122 



ACCESSORY APPARATUS. 



objective, although each division of the milled head, will thus 

 represent l-300,000th of an inch, yet the smallest measurable 

 space will be four times that amount, or 1-75, 000th of an inch. 

 With the l-12th inch objective, the smallest measurable space 

 may be about l-100,000th of an inch. 



46. The expensiveness of the cobweb-micrometer being an im- 

 portant obstacle to its general use, a simpler method is more com- 

 monly adopted, which consists in the insertion of a transparent 

 scale into the focus of the eye-piece, on which the image of the 

 object is seen to be projected. By Mr. Eoss, who first devised 

 this method, the "positive" eye-piece was employed, and a glass 

 plate ruled in squares was attached beneath its field-glass, at such 

 a distance that it and the image of the object should be in focus 

 together ; and the value of these squares having been determined 

 with each of the objectives, in the manner already described, the 

 size of the object was estimated by the proportion of the square 

 that might be occupied by its image. While the use of the posi- 

 tive eye-piece, however, renders the definition of the ruled lines 

 peculiarly distinct, it impairs the definition of the object; and 

 the " negative" or common Huyghenian eye-piece is now gene- 

 rally preferred. The arrangement devised by Mr. G. Jackson 

 allows the divided glass to be introduced into the ordinary eye- 

 piece (thus dispensing with the necessity for one specially adapted 

 for micrometry), and greatly increases the facility and accuracy 

 with which the eye-piece scale may be used. This scale is ruled 

 like that of an ordinary measure (i. e. with every tenth line long, 

 and every fifth line half its length), on a slip of glass, which is 

 so fitted into a brass frame (Fig. 33, B), as to have a slight motion 



towards either end ; 

 one of its extremities 

 is pressed upon by a 

 small fine milled-head- 

 ed screw which works 

 through the frame, and 

 the other by a spring 

 (concealed in the figure) 

 which antagonizes the 

 screw. The scale thus 

 mounted is introduced 

 through a pair of slits 

 in the eye-piece tube, 

 immediately above the 

 diaphragm (Fig. 33, A), 

 so as to occupy the 

 centre of the field ; and 

 it is brought accurately 

 into focus by unscrew- 

 ing the glass nearest to 

 the eye, until the lines of the scale are clearly seen. The value 



FIG. 33. 



Mr. Jackson's Eye-piece Micrometer. 





