of Bronzes from Machu Picchu, Peru. 575 



magnifications which are too high to permit the use of pris- 

 matic illumination through the objective requires the develop- 

 ment of considerable contrast upon the specimen by etching. 

 The difficulties in this direction have already been set forth. 

 In view of the fact that similar but more distinct secondary 

 structures obtained in the study of other objects will be 

 described and illustrated shortly, it has seemed appropriate to 

 forego the introduction of illustrative photo-micrographs here 

 and apply an annealing test to confirm the results of the original 

 examination relative to the distribution of the shaping forces 

 (hammer blows) over the surfaces of the specimen. 



A temperature of 700° and a time of one-half hour was 

 adopted for the anneal. The entire longitudinal section was 

 annealed and then explored under the microscope. The general 

 effects obtained in annealing different classes of material have 

 already been discussed. 



As may be anticipated from the original examination, the 

 regions b and d of the annealed metal are now nearly homo- 

 geneous, showing a grouping of irregularly bounded polygonal 

 grains without twin formation. This proves that the metal in 

 these regions had never been worked. On the other hand, the 

 regions a and c possess a fine-grained structure in which 

 twins are abundant, i. e., the type of structure which would 

 be developed after anneal of cold-worked metal. The two 

 conditions are represented by figs. 39 and 38, respectively. 

 The points at which the photo-micrographs were taken are shown 

 on the diagrammatic sketch. The structures are quite uniform 

 in the areas indicated with rather sharp transition zones. The 

 degree of homogeneity attained in the anneal was strictly com- 

 parable with that attained in the anneal of our own material 

 under similar conditions, i. e., only faint shadows accentuated 

 by changing the focus or moving the illuminating prism are 

 seen after anneal for one half hour at 700°. These are due 

 to slight changes in elevation developed by selective etching 

 determined by the varying copper-content at different points. 

 Some of these shadows are faintly seen in fig. 38, particularly 

 across the upper left-hand corner of the photo-micrograph. 

 The fact that this specimen has homogenized to about the 

 same extent as our own cast and hammered specimen proves that 

 any previous heat treatment was not drastic, as previously 

 deduced from other considerations. The size of the recrystal- 

 lized grains after anneal is also comparable with that obtained 

 under similar conditions with our own specimens. The num- 



