DETERMINATION OF THE MAGNIFYING POWER. 229 



them. The value of the divisions in the micrometer scale 

 and the magnifying powers of the different object-glasses are, 

 in most cases, supplied by the maker of the instrument — they 

 should be always so ; but for those who are anxious to work 

 the subjects out for themselves, it is hoped that the preceding 

 directions will not be deemed out of place. 



The late Mr. Coventry and Sir John Barton were cele- 

 brated for ruling micrometers of extreme delicacy ; some of 

 these, stiU extant, have as many as ten thousand lines in an 

 inch ; Mr. George Jackson, a gentleman whose name has been 

 so frequently mentioned in connection with the improvements 

 in the mechanical arrangements of the microscope, has also 

 paid considerable attention to this subject, and has succeeded 

 in ruling ten thousand lines in an inch, and in crossing these 

 at right angles with others precisely the same distance apart, 

 so that a series of squares are formed, each having a super- 

 ficial area of the one hundred millionth of an inch. These 

 are very remarkable as specimens of skill, but are far too 

 minute, either for measuring the magnifying power of an 

 instrument or the value of an eye -piece micrometer. 



In England, microscopists are in the habit of setting down 

 the measurements made by micrometers either in inches, or 

 in fractional or decimal parts of an inch ; but in France, and 

 some other parts of the continent, either a line or millimetre, 

 and fractional divisions of the same, are employed for a like 

 purpose. For the convenience of those who may wish to 

 compare foreign measures of length with our own, the follow- 

 ing tables have been drawn up, together with directions for 

 converting either English inches, or parts of an inch, into lines 

 or millimetres, or these last into English measures. 



Parts of an English Inch. 



A Paris line = .088815 or -^ of an English inch. 



A metre = 39.37100 inches English, or 3.281 feet. 



A centimetre ^ .39371 „ or a little less than f of an inch. 



A millimetre = .039371 „ or a little less than ^'s of an inch, 



