EDUCATIONAL VALUE OF PHOTOMICROGRAPHY. 151 



changing, that rocks are constantly being formed, metamorphosed and 

 disintegrated; that the earth has undergone radical changes, and that 

 the geography of the world in many particulars may be for one gen- 

 eration very different from that for the following one. Much informa- 

 tion on the details of these changes is gained from a minute study of 

 rocks and earth materials. 



By the aid of the microscope one is able to study the fine points in the 

 relation of different rock materials to one another, and by the aid of the 

 camera exhibit the result to others. As an illustration of this fact 

 Fig. 5 shows the position of a crystal of chloritoid in quartz, and Fig. 

 6, quartz, crossed by laminae of mica, taken with crossed Nicols to 

 properly differentiate the materials. These photographs are some of a 

 number made for the Geological Survey to be used in connection with 

 reports. The U. S. Geol. Report for 1899 on the geology of Yellowstone 

 Park shows many photomicrographs, admirably illustrating special 

 geologic features. 



Fig. 12. Cotton Fiber Injured in the Process of Ginning. X 40 Diameters. 



Much valuable information is also gained in this way of the structure 

 and properties of metals both in the ore and after smelting and refining. 

 Fig. 7 illustrates the appearance of iron whose tensile strength has been 

 exceeded, and it is easily possible to show also differences in composition 

 or quality of iron or steel by microscopic methods. 



The photography of microscopic sections of wood aid very materially 

 in a detailed consideration of forestry. It shows the character of the 

 small tubes or cells of which wood is made up, indicating the definite 

 way in which the cells of the new wood formed each year at the inner 

 surface of the cambium layer are arranged, depending upon the climate 

 where the tree grows. It shows the characteristic differences in cellular 

 structure of different woods, together with the lines separating the 

 growth of successive seasons in the trunk and bark ; the medullary rays, 

 which make the silver grain in quartered oak; and a host of other inter- 



