4SS Major-Qeneral Roy's Account of tls 



nicely ground to receive it, and thereby preventing all fhake* 

 To the fquare nut, one end of a watch chain is attached ; the 

 other end having palTed around is fixed to a barrel, ^which con- 

 tains a watch fpring colled up in the ufual manner. By this 

 contrivance, any lofs of time in the motion of the moveable 

 wire, fixed to the iquare flide, is effediually prevented, whe- 

 ther the fcrew be turned backwards or forwards. The fixed 

 wire, fo called becaufe it is only m^ade ufe of occafionaily, ap« 

 pears in the elevation to the left-hand of the former, and is 

 farther removed from the obferver, being attached to the oval 

 flide which bounds the field of the micrometer. This wire 

 is moved by theinfertion 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 micafurement of fmall differences of ex- 

 panfion, 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 inflirument would have performed very 

 well without it. 



The confl:ru6lion of the microfcopes will be readily un* 

 derftood, by referring to the figures under that head on 

 the right-hand lide of the plate ; where the relative fitua- 

 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 dim^enfions; but the diftances 

 from thefe to the objedl lenfes and marks refpedively, are con* 

 tracled or broken off, from want of fufficient room to delineate 

 them Gtherwife. To increafe the angle of vifion in microfcopes, 

 it is always nec^ffary that they fliould have at leafl two eye- 

 glafles, and the fixed microfcope in the plate (hews them in 



their 



