396 Dr. G. W. Royston-Pigott on [Mar. 23, 



stantly to embrace (unlike ordinary micrometers of the usual form) 

 precisely the same number of threads in every measurement, so that on 

 the average the same number of threads probably represented nearly the 

 same distance. In the case where a spring is more and more compressed 

 and the number of threads embraced by the nut is constantly increasing 

 (although some compensating action may arise), the screw and parts are 

 certainly submitted to very varying and uncertain conditions. Another 

 source of error arose, — shake of rotation. Having abandoned the usual 

 form of micrometer construction (a revolving nut with a constant 

 change in the number of threads embraced — a plan, one would think, 

 fatal to all delicate accuracy), the next difficulty was to insure to the 

 steel screw absolute advance and retreat without rotatory shake or 

 motion. For this purpose slides were also abandoned. This action, the 

 most important part of the instrument, should now be described. 



A lever is affixed to the cylinder of steel and bent at right angles ; it 

 carries an adjustable weight. This weight slides upon a flat edge formed 

 parallel to the axis of the steel screw by a most careful process, tested 

 by a carefully prepared spirit-level for parallelism. 



On lifting the weight slightly, the lever rotates the screw through a 

 small angle ; and this lever forms a constant test of the efficiency of the 

 screw action of the greatest sensitiveness. 



A further action put into motion by a fine screw gives to the advance 

 of the film-forming surface, or prism-lens, a movement of the millionth 

 of an inch. 



Supposing that the recording-wheels have advanced several turns, the 

 weight and lever also advance on the smooth edge already said to be 

 formed parallel with the axis of the screw. 



The constancy of the weight preserves the screw in one normal fidu- 

 cial position, as regards its liability to rotate on its axis. An error of 

 one hundredth of an inch in the sliding edge would produce an error 

 of the reading of less amount than the hundred-thousandth of an 

 inch *. But the lever advances so very slowly, as the wheels rotate the 

 nut upon the screw, that this error appears to be almost destroyed. 



An arrow-head shows upon the face of the differential wheels the 

 number of turns taken by the nut. The instrument is self-recording, 

 and reads to four places of decimals, from the hundredth of an inch to 

 the hundred-thousandth. Two wheels, divided into 98 and 100 teeth 

 respectively, run in gear at will with a long pinion of ten leaves, 

 carrying a wheel showing the hundred- thousandths of an inch. 



The prism end of the screw passes through the ground socket ; and this 

 socket carries a small stage accurately turned and ground, furnished with 

 stops and a spring to confine the small plate of glass, if necessary, in one 



* The path of the weight on the lever would be for a complete rotation about 

 20 inches ; the proportion of ~1-q to this is 2000, and the two thousandth part of a 

 revolution is two thousandth of yj-^- ^yUTTOTFQ" 



