260 A CENTUEY OF SCIENCE 



quent improvements in them and in ways of using them, 

 as exemplified in the delicately accurate methods intro- 

 duced by Wright. 27 The development of the microscope 

 itself as an instrument of research in this field and in 

 mineralogy deserves a further word in this connection. 

 The first step toward making the ordinary microscope of 

 special use in this way was taken by Henry Fox Talbot 

 of England, when he introduced in 1834 the employment 

 of the recently invented nicol prisms for testing objects 

 in polarized light. The modern instrument may be said 

 to date from the design offered by Rosenbusch in 1876. 

 Since that time there have been constant improvements, 

 almost year by year, until the instrument has become one 

 of great precision and convenience, remarkably well 

 adapted for the work it is called upon to perform, with 

 special designs for various kinds of use, and an almost 

 endless number of accessory appliances for research in 

 different branches of mineralogy and crystallography, as 

 well as in petrography proper. 28 This also calls to mind 

 the fact that for the convenience of those who are not able 

 to use the microscope special manuals of petrology have 

 been prepared in which rocks are treated from the 

 megascopic standpoint. 29 



Metamorphic Rocks. 



In this connection the metamorphic rocks should not 

 be forgotten. They afford indeed the most difficult 

 problems with which the geologist has to deal; every 

 branch of geological science may in turn be called upon to 

 furnish its quota for help in solving them. Under the 

 attack of careful, accurate and persistent work in the 

 field, under the microscope and in the chemical labora- 

 tory, with the aid of the garnered knowledge in petrol- 

 ogy, stratigraphy, physiography, and other fields of 

 geologic science, their mystery has in large part given 

 way. The inaugural work of Lehmann, Lossen, Barrois, 

 Bonney, Teall, and other European geologists, was par- 

 alleled in America by that of R. D. Irving, owing to whose 

 efforts the Lake Superior region became the chief place 

 of study of the metamorphic rocks in this country. 

 Irving soon obtained the assistance of G. H. "Williams, 

 who had been engaged in the study of such rocks, and the 



