to the 

 various papers published in the " Philosophical Transactions of 

 the Royal Society," by Sir William Roberts-Austen, M. Osmond, 

 Ewing and Rosenhain, and others. Work on steel, which, as 

 already mentioned, Las received a large amount of attention, 

 will be found in the Journals of the Iron and Steel Institute. 

 An excellent quarterly publication, the Metallographist, devoted 

 entirely to metallography, and containiilg reviews of all important 

 work, is published by the Boston Testing Laboratories, U.S.A. 



The following list contains some of the most important papeii^ 

 that have hitherto been published on this subject, and it may 

 be useful to note that extracts of most of these will be found in 

 the volumes of the Metallographist already referred to : — 

 SoRBY, H. C. " Microstructure of Iron and Steel." Journ. 

 Iron and Steel liistitute, 1887, p. 255. 



