252 A CENTURY OF SCIENCE 



The first thin sections appear to have been prepared in 

 1828 by William Nicol of Edinburgh, to whom we owe the 

 prism which carries his name. He undertook the making 

 of sections from fossil wood for the purpose of studying 

 its structure. The method he developed was in principle 

 the same as that employed to-day, where machinery is 

 not used ; that is, he ground a flat smooth surface upon 

 one side of a chip of his petrified wood, then cemented 

 this to a bit of glass plate with Canada balsam, and 

 ground down the other side until the section was suffi- 

 ciently thin. This method was used by others for the 

 study of fossil woods, coal, etc., but it was not applied to 

 rocks until 1850, when Sorby used it for investigating a 

 calcareous grit. Oschatz, in Germany, also about this 

 time independently discovered the same method. A fur- 

 ther advance was made in melting the cement, floating off 

 the slice, and transferring it to a suitable object-glass 

 with cover, a process still employed by many; though 

 most operators now cement the first prepared surface of 

 the rock chip directly to the object-glass, and mount the 

 section without transferring it. 



Next came the use of machinery to save labor in grind- 

 ing, and another step was made in the introduction of the 

 saw, a circular disk of sheet iron whose edge was fur- 

 nished with embedded diamond dust. This makes it 

 possible to cut relatively thin slices with comparative 

 rapidity, but the final grinding which requires experience 

 and skill must still be done by hand. Carborundum has 

 also largely replaced emery. The skill and technique of 

 preparers has reached a point where sections of rocks of 

 the desired thinness (0-001 inch), and four or five inches 

 square have been exhibited. 



The Era of Petrography. 



In these earlier days of the science, as noted above, 

 great difficulty was at first experienced in the recognition 

 of the minerals as they were encountered in the study of 

 rocks under the microscope. At that time the chemical 

 composition and outward crystal form of minerals were 

 relatively much better known than their physical and, 

 especially, their optical properties and constants. Some 



