ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



653 



the end of the screw ; the screw also being made hard and preferably small. 

 In another form (fig. 176 top view, with the top E removed ; fig. 177, section 

 of fig. 176 through K S), two screws are made on the same piece, each made 



of a different pitch, and a whole or split nut for each part of the screw, one 

 nut and the corresponding screw being attached to the frame that carries 

 the cross-hairs. 



He proposes to use wires of glass or other suitable material instead of 

 spider-webs, and to apply short cross-wires parallel with and opposite to 

 each other, leaving a space between them and in various positions, so that 

 the operator can have several points to guide him in adjusting the micro- 

 meter on the object to be measured. He says, — 



" It is well known to mechanics that a screw loose in the nut cannot 

 be depended upon for great accuracy and uniformity in measurements, 

 notwithstanding the slack may be taken up by a spring, as particles of 

 matter are liable to get between the threads and cause errors. That 

 difficulty is avoided in this improved micrometer. From experiments it is 

 believed that the cross-hairs in a micrometer made according to this 

 improvement can be adjusted to a line a number of times — say five, more 

 or less — within an error of • 00005 of an inch. It greatly facilitates the 

 adjusting of the cross-hairs to a line to have the screw move a considerable 

 part of a revolution for each division of the index-wheel. It is difficult to 

 move the screw made in the ordinary way little enough to adjust the cross- 

 hairs in the most accurate manner, and the difficulty in moving it little 

 enough often influences the operator to accept an adjustment as correct with 

 which he is not fully satisfied. 



In the drawings I have illustrated a screw made in two parts on the 

 same piece, one part being 20 pitch and the other 25 pitch, which gives 

 a movement to the cross-hair frame of 1/100 in. each revolution, this 

 being intended for ordinary work; but in a micrometer for very fine 

 measurements I should use a screw from 35 to 40 pitch. 36 and 

 37 • 087 pitches would be 3/4000 in. approximately, to each turn of the screw, 

 and the object being magnified fifteen times, and the index-wheel divided 

 into ten parts, one division on the wheel would be 1/200,000 in., and the 



