528 Supplement to Dr. Parry's Essay 



micrometer has its subdivisions magnified by its being placed in the focus of the 

 eye-glass; and whatever may have been the magnifying power of that glass, the 

 lineal magnifying power of the whole apparatus appears to have been 14 times as 

 great as that of the eye-glass.* In this case it is obvious, that if a filament of wool 

 placed on the stage of the microscope extends exactly from one parallel side of a 

 square in the micrometer to the next side, the filament so magnified is in diameter 

 precisely —To of a French inch, or about -j-tVt of an English inch. 



Now surely nothing can be more gross or inaccurate than this mode of exami- 

 nation. The filament must be so placed, as to lie precisely parallel to the sides of 

 some one square of the micrometer. If it presents itself diagonally, the micrometer 

 no longer affords a rule of mensuration. According to the common construction 

 of the compound microscope, the micrometer is included in the tube, and cannot 

 be shifted relatively to the object, while the eye is engaged in observing ; and with 

 regard to the filament itself, it mu5t be considered, that the space through which it 

 is moved on the stage is magnified proportionably to the compound magnifying 

 power of the whole apparatus relatively to that of the eye-glass. If, therefore, the 

 side of each square of the micrometer be -j— of an inch, and the magnifying power 

 of the microscope be 14 times as great as that of the eye-glass, if in attempting to 

 adjust the position of the filament on the stage, we shift the stage only -^— of an inch, 

 the apparent space through which the filament is moved will be -'^ of an inch, which 

 will be equal to 12 squares of the micrometer. 



Let the reader for a moment consider the difficulty of such an examination in 

 any case, but more especially in that of an object so subtle and tremulous as a 

 filament of wool, of which many hundreds may require to be succcs.-ivcly measured, 

 and he will readily admit the inapplicableness of this method of Daubenton. 



* Un micrometre repxesente un petit reseau. Cet instrument etant phcc dans >in microscope, 

 on juge de la grosseur du petit objet que 1' on observe, par 1' espac; q\ie cet objet parait occuper 

 ilans le micrometre, parceque 1' on sait quelle est V etendue dcs mailles du micronK-trc, et com- 

 bicn de fois le microscope grossit 1' olijet que 1' on y voit. Iiislruclioii pouv les Bergers, p. 338. 



II n'y a qii'un dixieme de ligne entre les cotes paralleles des carres dj micrometre dont je me 

 sers pour mesurcr la grosseur des filamens des laine.?. U est place au foyer de I'oculaire du micro- 

 scope. La lentillc grossit quatorze fois ; par consequent, la grosseur d'un filament de laine qui 

 est au foyer de cette lentillc, et qui parait occuper par sa largci:/ un carre entier du micrometre, 

 xi'est que de la cent-quarantieme partie d'une lignc. Ibidem, p. 340. 



