OF THK UNITKn STATES. 577 



ilie heavy brass truncated cone qq restina; on three less 

 like r. 



'I'lie steel tiiun2;le pp is moved vertically hy the strong 

 screw 5 of wliith the capstan head t is turned downwards, 

 and rests a'^ainst the smaller l)ase of an inverted hollow 

 cone int screwed fast to the solid [)iece q. This screw s 

 can he moved very gently, to bring the cobweb of the bar 

 ii.to the focus of the microscope, by a pin and handle fitted 

 to the holes of tlie capstan head t. 



'llie legs ;t. which support this apparatus, are of wood, 

 and a number (►f tliem ot dillerent lengths should be pre- 

 pared to suit the inequalities of the ground within the limits 

 ol the motion oi' the triangular bar />/;, by which the nearer 

 adjustment to the focus is made. 'I'hey screw in brass fer- 

 rules, which serve to connect them to the solid piece qq. 



Ill the actual use of the ap|)aratus, it is necessary to have 

 at least three such microscopic arrangements, that while two 

 of them are used before, the third may I)e left behind for re- 

 ference in case of accident. This arrangement will be taken 

 forward when the bar is laid oflTthe second time. 



'I'he point //. so determined in space, becomes that from 

 which, when the bar is moved forwards, the further mea- 

 surenient is to l)egin. By means of this apparatus, it remains 

 steady in its place, and the bar. when laid down in the next 

 position, is adjusted by making tl)e colnveb at the other end 

 c incide with the focus of the half lens corresponding to it, 

 as at^; all motions are then niade by the screw work on 

 the bar ap[)aratus, of which a description shall immediately 

 be given. 



The second and third requisites make it necessary that 

 the bar which is to measure the distance should be adjusta- 

 ble in all directions, ft will immediately be seen that the 

 longitudinal motion is attended with the most friction, and 

 must on that aecount be impeded as little as possible. This 

 precaution however seems to have been overlooked in the 



VOL. II. — o 2 



