II. LENGTH. 



Gl 



is visible through the telescope, each variation in the position of the point at 

 the end of the lever will be magnified to a degree indicated by the quotient, 

 the numerator of which represents the double distance of the mirror from the 

 scale, and the denominator that of the point from the axis of rotation. 



As the latter distance can be diminished to one centimeter in the apparatus, 

 and as, moreover, the telescope with the scale can be placed at any distance 

 within which distinct images will be seen, say five metres, a shifting of the 

 reflected image by one millimeter will be equal to displacement of the end- 

 surface of the bar to be measured by O'OOl millimeter. As, however, the 

 tenths of the millimeter even can be pretty accurately determined, the reading 

 will be correct as far as the 10,000th part. ' 



There is no doubt that the correctness of the reading with this apparatus 

 can be carried still further, if mirrors of superior quality and powerful 

 telescopes are employed. 



In measuring the variations in the length of a wire a flat surface must be 

 given to that part to which the wire is to be suspended, as well as to the part 

 OH which the weights are placed, and to which the levers are to be applied. 



