( HO ) 



In 1802 he was appointed Professor of Natural and Experimental 

 Philosophy at the Royal Institution. It is in the syllabus of these 

 lectures that there occurs the first publication of his most famous 

 discovery, the law of the interference of light, " one of the greatest 

 discoveries since the time of Newton, and which has subsequently 

 changed the whole face of optical science." Indeed, this discovery 

 exerted a profound influence on the rival theories of light — undulatory 

 versus the Newtonian hypothesis. The volumes published in 1807, .4 

 Course of Lectures on Natural Philosophy, and The Mechanical Arts in 

 Lecture XVII. " On Timekeepers " (Kelland's ed., p. 146, 1845), 

 contain the following passage. This marks the beginning of the 

 graphic method. 



" A chronometer may be constructed on this principle for measuring small portions 

 of time which appears to be capable of greater accuracy than Mr. Whitehurst's 

 apparatus, and by means of which an interval of a thousandth part of a second may 



possibly be rendered sensible. If two revolving 

 pendulums be connected with a vertical axis, in such 

 a manner as to move two weights backwards and 

 forwards according as they fly off to a greater or 

 smaller distance, the weights sliding, during their 

 revolution, on a fixed surface, a, a small increase of 

 velocity will considerably increase the distance of 

 the weight from the axis, and consequently the effect 

 of their friction, so that the machine will be im- 

 mediately retarded, and its motion may thus be 

 made extremely regular. It may be turned by a 

 string coiled round the upper part, and this string 

 may serve as a support to a barrel, sliding on a 

 square part of the axis, which will consequently 

 descend as it revolves. Its surface, being smooth, 

 may l)e covered either with paper or with wax, and 

 a pencil or a point of metal may be pressed against it 

 by a fine spring so as to describe always a spiral line 



YOUNG'S METHOD OF RECORDING , , ., .... 



MINUTE INTERVALS OP TIME BY 0n the baITe1 ' eXCe P* WheQ the S P V1Q % lS f ° rCed a 



means of a vibrating style little on one side by touching it slightly, either with 

 writing on a revolving fljg i ianc j j or ty means G f any body of which the 



motion is to be examined, whether it be a falling 

 weight, a vibrating cord or rod, or any other moving substance. In this manner, 

 supposing a barrel a foot in circumference to revolve in two seconds, each hundredth of 

 an inch would correspond to the six-hundredth part of a second, and the scale might be 

 still further enlarged if it were necessary. (Plate xv., Fig. 198.) By means of this 

 instrument we may measure, without difficulty, the frequency of the vibrations of 

 sounding bodies, by connecting them with a point, which will describe an undulated 

 path on the roller. These vibrations may also serve in a very simple manner for the 

 measurement of the minutest intervals of time ; for if a body, of which the vibrations 

 are of a certain degree of frequenay, be caused to vibrate during the revolution of an 

 axis, and to mark its vibrations on a roller ; the traces will serve as a correct index of 

 the time occupied by any part of a revolution, and the motion of any other body may 

 be very accurately compared with the number of alterations marked in the same time, 

 by the vibrating lx)dies. For many purposes, the machine, if heavy enough, might be 



