46 8 Major-Geneml Roy’s Account of the 
nicely ground to receive it, and thereby preventing all fhafe. 
To the fquare nut, one end of a watch chain is attached ; the 
other end having paffed around is fixed to a barrel, which con- 
tains a watch fpring coiled up in the ufual manner. By this 
contrivance, any lofs of time in the motion of the moveable 
wire, fixed to the fquare hide, is effectually prevented, whe- 
ther the fcrew be turned backwards or forwards. The fixed 
wire, fo called becaufe it is only made ufe of occafionally, ap- 
pears in the elevation to the left-hand of the former, and ii 
farther removed from the obferver, being attached to the oval 
Hide which bounds the field of the micrometer. This wire 
is moved by the infertion of a milled-headed key (although not 
reprefented in the plate) fitted to flip upon the fquare end of 
its proper fcrew, which may be feen, in the elevation, projecting 
above the micrometer head. It has but little motion, being 
only intended for the meafurement of fmall differences of ex- 
pansion, or any fmall fpace, by leaving it there, while the 
other wire is repeatedly brought to coincide with, and again 
depart from it. For particular purpofes this wire may be ufe- 
ful ; neverthelefs, the inftrument would have performed very 
well without it. 
The conffruCtion of the microfcopes will be readily un« 
derftood, by referring to the figures under that head on 
the right-hand fide of the plate ; where the relative fixa- 
tions of the different eye-glafles, with regard to the wires 
or place of the magnified image, as well as to the eye, are 
truly reprefented in their real dimenfions; but the diftances 
from thefe to the objeCt lenfes and marks refpedively, are con- 
traded or broken off, from want of fufficient room to delineate 
them otherwife. To increafe the angle of vifion in microfcopes, 
it is always" neceffary that they fhould have at leaf!: two eye- 
glafles, and the fixed microfcope in the plate fhews them in 
their 
