02 



Microscopical Essays* 



:be fuch a part of one-tenth, as is expreffed by their divifions. 

 Thus, if it be divided into ten equal parts, this will divide the 

 inch into one hundred equal parts; the firft divifion next c will 

 be equal to one hundredth part of an inch, becaufe it is the tenth 

 part of one-tenth of an inch. If thefe lines are divided into 

 twenty equal parts, the inch will be by that means divided into 

 two hundred equal parts. Laflly, if ab ca are made three 

 inches long, and divided into one hundred equal parts, we obtain 

 with eafe the cue thoufandth part. The fcale is reprefented as 

 folic! at Fig. 6, but as perforated at Fig. 5 and 7 ; fo that the light 

 palfes through the aperture, when the fe£loral part is placed on 

 the ftage. 



To ufe this fcale, firfl, fix the micrometer, Fig. 8, Plate IX. A, 

 to the body of the microfcope ; then fit the fect-oral fcale, Fig. 7, 

 in the ftage, and adjuft the microfcope to it's proper focus or 

 diftance from the .fcale, which is to be moved till the bafe appears 

 in the middle of the field of view ; then bring the needle point g, 

 Fig. 8, (by turning the fcrew L) to touch one of the lines c a ex- 

 actly at the point anfwering to 20 on the fecloral fcale. The 

 index a of the micrometer, Fig. 8, is to be fet to the nrft. divifion, 

 and that on the dial plate to 20, which is both the beginning and 

 end of it's divifions : we are then prepared to find the magnifying 

 power of every magnifier in the compound microfcope which we 

 ;are ufing. 



Example ; every thing being prepared agreeable to the fore- 

 going .directions, fuppofe you are defirous of afcertaining the 

 magnifying power of the lens marked No. 4 ; turn the micrometer 

 fcrew, until the point of the needle has paffed over the magnified 



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