344 MICROMETRY. 



small dial registers complete revolutions of the screw, the fractional 

 parts being indicated by graduations on the larger disc, whose 

 smallest divisions are equivalent to "002 mm. The screw measures 

 up to 10 mm., or nearly "4 inch. 



Micrometer eye-pieces. — These may conveniently be divided into 

 those with fixed scale and those with movable scale. The eye- 

 piece may be either positive or negative, the former giving the best 

 view of the micrometer, and the latter of the object. 



In micrometers with fixed scales, the lines may represent frac- 

 tional parts of a millimeter or of an inch, or, as is usually the case, 

 be some arbitrary scale, the value of which must be determined 

 for different combinations of objectives, oculars, and tube length. 

 The scales ruled on glass in the form of squares, and which are 

 copied from the lattice micrometers of the older microscopists, 

 are not to be recommended for general work, although they are 

 sometimes used in special cases. 



In 1840, Geo. Jackson devised an improved form of Martin's 

 eye-piece micrometer. The scale was on glass and mounted in a 

 metal frame, with a screw at one end and a spring at the other. 

 The eye-piece, a negative one, was provided with slits to allow the 

 scale to be moved across the field. The arrangement is shown in 

 Figs. 5 and 6, the former being the micrometer, and the latter its 

 position in the eye-piece. 



The method of using this micrometer is familiar to most, but 

 for the benefit of others it may not be out of place to give one 

 method, especially as the same applies to other ocular micrometers. 

 It is first necessary to standardise the eye-piece micrometer. This 

 is effected by bringing the scale into focus, placing a millimeter or 

 inch micrometer upon the stage, focussing, and then adjusting the 

 two scales so that the zero of the eye-piece scale shall exactly cor- 

 respond with one of the lines of the stage micrometer. Determine 

 how many divisions of one are equal to an even number of divi- 

 sions of the other, when a simple calculation will give the value of 

 each division of the ocular scale in terms of the stage micrometer. 

 Thus, suppose that 0*5 mm. of the stage micrometer are equivalent 

 to forty-six divisions in the eye-piece, each division of the latter is 

 therefore equal to o'oioS; mm. Replacing the stage micrometer 

 by the object to be measured, it is found that its length is equal to 



