MICROMETER EYE-PIECES 



271 



The most efficient piece of apparatus for micro-measurement is 

 without doubt the SCREW-MICROMETER EYE-PIECE ; it was invented 

 by William Gascoigne in 1639 for telescopes, and if well constructed 

 is a most valuable adjunct to the microscope. It is made by 

 stretching across the field of an eye-piece two extremely fine parallel 

 wires, one or both of which can be separated by the action of a 

 micrometer screw, the circumference of the brass head of which is 

 divided into a convenient number of parts, which successively pass 

 by an index as the milled head is turned ; it is seen in fig. 212, B. 

 A portion of the field of view on one side is cut off at right angles 

 to the filaments by a scale formed of a thin plate of brass having 

 notches at its edge, whose distance corresponds to that of the threads 

 of the screw, every fifth notch being made deeper than the rest to 

 make the work of enumeration easier. Formerly one filament was 

 stationary, the object being brought into such a position that one 

 of its edges appeared to touch tKe '"fixed wire, the other wire being 

 moved by the micrometer screw until it appeared to lie in contact 



m 



FIG. 212. The micrometer eye-piece. 



with the other edge of the object ; the number of entire divisions 

 on the scale then showed how many complete turns of the screw had 

 been made in the separation of the wires, while the number of 

 index -points on the edge of the milled head showed the value of the 

 fraction of a turn that might have been made in addition. Usually 

 a screw with 100 threads to the inch is employed, which gives to 

 each division in the scale in the eye-piece the value of x^yth of an 

 inch, whilst the edge of the milled head is usually divided into 100 

 parts. 



Both wires or filaments have since been made to move, a screw 

 and divided head being fixed to the stationary wire. There is no 

 advantage in this plan, and it involves needless complexity in calcu- 

 lation. The best method, there can be no doubt, is the one employed 

 by Mr. Kelson, which is to have one 1 thread fixed, but not in the 

 centre of the eye-piece, but five notches in the scale from the centre 

 on the side furthest from the screw-head. This not only permits of 

 a much larger object being spanned, but also keeps the average of 

 measurements in the middle of the ' field.' This is not only 



