DISCOVERY BY MEANS OF NEW INSTRUMENTS. 471 



or its forces, produces different effects, and every new 

 aspect of viewing a phenomenon unfolds new truths, so 

 every different instrument, by being employed in one or 

 other of these ways, may do so likewise. We must, however, 

 remember that the invention of a new instrument often 

 itself depends upon the previous discovery of some scientific 

 fact or principle ; for instance, Volta could not have in- 

 vented his pile had not Gralvani and himself previously 

 discovered the truths upon which the construction and 

 action of that instrument are based. In this way, as I 

 have already remarked, a principle of alternation, and of 

 action and reaction, operates in scientific research : we 

 discover a fact or principle, and then invent an instru- 

 ment or experiment based upon it ; then, by means of that 

 instrument or experiment, we discover other new facts, 

 and so on ; and in this way the great fabric of science has 

 been, is being, and will continue to be raised. Invention 

 is a condition of discovery, and discovery is a condition of 

 invention ,; and in this way invention renders immense aid 

 to discovery, and discovery makes invention possible. The 

 aid afforded to the cause of original research by instrument - 

 makers who have improved the accuracy and power of 

 instruments, also by men of business who have had con- 

 structed for commercial and manufacturing processes in- 

 struments of great size and power, has been exceedingly 

 great. 1 



An immense number of discoveries have been made by 

 means of new or improved indicators and measurers of 

 time, space, number, sequence, mass, motion, cohesive 

 power, light, heat, electricity, magnetism, chemical power, 

 nervous force, mental action, and of all their modes of 

 being : indicators and measurers of the orders and speeds 

 of succession in time ; of the distribution and arrangements 



1 Compare H. Spencer's Principles of Psychology, pp. 460-462. 



