Microscopical Essays. 63 



image of the tenth part of one inch ; then the divifion, where the 

 two indices remain, will mew how many revolutions, and parts- 

 of a revolution, the fcrew . has made, while the needle point 

 traverfed the magnified image of the one-tenth of an inch ; fup- 

 pofe the refult to be twenty-fix revolutions of the fcrew, and 

 fourteen parts of another revolution, this is equal to 26 multi- 

 plied by 20, added to 14 ; that is, 534 thoufandth parts of an 

 inch. 



The twenty-fix di virions found on the {trait fcale of the: 

 micrometer, while the point of the needle paffed over the magni- 

 fied image of one-tenth part of an inch, were multiplied by 20, 

 becaufe the circular plate C D, Fig. 8, is divided into 20 equaf 

 parts ; this produced 520 ; then adding the fourteen parts of the 

 next revolution, we obtain the 534 thoufandth parts of an inclv 

 or 5 tenths and 34 hundredth parts of another tenth, which 

 is the meafure of the magnified image of one-tenth of an inch, 

 at the aperture of the eye glaffes, or at their foci. Now if we 

 fuppofe the focus of the two eye glaffes to be one inch, the dou- 

 ble thereof is two inches ; or if we reckon in the thoufandth 

 part of an inch, we have two thoufand parts for the diflance of 

 the eye from the needle point of the micrometer. Again, if we 

 take the diftance of the image from the object, at the It age at fix 

 inches, or fix thoufandths, and add thereto two thoufand, double 

 the dill an ce of the focus of the eye glafs, we ftiall have eight 

 thoufand parts of an inch for the difiance of the eye from the ob- 

 je6fc ; and as from the proportion, page 5.5,. we gather that, the 

 glaffes double the image, we muft double the number 534 found; 

 upon the micrometer, which then makes 1068: then, by the 

 following analogy, we fhall obtain the number of times the 



microfcope 



