THE MICROSCOPE. 



tally. It will appear to pass gradually under the vertical wire 

 until its other extremity shall coincide with that wire. If then 

 the number of complete turns, and parts of a turn of the screw 

 be counted, the length of the object then will be known. Thus, 

 if at the end of ev^ry complete turn, the screw produce an 

 audible sound like the tick of a clock, the observer can count 

 the complete turns, and if the circumference of the head be 

 divided into 100 parts, and that an index be fixed upon the 

 stage to indicate the position of the head at the commence- 

 ment, the decimal parts of a turn can be ascertained, each division 

 of the head corresponding to the 100th part of a complete turn, 

 and therefore to the 10000th of an inch. 



By turning the stage so that the screw will cause the object 

 to move across the field in the direction of the vertical wire, its 

 dimensions in the other direction can be ascertained. 



59. A simple and ingenious micrometer for ascertaining the 

 dimensions of such objects as would bear a slight pressure without 

 change of form, was invented by M. Le Baillif. A plan and 

 vertical section or side view of this are shown in fig. 26. 



Two upright pieces, c c, are fixed in a slip of copper, formed 

 like one of the slides, having a circular hole in its centre, in 



Fig. 20. 



which is set a plate of glass, on which a scale o is engraved. 

 Upon this is placed a moveable piece, e e, having a similar hole 

 and plate of glass, with a fine line engraved upon it at right angles 

 to the scale, so that when it is moved from left to right this fine 

 line will coincide necessarily with all the divisions of the scale. 

 From this piece, two rods proceed, which pass through holes in the 

 upright pieces c c, and one of them is reacted upon by a piece of 

 watch-spring, f y while the other abuts against the end of a fine 

 screw, I, which moves in a nut, d. 



When an object is to be measured, the index line upon the 

 upper glass disc is brought to coincide with the first division or 

 54 



