^lEASUREMEXT. 



[ 494 ] 



MEASUREMENT. 



further explanation. Supposing, however, 

 that the divisions of the stage-micrometer 

 are equal to 1-lOOOth of an inch, and 

 those of the eyepiece-micrometer equal to 

 l-20,000th of an inch (i. e. twenty of them 

 cover one space in the former), an object 

 brought into focus and covering five of the 

 spaces of the eyepiece-micrometer, will be 

 l-4000th of an' inch in diameter ; and so 

 for other dimensions. "When the objects 

 are large, the compasses and the ivory scale 

 wiU suffice for their measurement ; but 

 sometimes this may be conveniently done 

 under a low power, for the 1-lOOths of an 

 inch are not very clearly discernible to all 

 eyes. 



In measuring objects, they must be co- 

 vered with thin glass, and not immersed in 

 too much liquid. 



It is a matter of great diSiculty, under 

 high powers, to adjust accurately the divi- 

 sions of the eyepiece-micrometer to those 

 of the stage-micrometer, or to the margins 

 of objects, by means of the movable stage ; 

 a very ingenious apparatus has been con- 

 trived by 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 com- 

 municated by the end of a screw working 

 against one e"nd of the slide, and resisted at 

 the other by a spring ; and as the magiii- 

 i'ying power with which the divisions of 

 the eyepiece-micrometer are viewed is 

 small, the adjustment is easily and accu- 

 ratelv effected. 



Hartnack's diagonal scale is very useful ; 

 and very minute investigations and mea- 

 surements may be taken with it and 

 with Jackson's eyepiece-micrometer to 

 l-100,000th of an inch. 



Other micrometers, as the "cobweb- 

 micrometer," are made ; but as they are 

 very expensive, we shall pass them over. 



Some authors express the measurement 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 known 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 

 ascertained without measuring with com- 

 passes 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 num- 

 bers near the figures. 



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, or of a Paris, 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. 



The following data wiU be found useful 

 in reducing the foreign to tlie English mea- 

 sures : — - 



A millimetre = 0'0393707 English inch ; 

 or rather less than l-2oth of an English 

 inch. 



A centimetre = 0"393707 Eng. inch ; or 

 rather more than l-3i'd Eng. inch. 



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

 rather more than 1-11 th 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, Q-2b^^ (millimetre) X 

 0-0303707=0-G0984267o Engl. inch. But 

 in most cases a few decimal places only 

 need be observed. In this way, however, 

 w-e get a rather long sum, which may be 

 avoided by the use of the following Table. 



Table for conversion of foreign into English 

 measures. 



in which the numbers in the first (or left- 

 hand) column correspond to the denomi- 

 nations expressed in the uppermost (head) 

 line of the three broader columns, while 

 the fractions opposite these numbers denote 

 their values in parts of the denrmiinations 

 of the lowermost (head) line. Thus, 1mm = 

 0-039.370 Eng. inch; 3""ii' = 0118112 ; 2 

 Prussian lines = 0171033 Eng. inch, and 

 so on. In using this Table, the decimal 



