Description of an improved Micrometer, 1-29 



scope, subtends an angle of 30 seconds, which being di- 

 vided by 575 the quotient is 52", the value oi' one inch of 

 the eye- tube. Consequently, 



•j—th of an inch on the eye-tube is = 



. •5jL.th of ditto is = 



Vu^th of ditto is = 



•j-^odlh of ditto is = 



The fractional parts of an inch on the eye-lube were ob- 

 tained very exactly, by means of a pair of fine pointed com- 

 passes and a diagonal scale. — But it may be alleged, that 

 the telescope cannot be adjusted a number of times to the 

 same object by means of the eye-piece, so that it shall be 

 drawn out preci'^ely to the same point. Errors in these ad- 

 justments will frequently take place, but they may be cor- 

 rected. The ailmnomer takes a mean of a number of his 

 observations ; and errors arising from adjusting the tele- 

 scope to I he object may be corrected in the same manner. 



The following observations, which were taken of the 

 sun's dlanieter with my 30-inch telescope, will show the 

 limits of these errors. 



The telescope was adjusted six different times to the sun, 

 so that his image extended over '■20 divisions on the micro- 

 nieter, and the lengths of the eye-tube drawn out each time, 

 in parts of an inch, were 042 . . .0-42 . . .0*44 . . 0'42 . . . 

 0-42. . .0'43; the mean of these different measures is 0'425, 

 which differs only 0"()15 from the extreme: and other sets 

 of observations were taken with the same precision. But, to 

 obtain correct observations, it is necessary that die telescope 

 should be a good one, and mounted upon a proper stand. 



The angular distance between two contiguous objects, at 

 a remote distance, may be determined thus : 



Direct the telescope to the objects, and if the angular 

 points do not fall upon two lines of the micrometer scale, 

 which will but seldom happen, draw out the eye-tube till 

 they do. Then multiply the value of one of the divisions 

 by the number of divisions, from which product subtract 

 the number of seconds indicated by the eye-tube, and thtt 

 remainder will be the angle sought. 



Example I. — Suppose the iniaire of the full moon ex- 

 tended over 18 divisions of the micrometer, when the eve- 

 tube was drawn out -j-Vudth parts of an inch : What was her 

 apparent diameter? 



Kach division contains 99 6 seconds; but when this value 



was determined the eye-liibe was drawn out n-'-^lb part of aix 



inch, which must always be deducted from the whole length 



\ ol. 38. No. 160. Jamst 181 1. I "of 



