INTRODUCTION 



[INTRO, 



hension of principles, and the reasons for what is done. For creative 

 work, then, knowledge of principles is indispensable. 



(2) Need of abundant practical work to go with the theoretical 

 part has been shown by all human experience. In all the crafts and 

 in all the fine arts mastery comes only with almost endless effort 

 and repetition, the most common example being the attainment of 

 facility in music. Hence in this work there have been introduced 

 many practical exercises so that the worker might gain the deftness 

 needed. It is also a part of human experience that in successfully 

 going through the manipulations necessary to demonstrate principles 



FIG. i. 



PROJECTION MICROSCOPE WITH ENLARGED REAL IMAGE 

 ON THE SCREEN. 



and methods, the principles and methods themselves become more 

 real. That is, comprehension of principles aids in the certainty with 

 which work can be done, and conversely the doing of the work helps 

 to increase the grasp on the principles. 



After observing the work of students in my own. and in other labora- 

 tories the conclusion was reached and expressed in the third edition 

 of this book (1891) that "simply reading a work on the microscope, 

 and looking a few times into an instrument completely adjusted by 

 another, is of very little value in giving real knowledge. In order 

 that the knowledge shall be made alive, it must be a part of the 

 -student's experience by actual experiments carried out by the 

 student himself." 



Beale, in his work on the microscope, expresses it thus: " The num- 



