SCIENCE IN THE LAST HALF CENTURY. 89 



ligbtiiig is another great gift of science to civilization, the practical 

 effects of which have not yet been fully developed, largely on account 

 of its cost. But those whose memories go back to the tinder-box pe- 

 riod, and recollect the cost of the first lucifer matches, will not despair 

 of the results of the application of science and ingenuity to the cheap 

 production of anything for which there is a large demand. 



The influence of the progress of electrical knowledge and invention 

 upon that of investigation in other fields of science is highly remark- 

 able. The combination of electrical with mechanical contrivances has 

 produced instruments by which not only may extremely small intervals 

 of time be exactly measured, but the varying rapulity of movements, 

 which take place in such intervals and appear to the ordinary sense in- 

 stantaneous, is recorded. The duration of the winking of an eye is a 

 proverbial expression for an instantaneous action ; but, by the help of 

 the revolving cylinder and the electrical-marking apparatus, it is possi- 

 ble to obtain a graphic record of such an action, in which, if it endures 

 a fraction of a second, that fraction 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-microscopes. Such appliances have not only effected a 

 revolution in physiology by the power of analyzing 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 microphone, 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. 



PHOTOGRAPHY. 



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 never have attained its present important development had not 

 the progress of invention substituted paper and glass for the silvered 

 plates then in use. It is not my aff"air to dwell upon the practical appli- 

 cation of the photography of the present day, but it is germane to my 

 purpose to remark that it has furnished a most valuable accessory to 

 the methods of recording motions and lapse of time already in exist- 

 ence. In the hands of the astronomer and the meteorologist it has 

 yielded means of registering terrestrial, solar, planetary, and stellar 

 phenomena, independent of the sources of error attendant on ordinary 

 observation ; in the hands of the physicist not only does it record spec- 

 troscopic phenomena w:ith unsurpassable ease and precision, but it has 



