L. V. Pirsson — Rise of Petrology as a Science. 233 



Eosenbusch with the many modern treatises on petro- 

 graphic methods to be assured of this. 26 It is due on the 

 one hand to the vast amount of careful work which has 

 been done in accurately determining the physical con- 

 stants of rock-minerals* and in arranging these for their 

 determination microscopically, as in the remarkable 

 studies on the feldspars by Michel-Levy, and on the other 

 in researches on the apparatus employed, and in conse- 

 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 Eosenbusch 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 Eocks. 



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 



* We may mention here, for example, the work in mineralogy of Pen- 

 field, noticed in the accompanying chapter on mineralogy. In addition to 

 the accurate determination of the composition and constants of many 

 minerals, some of which have importance from the petrographic standpoint, 

 we owe to him more than anyone the recognition of fluorine and hydroxy! 

 in a variety of species, and thereby the perception of their pneumatolytie 

 origin. His papers have been published almost entirely in this Journal. 



