1 2 8 Description -of an improved Micromcler. 



with ihe point of a small diamond, at the distance of -j-i^dth 

 part of an inch from one another. This micrometer is 

 placed in the focus of the eye-glass of a telescope or mi- 

 croscope, just where the image is formed when the instru- 

 n)ent is adjusted to the object. 



The telescope to which I have adnpted this micrometer is 

 a 30 inch achromatic with a compound eye-piece, consist- 

 ing of a sliort tuhe containing two glasses*. Now it is a 

 property of telescopes of this construction, that as the eye- 

 piece is drawn out the mao;ntfving power of the telescope 

 is increased, and consequently the image of the object upon 

 the micromeier is enlarged. By this means the fractional 

 parts of a division on the scale may be determined in a very 

 easy and expeditious manner, and to a very great degree of 

 exactness. 



The value of the divisions of the micrometer in the tele- 

 scope which I use, magnifying 23 times, was ascertained 

 by observations on the sun, in the following manner. 



When the eye-tube of the telescope was drawn out-^Vth of 

 an inch, the solar image extended over 19 divisions of the 

 micrometer. Then the diameter of the sun on the day of 

 observation, being taken out of the Nautical Almanack, and 

 divided by 19, gave 99"*6, which is the value of one division 

 of the nucrometer. After this value had been determined 

 by a great number of observations, I drew out the eye-piece 

 and re-adjusted the telescope until the image of the sun ex- 

 tended over 20 divisions on the micrometer ; and on mea- 

 suring the eye-piece I found that it had been drawn out 



rWodlh parts of an inch further than before. Then -?^ = 

 304''^"5 = one inch of the eye-piece. Consequently, 

 ■^th of an inch on the eye-piece is = 3o''*43"^ .i . 



■j^j-th of ditto is = l.5'"-22 1^1 



3?oth vf ditto is = 6"-09 ( "^ | 



Y^^dth of ditto is = 3"* J o S 



A micrometer, with lines drawn at the distance of -p^-^dth 

 of an inch from one another, was adapted to a 3| feet achro- 

 matic telescope magnifying from 45 to fio times, with the 

 same glasses. When the eye-j-jicce wasdrawn out-j-^thsof an 

 inch a remote land object extended over 23 divisions of the mi- 

 crometer, and when it was drawn out YVcV^l'h parts of an inch 

 further than before, the same object cxitudcd over 24 divi- 

 sions of the micrometer. Elach division of the scale, in this 



* Calletl the M'j\gonian eyc-picce. 



telescope. 



