WITH THE MICROSCOPE. 39 



5. Standard* of Measurement. In this country we usually 

 employ the English inch, but on the continent the Paris line 

 = '0888, or about i-nth of an English inch, is very generally 

 used. The sign'" is used to signify " of a line," and has been 

 employed by Professor Kolliker in his works, while " signifies " of an 

 inch." 



6G. Conversion of Foreign Standards of Measurements. In 

 order to compare the researches of different authors, it is often 

 necessary to convert one expression of measurement into another. 

 The accompanying table of Dr. Robertson's (" Edin. Month. Jour, 

 of Science," Jan. 1852) will be found of great use in making these 

 calculations. See Table p. 40. 



Deputy Inspector-General Lawson gives the following rules for 

 converting different standards of measurement in a paper communi- 

 cated to my "Archives" (vol. II, page 292). A unit is required 

 that will admit of microscopic measurements being expressed in the 

 smallest number of figures, and permit of foreign measures being easily 

 converted into English, and vice versa, and the decimal notation 

 should be adopted to facilitate comparison between the measurements. 



Most microscopic measurements are under the hundredth of an 

 inch, one hundred-thousandth of an inch can only be measured with 

 certainty when magnified by the -^th or -g^th. See part V. The 

 requirements of the case therefore may be stated in decimals of an 

 English inch by -ooioi, and if the two ciphers next the decimal 

 point be struck out, and the first number be considered the unit, it may- 

 be written i t- oi, in which a thousandth of an inch is the unit. This 

 method will embrace nearly every microscopic magnitude in three 

 consecutive figures. 



The foreign measures are the millimetre and the French and Prus- 

 sian lines. The two latter are so nearly equal, that the same rule 

 will serve for the conversion of both. 



A millimetre contains "03937 English inch or 39 t- 37 ; accord- 

 ing to the method proposed, the length to be converted will seldom 

 amount to one fourth of this. To convert millimetres into thou- 

 sandths, shift the decimal point one place to the right and multiply 

 by 4 ; if greater accuracy be required, subtract i^ from the second 

 place of decimals for each of the nearest numbers 'of units of the 

 product. Thus o mm- 25o becomes 2-50 which X 4 10^00, from 

 which subtract ^15 ; and 9^35 is obtained as the value in thousandths 

 of an English inch, while o mm> 2^ is equal to 9^84, which differs from 

 the former by a quantity too small to measure. 



To convert thousandths of English inches into millimetres, add 

 i TT in the second place of decimals for the nearest number of units 



