THE MICROMETER. 



41 



ing microscopical measurements ; but I am of opinion that a decimal 

 notation -would be preferable, if universally adopted. Take the instance 

 supposed, and let the microscope be furnished -with a draw-tube, 

 marked on the side with inches and tenths. By drawing this out a 

 short distance, the image of the stage-micrometer may be expanded 

 until one division is covered by twenty in the eye-piece. These will 

 then have the value of two-thousandths of an inch, and the object 

 ■which before measured nine will then measure ten ; which, divided by 

 2000, gives the decimal fraction -005. 



Enter in a table the length to which the tube is drawn out, and the 

 number of divisions on the eye-piece micrometer equivalent to an inch 

 on the stage ; and any measurements afterwards taken with that mi- 

 crometer and object-glass may, by a short process of mental arithmetic, 

 be reduced to the decimal parts of an inch, if not actually observed in 

 them. 



In ascertaining the value of the micrometer with a deep object- 

 glass, the hundreth of an inch on the stage will occupy too much of 

 the field ; the two-hundredth or five-hundreth should then be used, and 

 the number of divisions corresponding to that quantity be multiplied 

 by two hundred or five hundred, as the case may be. 



The micrometer should not be fitted into too deep an eye-piece, for 

 it is essential to preserve clear definition. The middle eye-piece is for 

 most purposes the best, provided the object-glass be of the first quality; 

 otherwise use the eye-piece of lowest power. The lens above the micro- 

 meter should not be of shorter focus than three-quarters of an inch, 

 even with the best object-glasses ; and the slit cut in the tube can be 

 closed at any time by a small sliding bar, as at I, fig. 34. 



We subjoin the following comparative micrometrical measures 

 given by Dr. Hannover, as a reference-table. 



The wonderful tracing on glass executed by M. Nobert, of Earth, 

 in Prussia, deserves attention. The plan adopted by him is, to trace 

 on glass ten separate bands at equal distances from each other, each. 



