420 Mr. ramsden’s Defcription , of 
to me a matter of fome importance, to inveftigate the 
caufes of the uncertainty which has been found in the 
obfervations made with the micrometer with a divided 
objedt-glafs. The refult of my examination convinced 
me, that were it poffible to execute the conftrudtion of 
that micrometer, with the degree of accuracy required, 
it muft hill be fubjedt to inaccuracy from its prin- 
ciple. 
By the pofition of the micrometer every error of its 
glafs is magnified by the telefcope ; and if each furface 
of the micrometer glafs has not, in every part, precifely 
the fame radius (which opticians muft allow to be ex- 
ceedingly difficult) there will be a confiderable error in 
the angle to be meafured, and the eye applied to the dif- 
ferent parts of the pencil will, without moving the mi- 
crometer, fee the images of the objedt in the telefcope 
fludluating, fometimes appearing to overlap, and fome- 
times to feparate from each other. 
But fuppofing the glafs itfelf to be perfedt in its fub- 
ftance and in its curvature, there will yet remain imper- 
fedtions which arife from its principle. A micrometer glafs 
applied to a telefcope caufes a very confiderable abberra- 
tion. If the focus of the glafs is pofitive, the extreme 
aberration will be within the geometrical focus ; if nega- 
tive, it will be beyond it: and the aberration not only 
affedts 
