MICROMETER. 



221 



value of the sides of the squares, with the different object- 

 glasses. The plan adopted by Mr. Eoss is here shown. The 

 upper line indicating the value of the divisions in frac- 

 tions of an inch, the lower line the same value in a decimal 

 notation. 



To find the Value of each Revolution of the Screw, or parts 

 of a Revolution of the same, in the Cobweb Micrometer. — y.^hen 

 about to be used, the cylindrical portion containing the eye 

 and field lens is to be placed in the end of the compound 

 body, in the same manner as an ordinary eye-piece, and a 

 micrometer divided into hundreds and thousands, as employed 

 with the other instruments, placed on the stage, and its lines 

 brought into the focus of the object-glass. The graduated 

 head of the micrometer being set at zero, and the cobwebs 

 exactly coinciding with each other, the milled head is now to 

 be turned, and notice taken, how many revolutions or parts 

 of a revolution have been made when the cobwebs are opened 

 sufficiently wide, to cover a certain number of the divisions of 

 the image of the stage micrometer. It having been previously 

 stated, that the screw employed to separate the cobwebs is 

 provided with a graduated circle divided into a hundred parts ; 

 should it be found that five revolutions of the screw cause the 

 cobwebs to open, so as to cover ten divisions in that part of the 

 stage micrometer divided into thousands, it follows that one 

 revolution of the screw wotdd be equal to the -^^-^ of an inch, 

 and one division of the graduated head would be ^^^ of ^-^ q 

 or s-^.^TyTj- of an inch. The comb at the bottom of the field of 

 view, as shown by fig. 144, wiU indicate the number of the 



