110 THE PROGRESS OF SriKNVF n 



and the electrical marking-apparatus, it is possible 

 to obtain a graphic record of such an action, in 

 which, if it endures a second, that second shall be 

 subdivided into a hundred, or a thousand, equal 

 parts, and the state of the action at each hundredth, 

 or thousandth, of a second exhibited. In fact, 

 these instruments may be said to be time-micro- 

 scopes. Such appliances have not only effected a 

 revolution in physiology, by the power of analysing 

 the phenomena of muscular and nervous activity 

 which they have conferred, but they have furnished 

 new methods of measuring the rate of movement 

 of projectiles to the artillerist. Again, the micro- 

 phone, which renders the minutest movements 

 audible, and which enables a listener to hear the 

 footfall of a fly, has equipped the sense of hearing 

 with the means of entering almost as deeply into 

 the penetralia of Nature, as does the sense of 

 sight. 



That light exerts a remarkable influence in 

 bringing about certain chemical combinations and 

 decompositions was well known fifty years ago, 

 and various more or less successful attempts to 

 produce permanent pictures, by the help of that 

 knowledge, had already been made. It was not 

 till 1839, however, that practical success was 

 obtained; but the "daguerreotypes" were both 

 cumbrous and costly, and photography would 

 have attained its present important development 

 had not the progress of invention substituted 



