Manchester Memoirs, Vol. li. (1907), No. 15. 3 



tion of 100 diameters.* Notice the distinction in texture 

 of the various grains, and the irregular boundaries 

 between them. 



In this example the microscope was looking directly 

 at the specimen, of course, and the light by which the 

 specimen was illuminated passed down through the tube 

 of the microscope directly on to the specimen, perpen- 

 dicular to the surface, and back again into the lens by 

 reflection. But we can illuminate the specimen in a 

 different way. We can throw upon it light from the side, 

 and only a portion of the light dispersed by the different 

 granules will be sent up into the microscope. When we 

 do that we find a very striking difference in appearance. 

 Here is an example {Fig. 2, Plate II). It happens that 

 this is a photograph of cast lead, but the difference 

 between it and Fig. 1 is mainly an effect due to the oblique 

 illumination. You see again the separate grains or 

 granules with their boundaries, but you now find that 

 some of them are very dark, others very bright, and others 

 of intermediate shades. You also find when you move 

 the light about, so as to make it fall first from one 

 direction and then from another, and watch the same 

 grains under the microscope, that a grain which is bright 

 at one moment will be dark at the next, when the light 

 is moved into a different position, and on the contrary, a 

 grain which was dark before will turn out of an inter- 

 mediate shade, or perhaps flash out quite bright. This 

 becomes intelligible as soon as we consider of what it is 

 that these grains consist. 



We are accustomed to think of a crystal as a thing 

 with well defined geometrically formed external boundaries. 

 Xhe grains that build up a piece of metal do not possess 

 that characteristic, but for all that they are true crystals. 



* Ewing and Rosenhain, Phil. Trans., ser. A, vol. 193, p. 553, 1899. 



