THE MICROMETER. 



SO 



tlie image is ten times larger than the object, and the dimensions of 

 the object are ten times less than indicated by the micrometer. If 

 the divisions on the micrometer and on the magnified scale were not 

 equal, it becomes a mere rule-of-three sum; but in general this trouble 

 is taken by the maker of the instrument, who furnishes a table show- 

 ing the value of each division of the micrometer for every object-glass 

 with which it may be used. 



While on the subject of measuring, it may be well to explain the 

 mode of ascertaining the magnifying power of the compound micro- 

 scope, which is generally taken on the assumption before mentioned, 

 that the naked eye sees most distinctly at the distance of ten inches. 



Place on the stage of the instrument, as before, a known divided 

 scale, and when it is distinctly seen, hold a rule at ten inches distance 

 from the disengaged eye, so that it may be seen by that eye over- 

 lapping or lying by the side of the magnified picture of the other 

 scale; then move the rule till one or more of its known divisions 

 correspond with a number of those in the magnified scale, and a com- 

 parison of the two gives the magnifying power. 



Mr. Jackson has adopted a simpler and cheaper form of micro- 

 meter, represented in fig. 34, which is thus described by him in the 

 Microscopical Society's Transactions: It consists of a slip of glass 

 placed in the focus of the eye- 

 glass, and can be used with the 

 divisions sufficiently fine to have 

 the value of the ten-thousandth 

 of an inch with the quarter-inch 

 object-glass, and the twenty- thou- 

 sandth with the eighth ; and at 

 the same time the half, or even 

 the quarter of a division maybe es- 

 timated, thus affording the means 

 of attaining all the accuracy that 

 is really available. It may there- 

 fore entirely supersede the more 

 complicated and expensive screw- 

 micrometer, being much handier 

 to use, and not liable to derange- 

 ment in inexperienced hands. 



The positive eye-piece gives 

 the best view of the micrometer, 

 the negative of the object. The 



fig. 34. 



