ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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or " 5 mm. per turn, as the case may be, that amount will be recorded 

 on the scale with unerring - accuracy, and thereby enable fractions to 

 be read on the drum of the micrometer. To obviate backlash, the screw 

 works against a spring which is coiled in a box, thereby abolishing 

 the inaccuracies that creep in on account of strain and backlash. The 

 range of screw movement with the micrometer is one inch. If a 

 greater length is required than this, the stage is moved by means of the 

 quick-acting screw for a further inch, and a pivot bar F, exactly one 

 inch in length is interposed between the end of the micrometer and the 



Fig. 7. 



stage plate. This enables measurements up to 2 inches in length to be 

 taken. The vertical movement is effected by means of a rack-and-pinion 

 which again is maintained with a tension coiled spring shown at G. A 

 divided scale of the same accuracy, reading by means of a vernier to 

 ^ mm. or ^ in., whichever may be preferred, is fitted. The greater 

 portion of the measurements are intended to be made in conjunction 

 with the horizontal movement, hence the vertical movement is not pro- 

 vided with the same conveniences for fine measurement. These could, 

 however, be provided, if required. The eyepiece is fitted with a single 

 line for setting and reading, the two ends of the subject to be measured 



