MEASUREMENT. 



[ 418 ] 



MEASUREMENT. 



sions of the eyepiece micrometer to those 

 of the stage-micrometer, or to the margins 

 of objects, by means of the moveable stage; 

 a very ingenious apparatus has been con- 

 trived by Mr. Jackson to overcome the diffi- 

 culty. It consists of a little brass frame, 

 in which the eyepiece micrometer slides 

 from side to side, the motion being commu- 

 nicated by the end of a screw working 

 against one end of the slide, and resisted at 

 the other by a spring; and as the magnifying 

 power wuth which the divisions of the eye- 

 piece micrometer are viewed is small, the 

 adjustment is easily and accurately effected. 

 Other micrometers, as the ' cobweb-micro- 

 meter,' are made ; but as they are very ex- 

 pensive and not necessary, we shall pass 

 them over. 



Some authors express the measm-ement 

 of objects by means of a ruled scale appended 

 to the figures or plates of them, the scale 

 consisting of divisions of a stage-micrometer 

 of knowTi value traced off under the same 

 power as the objects themselves ; or some- 

 times the divisions are ruled over the figures. 

 These methods are very objectionable, be- 

 cause the size of the objects cannot be ascer- 

 tained without measuring with compasses 

 and calculation, which is almost as bad as 

 the size being omitted altogether. 



Whenever figures of objects are given, the 

 magnifying power with which they are drawn 

 should always be expressed in numbers near 

 the figures. Many or even most authors 

 omit all notice of dimensions, so that whe- 

 ther an object figured be as large as an ox 

 or as small as a mite, is known only to them- 

 selves and their friends ; the student will 

 find for himself this to be the greatest difli- 

 culty in identifying objects in the study of 

 natural history, because the visible structure 

 of objects varies according to the power 

 under which these are seen. Other writers 

 state the magnifying power in a note in the 

 substance of the book, or in some obscure 

 and inconvenient place. 



Measures. — The measures in which the 

 dimensions of objects are expressed should 

 consist of parts of an English inch, and not 

 of a line. On the continent, fractions of a 

 millimetre, of a Paris or French line, and 

 of a Rhenish or Prussian line are used. 

 When fractions of a millimetre are adopted, 

 this is usually denoted by the addition of "^'^ 

 to the figure or figures. In France the mil- 

 Umetre and the Paris line are both used; in 

 Germany fractions of a line are expressed ; 

 but whether this is the Paris line or the 



Prussian line, w^e have never seen stated in 

 any of the works, although we believe the 

 Paris line to be generally signified. 



The following data will be found useful in 

 reducing the foreign to the English mea- 

 sures '. — 



A millimetre = 0-0393707 Enghsh inch ; 

 or (roughly) rather less than l-2ath of an 

 English inch, ss 



A centimetre = 0*393707 Eng. inch ; 

 (roughly) rather more than l-3rd Eng. inch. 



A Paris line = 0-088815 Eng. inch; or 

 rather more than 1-1 1th Eng. inch, to which 

 vulgar fraction it is nearest. 



To convert a foreign into the English 

 measure, the former must be multiplied by 

 its unit value ; thus, 0*25'*'" (millimetre) X 

 0-0393707 = 00098^2675 Engl. inch. But 

 in most cases a few" decimal places only need 

 be observed. In this way, however, we get 

 a rather long sum, which may be avoided by 

 the use of the following Table, in which the 



Table for conversion of foreign into English 

 measures. 



numbers in the first (or left-hand) column 

 correspond to the denominations expressed 

 in the uppermost (head) line of the three 

 broader columns, while the fractions oppo- 

 site these numbers denote their values in 

 parts of the denominations of the lowermost 

 (head) line. Thus, 1"^^ = 039370 Eng. 

 inch; 3°^^= 0-118112; 2 Prussian lines 

 =0-171633 Eng. inch, and so on. In using 

 this table, the decimal fraction to be con- 

 verted into parts of an English inch must 

 be broken up into its decimal parts, and 

 each valued separately from the table ; thus, 

 to convert 0'75^^ into a fraction of an 

 English inch — 



Q.^mm _ 00275595 



0-05"^ = 0-00196853 



I (by the table). 



Q.^smm _ 0-02952803 Eng. inch. 



