Two new Micrometers. 429 
feci both the colours and fpherical aberration of the firft 
eye glafs. 
This micrometer is applied to the ereCt eye tube of a 
refracting telefcope, and is placed in the conjugate focus 
of the firit eye glafs : hence arifes its great fuperiority to the 
object glafs micrometer. It has been before obferved, that 
if a micrometer is applied at the object glafs, the imper- 
fections of its glafs are magnified by the whole power of 
the telefcope ; but in this pofition, the image being con- 
liderably magnified before it comes to the micrometer, 
any imperfection in its glafs will be magnified only by 
the remaining eye glafles, which in any telefcope feldonx 
exceeds five or fix times. 
By this pofition the fize of the micrometer glafs will 
not be the r^th part of the area which would be re- 
quired if it was placed at the objeCt glafs ; and, notwith- 
ftanding this great difproportion of fize, which is of 
great moment to the practical optician, the fame extent 
of fcale is preferved, and the images are uniformly 
bright in every part of the field of the telefcope. 
Fig. 4th, reprefents the glafles of a refracting tele- 
fcope; xy the principal pencil of rays from the objeCt 
glafs o; tt and uu the axis of two oblique pencils; a the 
firft eye glafs; m its conjugate focus, or the place of the 
micrometer ; b the fecond eye glafs, c the third, and d the 
Vol. LXIX. Lll fourth. 
